Twin dump, single dump downpipe???
28 November 2001, 05:45 PM
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Hi, I'm after getting a downpipe but which do I go for???
Was going for a hks item as this is the make of the current exhaust, however on speaking to a helpfull chap at a suppliers he said they do there own twin dump, single dump or the hks one I was after.
Anyone have any experience with any of these that can advise me which one to get??
Prices are very similer.
Cheers, crypt.
Was going for a hks item as this is the make of the current exhaust, however on speaking to a helpfull chap at a suppliers he said they do there own twin dump, single dump or the hks one I was after.
Anyone have any experience with any of these that can advise me which one to get??
Prices are very similer.
Cheers, crypt.
28 November 2001, 05:55 PM
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Scoobysport 3 inch downpipe is supposed to be the one that gives the best top end gain
I have a BPM twin dump and am changing to a Sccobysport downpipe with HKS Center and rear.
This is partly to do with power but just as much to do with the sound of the bpm finally driving me insane
I have a BPM twin dump and am changing to a Sccobysport downpipe with HKS Center and rear.
This is partly to do with power but just as much to do with the sound of the bpm finally driving me insane
28 November 2001, 06:20 PM
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I'm in a similar situation. Currently have a super drager with the HKS single dump downpipe on my MY95 WRX and am thinking of going for another downpipe. Initially I was thinking of getting the scoobymania one (225 +vat) but have now thought I may be better considering a few options first, not really knowing which is best.
I was thinking of maybe going for a scoobysport downpipe and centre (270 +vat) and putting my original back box on. Then I could sell the HKS and then go for the SS backbox. This would probably not work out much more money wise. then there's the question of noise - I think there is enough of it now so probably don't want to make it any louder.
But I'm not clued up enough to know what is the best way to go. To be honest I'm not even sure what downpipe will match up with the HKS. My objective is to improve the low end response as I will use this more than the top end, if possible I would like to find out how much of an improvement I could expect from doing this to see if it is worthwhile.
If someone is able to help me decide it would be much appreciated.
Thanks
Ken
I was thinking of maybe going for a scoobysport downpipe and centre (270 +vat) and putting my original back box on. Then I could sell the HKS and then go for the SS backbox. This would probably not work out much more money wise. then there's the question of noise - I think there is enough of it now so probably don't want to make it any louder.
But I'm not clued up enough to know what is the best way to go. To be honest I'm not even sure what downpipe will match up with the HKS. My objective is to improve the low end response as I will use this more than the top end, if possible I would like to find out how much of an improvement I could expect from doing this to see if it is worthwhile.
If someone is able to help me decide it would be much appreciated.
Thanks
Ken
28 November 2001, 06:41 PM
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Ken, I have the same system as you and the fellow I was speaking to said that the hks system has the same flange so most makes of downpipe will fit, they currently make there own and he asures me there isnt much in any of the makes but his at £145 all in seems to be a bargin also said he'd get some exhaust wrap on it to keep the temps down under the bonnet.
I'd keep the superdragor, best looking backbox on the market IMO.
I'd keep the superdragor, best looking backbox on the market IMO.
28 November 2001, 10:00 PM
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the hks mid and back box is great but the downpipe is pants. It blocks the wastegate entirely and as such doesnt provide much more than the standard setup.
In my mind the bpm is the downpipe to go for. If not then the scoobysport or scoobymania.
In my mind the bpm is the downpipe to go for. If not then the scoobysport or scoobymania.
29 November 2001, 11:35 AM
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There are 3 basic designs for aftermarket downpipe.
Twin Dump (BPM, PE, Scoobymania, Magnex)
Closed Neck (HKS, Graham Goode)
Open Neck (Scoobysport, Zero Sport)
The first designs were large bore closed neck designs such as the HKS. This design is very similar to the OE downpipe. The next design to follow was the Scoobysport and more recently the twin dump has come onto the market.
The aim of any after market downpipe is to provide a smooth exit for turbine gas. This aids spool up (reduces trubo lag) and provides better throttle response and power throughout the rev range.
The standard design is flawed in that the gas leaves the wastegate and is directed into the gas leaving the turbine at virtually 90 degrees, directly behind the turbine. This creates a pressure build up at the turbine exit, exactly where you don't want one.
The open neck designs collect from the entire back of the turbo housing and allow the turbine gas a free exit. The wastegate gasses tumble around the wastegate valve and gradually join with the main flow where the pipe tapers down to 3".
The twin dump designs keep the wastegate gas separate, until it is join the main flow further down the main pipe. Some join approx 150mm down, others over 300mm down.
The design of the twin dump looks like an advance, but as Impreza tuning has advanced on it has presented tuners with a new problem.
To achieve high power outputs, without getting det, very accurate boost control is required. With accurate boost control, fine tuning of fueling and timing can give the figures seen at the "No Excuses" rolling road day. The most powerful Imprezas on the day were all using open neck downpipes and as an example, Steve McCulloch's car was virtually identical to Trout's except Steve was using a BPM downpipe.
It was Bob Rawle that discovered the problem. Although a twin dump prevents turbulence at the back of the turbo, the wastegate gas is severely restricted in its path. The wastegate valve opens to approx 30 degrees max where the gas leaves the valve and then hits the separating flap before finding its way back around the valve flap into the small, second exit pipe. As a result of this flow path there is a restriction on the wastegate gas flow, wastegate control is much less accurate and the ECU is less able to regulate surges in boost. This means that although high boost can be achieved, the inability to accurately control boost requires very conservative fueling and timing to prevent det. The result is less power.
This reduction in boost control is unlikely to cause det on a car running standard boost on the original ECU.
Trout was using a 3" straight through Scoobysport downpipe, as does Bob Rawle (347bhp). Andy Tang and Moe (both with UK engines) have the traditional Scoobysport downpipe that tapers to 2.5" at the centre section flange, as does Nigel Bowles' mad, mad Nitrous breathing monster.
Twin Dump (BPM, PE, Scoobymania, Magnex)
Closed Neck (HKS, Graham Goode)
Open Neck (Scoobysport, Zero Sport)
The first designs were large bore closed neck designs such as the HKS. This design is very similar to the OE downpipe. The next design to follow was the Scoobysport and more recently the twin dump has come onto the market.
The aim of any after market downpipe is to provide a smooth exit for turbine gas. This aids spool up (reduces trubo lag) and provides better throttle response and power throughout the rev range.
The standard design is flawed in that the gas leaves the wastegate and is directed into the gas leaving the turbine at virtually 90 degrees, directly behind the turbine. This creates a pressure build up at the turbine exit, exactly where you don't want one.
The open neck designs collect from the entire back of the turbo housing and allow the turbine gas a free exit. The wastegate gasses tumble around the wastegate valve and gradually join with the main flow where the pipe tapers down to 3".
The twin dump designs keep the wastegate gas separate, until it is join the main flow further down the main pipe. Some join approx 150mm down, others over 300mm down.
The design of the twin dump looks like an advance, but as Impreza tuning has advanced on it has presented tuners with a new problem.
To achieve high power outputs, without getting det, very accurate boost control is required. With accurate boost control, fine tuning of fueling and timing can give the figures seen at the "No Excuses" rolling road day. The most powerful Imprezas on the day were all using open neck downpipes and as an example, Steve McCulloch's car was virtually identical to Trout's except Steve was using a BPM downpipe.
It was Bob Rawle that discovered the problem. Although a twin dump prevents turbulence at the back of the turbo, the wastegate gas is severely restricted in its path. The wastegate valve opens to approx 30 degrees max where the gas leaves the valve and then hits the separating flap before finding its way back around the valve flap into the small, second exit pipe. As a result of this flow path there is a restriction on the wastegate gas flow, wastegate control is much less accurate and the ECU is less able to regulate surges in boost. This means that although high boost can be achieved, the inability to accurately control boost requires very conservative fueling and timing to prevent det. The result is less power.
This reduction in boost control is unlikely to cause det on a car running standard boost on the original ECU.
Trout was using a 3" straight through Scoobysport downpipe, as does Bob Rawle (347bhp). Andy Tang and Moe (both with UK engines) have the traditional Scoobysport downpipe that tapers to 2.5" at the centre section flange, as does Nigel Bowles' mad, mad Nitrous breathing monster.
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29 November 2001, 11:58 AM
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Pete, if this is the case, why do all the top power scoobs on the wrx list use twin dumps?
Are you effectively saying that if steve chages his downpipe to the scoobysport downpipe we can expect him to see the same figures as rannoch/trout/david? (rtd from ow on
)
joking aside, I would be seriously interested to see a back to back swap test at power engineering of steves car before and after the downpipe swap. I have heard bob talk about this before but have never been convinced having seen Marks experience (R19KET) with the BPM I gave it a go myself.
Bob at the time was also recommending it having switched away from the scoobysport design. I dont know if I can afford to keep changing each time a top tuner changes his mind
but if this can be proven to me I would certainly be a lot happier.
The only problem with my suggestion is if boost control were seriously affected then a remap would be required after the swap and so changing it over in one day and running again may not be quite so easy.
All I know is that on my standard ecu when I changed from scoobysport to my current bpm downpipe the improvement in drivability and response was pronounced.
Are you effectively saying that if steve chages his downpipe to the scoobysport downpipe we can expect him to see the same figures as rannoch/trout/david? (rtd from ow on
)joking aside, I would be seriously interested to see a back to back swap test at power engineering of steves car before and after the downpipe swap. I have heard bob talk about this before but have never been convinced having seen Marks experience (R19KET) with the BPM I gave it a go myself.
Bob at the time was also recommending it having switched away from the scoobysport design. I dont know if I can afford to keep changing each time a top tuner changes his mind
but if this can be proven to me I would certainly be a lot happier.The only problem with my suggestion is if boost control were seriously affected then a remap would be required after the swap and so changing it over in one day and running again may not be quite so easy.
All I know is that on my standard ecu when I changed from scoobysport to my current bpm downpipe the improvement in drivability and response was pronounced.
29 November 2001, 12:29 PM
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Adam I don't read the wrx list, but think this is the Australian one(?). I'm not sure that a 3" open neck down pipe is made by any of the Aussie tuning companies, which may explain the dominance of twin dumps (an Aussie invention).
Mark tried our 3" open neck downpipe and told me it could not be as good as he was able to take fuel out at the top end, compared to his BPM. At the time, it didn't occur to either of us that he may not have needed to run such a large amount of "over fuel", because of the better boost control.
If you do need to run lots of fuel for safety, it will have the same effect as driving a car on cold start enrichment. Det prevention measures will mean burn efficiency is reduced for a given target boost.
Mark tried our 3" open neck downpipe and told me it could not be as good as he was able to take fuel out at the top end, compared to his BPM. At the time, it didn't occur to either of us that he may not have needed to run such a large amount of "over fuel", because of the better boost control.
If you do need to run lots of fuel for safety, it will have the same effect as driving a car on cold start enrichment. Det prevention measures will mean burn efficiency is reduced for a given target boost.
29 November 2001, 01:03 PM
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thanks for reply pete.
Overfuelling seems to be a bit of an issue.
I am having serious trouble understanding why we are all running so much fuel.
Theoretically 380s are good for 300bhp, meaning 760s are good for 600bhp. Bob, steve mcculloch and trout are running 740s as 550s dont appear to be good enough even though they should be good for 440bhp. Anders is about to be running 850s!!!!
Something is seriously amiss.
hyperex's 2.5 running at roughly 500bhp is sitting comfortably on 740 so why are we needing anything like this size of injector. the cossie boys also dont seem to be injecting anyway near as much fuel as us.
I just wonder whether our lambda values are off or something else perhaps is causing us tobelieve we need more fuel than we actually do. We both now that extra fuel saps power enormously as you stated with regard to the boost control mentioned above.
Perhaps the sensors are far more affected by temperature than we realise and the only safe way to combat this is through use of wide band.
Incidentally bob, can you please let me know how much the bosch lsu 4 wbls really is. Your answer may sway an m800 or pectel type purchase for me.
Aside from these overfuelling issies pete, I am still not fully understanding how boost control can lead to over fuelling.
When mark would have added his bpm exhaust and put in extra fuel, he would have done that because it was running lean. It would only be running leaner because there was more oxygen in the exhaust. To maintain lambda he would need to add fuel or risk damage to the engine. I am failing to understand how more air does not equate to mroe power, the fuel had to go somewhere, and if it was running at the same lambda as it was before the downpipe change then extra fuel burning means more energy in the combustion chamber.
Please explain.
Overfuelling seems to be a bit of an issue.
I am having serious trouble understanding why we are all running so much fuel.
Theoretically 380s are good for 300bhp, meaning 760s are good for 600bhp. Bob, steve mcculloch and trout are running 740s as 550s dont appear to be good enough even though they should be good for 440bhp. Anders is about to be running 850s!!!!
Something is seriously amiss.
hyperex's 2.5 running at roughly 500bhp is sitting comfortably on 740 so why are we needing anything like this size of injector. the cossie boys also dont seem to be injecting anyway near as much fuel as us.
I just wonder whether our lambda values are off or something else perhaps is causing us tobelieve we need more fuel than we actually do. We both now that extra fuel saps power enormously as you stated with regard to the boost control mentioned above.
Perhaps the sensors are far more affected by temperature than we realise and the only safe way to combat this is through use of wide band.
Incidentally bob, can you please let me know how much the bosch lsu 4 wbls really is. Your answer may sway an m800 or pectel type purchase for me.
Aside from these overfuelling issies pete, I am still not fully understanding how boost control can lead to over fuelling.
When mark would have added his bpm exhaust and put in extra fuel, he would have done that because it was running lean. It would only be running leaner because there was more oxygen in the exhaust. To maintain lambda he would need to add fuel or risk damage to the engine. I am failing to understand how more air does not equate to mroe power, the fuel had to go somewhere, and if it was running at the same lambda as it was before the downpipe change then extra fuel burning means more energy in the combustion chamber.
Please explain.
29 November 2001, 01:48 PM
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OK Adam, here's why its thought the extra fuel is needed (as explained to me by Bob).
Lets say you are using a downpipe that doesn't help accurate boost control and have set your target boost at 1.3 bar. When nailing the car, the reduced accuracy of boost control means that you hit 1.45 bar before settling at something that fluctuates around 1.3. The fluctautions may be as much as 0.1 bar up and down as the burn characteristics change through the rev range. Fuelling and timing to run at 1.3 bar will not be safe for the peaks encountered and your knocklink will go mad. So you add fuel and/or retard the ignition until it becomes safer again. Basically, you are mapping for the over boost and are a long way out for your target.
If the boost were accurately held at any given level, you could map more accuarately. You would not need as much over fuel and you would produce more power for the given level of boost. As you said, too much fuel really does sap the power.
On the standard ECU, most aftermarket downpipes will give increased peak boost, as the boost control is expecting the lardy reactions of the standard set up. If the downpipe used has a very restrictive wastegate flow, you may over shoot quite considerably and the car will feel very quick. How safe the peaks are will vary between different engine/turbo setups.
Lets say you are using a downpipe that doesn't help accurate boost control and have set your target boost at 1.3 bar. When nailing the car, the reduced accuracy of boost control means that you hit 1.45 bar before settling at something that fluctuates around 1.3. The fluctautions may be as much as 0.1 bar up and down as the burn characteristics change through the rev range. Fuelling and timing to run at 1.3 bar will not be safe for the peaks encountered and your knocklink will go mad. So you add fuel and/or retard the ignition until it becomes safer again. Basically, you are mapping for the over boost and are a long way out for your target.
If the boost were accurately held at any given level, you could map more accuarately. You would not need as much over fuel and you would produce more power for the given level of boost. As you said, too much fuel really does sap the power.
On the standard ECU, most aftermarket downpipes will give increased peak boost, as the boost control is expecting the lardy reactions of the standard set up. If the downpipe used has a very restrictive wastegate flow, you may over shoot quite considerably and the car will feel very quick. How safe the peaks are will vary between different engine/turbo setups.
29 November 2001, 02:05 PM
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nutter,
Power Engineering sell a manifold made by a company in japan, I believe you can source this elsewhere to. There are several other replacement manifolds available.
* Please open a new topic to ask about manifolds. *
Moray
[Edited by MorayMackenzie - 11/29/2001 2:08:06 PM]
Power Engineering sell a manifold made by a company in japan, I believe you can source this elsewhere to. There are several other replacement manifolds available.
* Please open a new topic to ask about manifolds. *
Moray
[Edited by MorayMackenzie - 11/29/2001 2:08:06 PM]
29 November 2001, 02:12 PM
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Surely a decent remappable ecu will let you map seperate fuel zones for 1.3 bar, 1.4bar and 1.5bar or some similar graduation, hence, assuming the ecu responds to map input quickly enough, you will be able to run correct fuelling at all zones and let the ecu control system deal with selecting the correct fuel setting for the given map signal and rpm. Why would you need to map 1.3bar rich to cope with boost overshoot if you can map for the higher boost levels in different zones in the map?
29 November 2001, 02:23 PM
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Hi,
I ran scoobysport systems on my first 2 UK turbos and found them to be amazing. I also used the same system on my P1 for a little while but I fancied a change (as you do from time to time).
I am now running a full HKS System with the closed neck downpipe (3" from start to finish). I haven't really noticed any great differences since switching over.
I can imagine a twin-dump system may make a difference in higher tuned cars but I'm running a practically stock car (std ecu, turbo, intake pipes, motor and injectors).
I am not a mechanic so I don't understand the physics of gas flow/turbulence/backpressure in exhaust systems so I'm going to keep this short.
I often wonder about the benefits of this things...does anybody know the angle that the wastegate opens to on the turbo ???? is it 45degrees ???? is it 90degrees ???...I bet not. Surely if the wastegate only opens < 30 degrees then the wastegate gases will meet with the exhaust gasses just the same.
Twin dump systems must use a divider that sits between the wastegate vent and the exhaust port on the turbo to keep the gasses separate...but they still have to join back together at some point.
Anyway...I don't know where I'm really going with this but I would be interested in the various fors and againsts of each system.
I was always happy with the scoobysport system (in fact its in my garage for safe keeping) but I am pretty happy with the HKS too.
Just for reference my car managed to produce more power and torque than all of the phase 2 cars (ecu, exhaust and breathing) in a recent power engineering rolling road day, and even fared very well against some of the phase 3 cars.
The car produced 315bhp @ 7,400 rpm. Peak torque was 278 at approx 3,700rpm. I know rolling road figures are meaningless (the car will not REALLY have 315 ponies) but it made for an interesting yardstick against other cars on the day.
I will watch this thread with interest.
regards
Ian
[Edited by igratton - 11/29/2001 2:31:57 PM]
I ran scoobysport systems on my first 2 UK turbos and found them to be amazing. I also used the same system on my P1 for a little while but I fancied a change (as you do from time to time).
I am now running a full HKS System with the closed neck downpipe (3" from start to finish). I haven't really noticed any great differences since switching over.
I can imagine a twin-dump system may make a difference in higher tuned cars but I'm running a practically stock car (std ecu, turbo, intake pipes, motor and injectors).
I am not a mechanic so I don't understand the physics of gas flow/turbulence/backpressure in exhaust systems so I'm going to keep this short.
I often wonder about the benefits of this things...does anybody know the angle that the wastegate opens to on the turbo ???? is it 45degrees ???? is it 90degrees ???...I bet not. Surely if the wastegate only opens < 30 degrees then the wastegate gases will meet with the exhaust gasses just the same.
Twin dump systems must use a divider that sits between the wastegate vent and the exhaust port on the turbo to keep the gasses separate...but they still have to join back together at some point.
Anyway...I don't know where I'm really going with this but I would be interested in the various fors and againsts of each system.
I was always happy with the scoobysport system (in fact its in my garage for safe keeping) but I am pretty happy with the HKS too.
Just for reference my car managed to produce more power and torque than all of the phase 2 cars (ecu, exhaust and breathing) in a recent power engineering rolling road day, and even fared very well against some of the phase 3 cars.
The car produced 315bhp @ 7,400 rpm. Peak torque was 278 at approx 3,700rpm. I know rolling road figures are meaningless (the car will not REALLY have 315 ponies) but it made for an interesting yardstick against other cars on the day.
I will watch this thread with interest.
regards
Ian
[Edited by igratton - 11/29/2001 2:31:57 PM]
29 November 2001, 03:41 PM
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Everyone
Thanks for all the replies. From what I can gather, not knowing a great deal about gas flows and stuff,
the HKS downpipe is probably the least effective design of the three.
The twin dump pipes sound like they are most effective with a mappable ecu and require more accurate boost control.
The scoobysport pipe is more effective than the HKS and doesn't require the boost control of the twin dump.
I should be able to get any of the downpipes and they will mate up with my Super Drager.
My car has a standard ecu and engine wise is standard except for the exhaust and a panel filter. It sounds like there is more chance of det with a twin dump so the scoobysport type is possibly a safer bet. It is also more cost effective than a twin dump, and the BPM sounds like it would be excessively noisy.
I will probably give Pete a call in the next few days provided it will fit. Pete - please can you confirm cos I understood your system was 2.5".
Thnaks again
Ken
Thanks for all the replies. From what I can gather, not knowing a great deal about gas flows and stuff,
the HKS downpipe is probably the least effective design of the three.
The twin dump pipes sound like they are most effective with a mappable ecu and require more accurate boost control.
The scoobysport pipe is more effective than the HKS and doesn't require the boost control of the twin dump.
I should be able to get any of the downpipes and they will mate up with my Super Drager.
My car has a standard ecu and engine wise is standard except for the exhaust and a panel filter. It sounds like there is more chance of det with a twin dump so the scoobysport type is possibly a safer bet. It is also more cost effective than a twin dump, and the BPM sounds like it would be excessively noisy.
I will probably give Pete a call in the next few days provided it will fit. Pete - please can you confirm cos I understood your system was 2.5".
Thnaks again
Ken
29 November 2001, 03:51 PM
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Ken,
As mentioned by Pete, both myself and Moe (TopCat) run Scoobysport 2.5" downpipes with our HKS Hiper exhaust systems.
We have both obtained quite good results!!
Not bad for UK cars with *minor* modifications!!
Cheers
Andy
As mentioned by Pete, both myself and Moe (TopCat) run Scoobysport 2.5" downpipes with our HKS Hiper exhaust systems.
We have both obtained quite good results!!
Not bad for UK cars with *minor* modifications!! Cheers
Andy
29 November 2001, 03:59 PM
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Adam
Dont worry I'll be doing a back to back when I swap the system - however it wont be entirely accurate as I'll have the HKS system instead of the reamining BPM gear - that said it should not be too different - so we'll see
However Trout has already done the comparison - Trout said that he got loads more torque - not sure about bhp - Bob noticed the same
I've been told to change the bpm system - so thats what I'm doing
(Cost is not an issue) - but we'll see when I do the RR.
My boost fluctuates between 1.3 bar and 1.5 bar and Bob finds it very difficult to map
Its not a case of getting overboost and it then say, settling at 1.3 bar - it simply fluates between 1.3 and 1.5 bar on acceleration which makes it a pain to map...
If Andy Tang can get more power with less advance, less boost and a very similar turbo (his is slightly smaller I believe) then theres a serious restriction in my system
At least I know its nothing to do with the injectors. My rolling road prints between Well Lane and PowerEngineering are almost identical...
They show a severe restiction at 5900 on PE's rollers and at about 6100 on Well Lanes (my power peaked at 7600 at PE but I suspect it was only a couple of bhp above the power reading at 5900)- well at least that how my graph reads
Interesting about the tapered neck on Andy tangs - I thought that would restrict top end at the benefit of the bottom end. I'm at a loss as to how Andy's produced so much more power - that said I think Trout is too (based on turbo sizes).
Would be interesting to know on the fuelling side. I agree that the Escort Cosworth I had got more miles to the gallon! than my Scoob well after bthe Green 803 injectors had been put in (I think thats what they were... was over 2 yrs ago....)
Dont worry I'll be doing a back to back when I swap the system - however it wont be entirely accurate as I'll have the HKS system instead of the reamining BPM gear - that said it should not be too different - so we'll see
However Trout has already done the comparison - Trout said that he got loads more torque - not sure about bhp - Bob noticed the same
I've been told to change the bpm system - so thats what I'm doing
(Cost is not an issue) - but we'll see when I do the RR.
My boost fluctuates between 1.3 bar and 1.5 bar and Bob finds it very difficult to map
Its not a case of getting overboost and it then say, settling at 1.3 bar - it simply fluates between 1.3 and 1.5 bar on acceleration which makes it a pain to map...
If Andy Tang can get more power with less advance, less boost and a very similar turbo (his is slightly smaller I believe) then theres a serious restriction in my system
At least I know its nothing to do with the injectors. My rolling road prints between Well Lane and PowerEngineering are almost identical...
They show a severe restiction at 5900 on PE's rollers and at about 6100 on Well Lanes (my power peaked at 7600 at PE but I suspect it was only a couple of bhp above the power reading at 5900)- well at least that how my graph reads
Interesting about the tapered neck on Andy tangs - I thought that would restrict top end at the benefit of the bottom end. I'm at a loss as to how Andy's produced so much more power - that said I think Trout is too (based on turbo sizes).
Would be interesting to know on the fuelling side. I agree that the Escort Cosworth I had got more miles to the gallon! than my Scoob well after bthe Green 803 injectors had been put in (I think thats what they were... was over 2 yrs ago....)
29 November 2001, 04:03 PM
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Hi All!
The reason that downpipes affect the way that the PossumLink ECU is mapped is because its load sites do not extend beyond 1 bar of boost, so if one were to try to run 1.3 bar, say, then the ignition timing conducive to maximum bmep without det would need to be entered into row 6 (the top row in a normal Link). This would mean that at 1 bar of boost, the timing is slightly sub-optimal. It also means that at 1.45 bar (say, with overboost) the timing is too aggressive... the car will det as a result, UNLESS the fuelling is altered.
A quick glance of maximum ignition advance against Air Fuel Ratio will show that the "perfect" AFR for power (about 12:1) is also "perfect" for det... ie if the fuelling is spot on to achieve maximum bmep then it is also most likely to det. Bummer. One can either a) try to control boost better or b) add more fuel. The addition of fuel will reduce bmep slightly and also help cool the combustion space (more latent heat). One can thus run the same boost (and a bit of overboost) without risking damage.
Of course, with an ECU which has more load sites, such as the MoTec, Pectel and even the later Links it is possible to drop the timing off a little in the overboost load region, thus making it possible to have "perfect" timing and fuelling on normal boost levels.
Boost control is a tricky subject because it is an interaction of electronics, fluid dynamics and mechanical components. Ideally, the actuator should be chosen such that it opens close to the chosen boost level. The method of boost control on the Subaru is such that a solenoid alters the amount of boost "seen" by the actuator. As the ratio of actual boost pressure to wastegate actuator cracking pressure is increased, it becomes increasingly more difficult to control boost accurately, as even the slightest change in the solenoid duty cycle will create a large difference in actual boost pressure. It is also worth remebering that like any moving mechanical system, there is inertia in the turbo's shaft, compressor and turbine. This means that any change made to the solenoid duty ratio now will not be reacted to by the turbo immediately, but with a delay. This is why there is overboost.... it takes time to pressurise the actuator... it takes time for the wastegate to open etc etc.
In an ideal world, one would set the system up then measure the step response function, find its phase reversal point and adjust the boost control parameters so as not to exceed unity gain (prefereably be somewhat below) at this point. This will ensure critial damping (Butterworth function) of the boost rise. It's a lot of hard work, though.
The effect of the downpipe on boost control will, naturally, be quite significant, since any change in the way that the wastegate can reduce exhaust manifold pressure (and therefore turbine driving power) will affect the step response function of the system, and thereby also require changes to the boost control algorithm parameters.
With regard to the wastegate gasses rejoining the main gas stream, this is bound to cause turbulence, regardless of where it actually happens. The ideal situation, then, is to either a) do not return it to the main gas stream (as in an external wastegate with separate tailpipe) or to create a venturi and to have that literally "suck" the wastegate gas back into the main stream. It is, perhaps, also worth remembering that exhaust gas flow, by its very nature, at the turbine exit is turbulent, NOT laminar. This goes a long way to explain why the ScoobySport downpipe works so well.... rather than forcing the wastegate cas to rejoin the main flow at 90 degrees it allows it to rejoin a turbulent flow in such a way as to minimise any additional disruption to flow.
With regard to fuelling changes as a result of downpipe fitment, there are only few regions where a reduction in fuelling could be considered a "good thing"... this is generally on vacuum. Basically, higher back pressure means less exhaust gas is expelled and thus less fresh air is drawn in during the induction phase. This is good because it means that for relatively less air once can still achieve the same (or better) bmep since the effective compression ratio is higher, and thermal efficiency is greatly affected by compression ratio (this is why EGR is used to increase fuel economy and reduce emissions... it effectively increases compression and thus efficiency). To illustrate this, consider that a cylinder is 500cc and that on cruise at 0.5 bar vacuum it gets completely scavenged and re-filled (but at 0.5 bar vacuum it would only get filled with the equivalent of 250cc at atmospheric pressure). Now imagine that 125cc (at atmospheric) actually remains in the cylinder, and that 250cc (at atmospheric) is drawn in. The mass of gas in the cylinder now equates to 375cc at atmospheric, but there is still only 250cc (at atmospheric) of fresh air. So same amount of air, same amount of fuel. But when the piston reaches top dead centre, in the first case it would compress 4:1 (because we have 250cc at atmospheric and 500cc at atmospheric would give 8:1). In the latter case the actual compression is increased to 6:1... with the same fresh air and fuel it actually makes more power as the thermal efficiency is higher. (Note to pedants: lots of technicalities skipped, actual CRs will be different as clearance volume is not fully taken into account on above, not are the effects achievable by stratified charge).
None of this realy helps answer the original question, sorry But as with everything real there isn't a perfect answer, only relative levels of suitability to purpose. Thus the "right" downpipe for a particular application will always be heavily dependant on the aims trying to be achieved. The Scoobysport downpipe has always been a good all-rounder... the BPM has triditionally been better at top end power (but then being 3" rather than 2.5" as most Scoobysport downpipes are, it will naturally flow more gas).
I have yet to try a Scoobysport 3" downpipe, the only reports I have had are from Mark, who as previously noted, had to remove fuel at the top end. I believe this was when he was using a MoTeC (so no need to massage the numbers as in the case of a 6 row Link). Unfortunatley no RR figures are available for a back to back comparison... this would be an interesting thing to try...
Hope this helps,
Pat.
The reason that downpipes affect the way that the PossumLink ECU is mapped is because its load sites do not extend beyond 1 bar of boost, so if one were to try to run 1.3 bar, say, then the ignition timing conducive to maximum bmep without det would need to be entered into row 6 (the top row in a normal Link). This would mean that at 1 bar of boost, the timing is slightly sub-optimal. It also means that at 1.45 bar (say, with overboost) the timing is too aggressive... the car will det as a result, UNLESS the fuelling is altered.
A quick glance of maximum ignition advance against Air Fuel Ratio will show that the "perfect" AFR for power (about 12:1) is also "perfect" for det... ie if the fuelling is spot on to achieve maximum bmep then it is also most likely to det. Bummer. One can either a) try to control boost better or b) add more fuel. The addition of fuel will reduce bmep slightly and also help cool the combustion space (more latent heat). One can thus run the same boost (and a bit of overboost) without risking damage.
Of course, with an ECU which has more load sites, such as the MoTec, Pectel and even the later Links it is possible to drop the timing off a little in the overboost load region, thus making it possible to have "perfect" timing and fuelling on normal boost levels.
Boost control is a tricky subject because it is an interaction of electronics, fluid dynamics and mechanical components. Ideally, the actuator should be chosen such that it opens close to the chosen boost level. The method of boost control on the Subaru is such that a solenoid alters the amount of boost "seen" by the actuator. As the ratio of actual boost pressure to wastegate actuator cracking pressure is increased, it becomes increasingly more difficult to control boost accurately, as even the slightest change in the solenoid duty cycle will create a large difference in actual boost pressure. It is also worth remebering that like any moving mechanical system, there is inertia in the turbo's shaft, compressor and turbine. This means that any change made to the solenoid duty ratio now will not be reacted to by the turbo immediately, but with a delay. This is why there is overboost.... it takes time to pressurise the actuator... it takes time for the wastegate to open etc etc.
In an ideal world, one would set the system up then measure the step response function, find its phase reversal point and adjust the boost control parameters so as not to exceed unity gain (prefereably be somewhat below) at this point. This will ensure critial damping (Butterworth function) of the boost rise. It's a lot of hard work, though.
The effect of the downpipe on boost control will, naturally, be quite significant, since any change in the way that the wastegate can reduce exhaust manifold pressure (and therefore turbine driving power) will affect the step response function of the system, and thereby also require changes to the boost control algorithm parameters.
With regard to the wastegate gasses rejoining the main gas stream, this is bound to cause turbulence, regardless of where it actually happens. The ideal situation, then, is to either a) do not return it to the main gas stream (as in an external wastegate with separate tailpipe) or to create a venturi and to have that literally "suck" the wastegate gas back into the main stream. It is, perhaps, also worth remembering that exhaust gas flow, by its very nature, at the turbine exit is turbulent, NOT laminar. This goes a long way to explain why the ScoobySport downpipe works so well.... rather than forcing the wastegate cas to rejoin the main flow at 90 degrees it allows it to rejoin a turbulent flow in such a way as to minimise any additional disruption to flow.
With regard to fuelling changes as a result of downpipe fitment, there are only few regions where a reduction in fuelling could be considered a "good thing"... this is generally on vacuum. Basically, higher back pressure means less exhaust gas is expelled and thus less fresh air is drawn in during the induction phase. This is good because it means that for relatively less air once can still achieve the same (or better) bmep since the effective compression ratio is higher, and thermal efficiency is greatly affected by compression ratio (this is why EGR is used to increase fuel economy and reduce emissions... it effectively increases compression and thus efficiency). To illustrate this, consider that a cylinder is 500cc and that on cruise at 0.5 bar vacuum it gets completely scavenged and re-filled (but at 0.5 bar vacuum it would only get filled with the equivalent of 250cc at atmospheric pressure). Now imagine that 125cc (at atmospheric) actually remains in the cylinder, and that 250cc (at atmospheric) is drawn in. The mass of gas in the cylinder now equates to 375cc at atmospheric, but there is still only 250cc (at atmospheric) of fresh air. So same amount of air, same amount of fuel. But when the piston reaches top dead centre, in the first case it would compress 4:1 (because we have 250cc at atmospheric and 500cc at atmospheric would give 8:1). In the latter case the actual compression is increased to 6:1... with the same fresh air and fuel it actually makes more power as the thermal efficiency is higher. (Note to pedants: lots of technicalities skipped, actual CRs will be different as clearance volume is not fully taken into account on above, not are the effects achievable by stratified charge).
None of this realy helps answer the original question, sorry
But as with everything real there isn't a perfect answer, only relative levels of suitability to purpose. Thus the "right" downpipe for a particular application will always be heavily dependant on the aims trying to be achieved. The Scoobysport downpipe has always been a good all-rounder... the BPM has triditionally been better at top end power (but then being 3" rather than 2.5" as most Scoobysport downpipes are, it will naturally flow more gas).I have yet to try a Scoobysport 3" downpipe, the only reports I have had are from Mark, who as previously noted, had to remove fuel at the top end. I believe this was when he was using a MoTeC (so no need to massage the numbers as in the case of a 6 row Link). Unfortunatley no RR figures are available for a back to back comparison... this would be an interesting thing to try...
Hope this helps,
Pat.
29 November 2001, 04:08 PM
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Blimey Pat!!
Mine is the 10 row chip so it doesn't expalin the problem of boost control with the old model
I am obviously very concerned about my power.........
Then again we'll see who's laughing at the next meet up....
Mine is the 10 row chip so it doesn't expalin the problem of boost control with the old model
I am obviously very concerned about my power.........
Then again we'll see who's laughing at the next meet up....
29 November 2001, 04:23 PM
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Ken
Almost all aftermarket centre sections are compatable with the OE downpipe as manufacturers had to allow for people keeping the OE catted downpipe. Now, most aftermarket downpipes use the standard fitment at their exit so they mate with the aftermarket centre sections.
The HKS centre is compatable with the OE downpipe so will fit almost all aftermarket downpipes.
Almost all aftermarket centre sections are compatable with the OE downpipe as manufacturers had to allow for people keeping the OE catted downpipe. Now, most aftermarket downpipes use the standard fitment at their exit so they mate with the aftermarket centre sections.
The HKS centre is compatable with the OE downpipe so will fit almost all aftermarket downpipes.
29 November 2001, 04:27 PM
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Oh dear you've run into the worst fait on the quest your stranded in drivetrain!! will you ever survive????????
Your brain is now fryed.
Wow, well thats a little confusing, but thanks for the replys.
So which one do I get?? I'm running at the moment a standerd wrx95 apart from hks filter and exhaust, however after xmas I'm putting on a apexi hybred turbo controlled by apexi avcr and afc.
Anyone any ideas of which to get??
[Edited by cryptwalk - 12/28/2001 1:39:25 AM]
Your brain is now fryed.Wow, well thats a little confusing, but thanks for the replys.
So which one do I get?? I'm running at the moment a standerd wrx95 apart from hks filter and exhaust, however after xmas I'm putting on a apexi hybred turbo controlled by apexi avcr and afc.
Anyone any ideas of which to get??
[Edited by cryptwalk - 12/28/2001 1:39:25 AM]
29 November 2001, 05:00 PM
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Wow pat, that made for interesting reading.
I know I will never have the knowledge required to truely understand all this but it does help . Thanks for taking the time to give us all such generous information.
Maybe I should be thinking about the best of both worlds... open flanged Scoobysport 3" pipe + my HKS System.
regards
Ian.
I know I will never have the knowledge required to truely understand all this but it does help . Thanks for taking the time to give us all such generous information.
Maybe I should be thinking about the best of both worlds... open flanged Scoobysport 3" pipe + my HKS System.
regards
Ian.
29 November 2001, 06:41 PM
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Steve,
there are hundreds of things that can affect the power output, the only conclusive evidence one can hope to gain is to do a back to back on the same car; then one can conclude what works best ON THAT CAR. Take another car, with identical setup and it may go the other way. Such is the nature of manufacturing tolerance.... but it is possible to try to establish *trends*, or the statistical probability that given components X,Y and Z then downpipe A would be more likely to give good figures than downpipe B...
The interesting thing about Andy's figures are the RPM figures... a UK engine being revved to 7900 RPM is unusual, but given that the
STi VII has gone back to over bucket shims on the buckets then it stands to reason that the valvetrain in the Boxer Master Phase II engine is good to 8000 RPM even in the UK variant. What is less clear is the effect on the cast pistons... STis should have forged items, thus be a bit stronger. Still, it is the rods that take the most hammering from high RPM. IIRC they are subjected to several thousand "G" during the reversal at the end of the exhaust stroke
The trouble with engines is they are extremely complex in their interactions, and any simulations, even those run on supercomputers, are mere approximations... to fully simulate everything would be pretty much impossible. So most of the time one just has to go with gut feeling / past experience.
Also, rolling road figures are pub figures, simply because they cannot replicate accurately enough the conditions of real world driving. Where one might deliberately map a bit of overboost into a car for road use, this may not happen on the rollers; thus the car would actually "go better" than the figures would suggest.
I guess that, at the end of the day, it all boils down to how well a car goes on the road and whether the owner is happy with it. Each "flaw" that an owner may point out will have a vast number of possible "cures" and each of these will have "side effects". A lot can be achieved just by mapping, anything further will require mechanical intervention.
Generally, then, I would recommend the Scoobysport downpipe over the BPM item for a few reasons.... a) it does not rust , b) it provided good results on just about every Impreza it has ever been fitted to and c) it doesn't lose much (if anything) in the way of bottom end torque (the BPM does, but may have a slight edge at the very top).
Having said that, it is entirely possible that an "oddball" car may react quite differently.... only testing will give the definitive answer *for that car*...
Hope this isn't too confusing,
Pat.
there are hundreds of things that can affect the power output, the only conclusive evidence one can hope to gain is to do a back to back on the same car; then one can conclude what works best ON THAT CAR. Take another car, with identical setup and it may go the other way. Such is the nature of manufacturing tolerance.... but it is possible to try to establish *trends*, or the statistical probability that given components X,Y and Z then downpipe A would be more likely to give good figures than downpipe B...
The interesting thing about Andy's figures are the RPM figures... a UK engine being revved to 7900 RPM is unusual, but given that the
STi VII has gone back to over bucket shims on the buckets then it stands to reason that the valvetrain in the Boxer Master Phase II engine is good to 8000 RPM even in the UK variant. What is less clear is the effect on the cast pistons... STis should have forged items, thus be a bit stronger. Still, it is the rods that take the most hammering from high RPM. IIRC they are subjected to several thousand "G" during the reversal at the end of the exhaust stroke
The trouble with engines is they are extremely complex in their interactions, and any simulations, even those run on supercomputers, are mere approximations... to fully simulate everything would be pretty much impossible. So most of the time one just has to go with gut feeling / past experience.
Also, rolling road figures are pub figures, simply because they cannot replicate accurately enough the conditions of real world driving. Where one might deliberately map a bit of overboost into a car for road use, this may not happen on the rollers; thus the car would actually "go better" than the figures would suggest.
I guess that, at the end of the day, it all boils down to how well a car goes on the road and whether the owner is happy with it. Each "flaw" that an owner may point out will have a vast number of possible "cures" and each of these will have "side effects". A lot can be achieved just by mapping, anything further will require mechanical intervention.
Generally, then, I would recommend the Scoobysport downpipe over the BPM item for a few reasons.... a) it does not rust
, b) it provided good results on just about every Impreza it has ever been fitted to and c) it doesn't lose much (if anything) in the way of bottom end torque (the BPM does, but may have a slight edge at the very top).Having said that, it is entirely possible that an "oddball" car may react quite differently.... only testing will give the definitive answer *for that car*...
Hope this isn't too confusing,
Pat.
29 November 2001, 08:14 PM
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Thanks Pat
I'll be doing the change and see what effect it has. I'm not gonna do another RR like I said as the results were spookily similar to Well Lanes...
I'll find out early January and report it to all!
I'll be doing the change and see what effect it has. I'm not gonna do another RR like I said as the results were spookily similar to Well Lanes...
I'll find out early January and report it to all!
29 November 2001, 09:52 PM
#27
Ecu Specialist
Patrick dear boy we are all using 10 row links these days, least those with MY99 cars onwards are ... not quite as granular as your Pectel or my M800 but pretty good as can be seen.
Back to downpipes, haveing fitted and removed the two types five times over a period of three months to check on the differing effects I would definately plump for the open kneck design, on every occasion I have fitted a twin dump during the period of experimentation I had to REMOVE fuel to maintain afr's. The point about boost control is as Pete says ... twin dump restricts the "dump" pipe as the wastegate opens forcing air thro the turbo at higher velocity, this causes erratic boost control as the turbo overshoots and struggles to cope, on part throttle it can over speed easily as well. The open kneck does not restrict as the wastegate opens and therefore has a more balanced performance, I had to add fuel whenever I used an open kneck version, I have settled on the 3" open kneck pipe coupled to the HKS Hiper. It has needed a noticeable increase in fuel to maintain afr values.
Just to be clear, I am not talking about running a richer mixture here, every setup was mapped to achieve the same air/fuel ratios. Timing maps were kept the same, boost control settings were optimised with each to get best spool up without overshoot.
One final thing, the wastegate restiction means that there is more pressure build up within the turbo housing, this has the effect of trying to hold the wastegate open when standard actuators are in use, this again aggravates boost control. (Both Motec and Link suffer with this). Also bear in mind that we are talking about a car running at 1.4/1.5 bar boost here as well.
I was very critical during the testing and explored all the differences thoroughly. To be honest, I too expected the twin dump to be better ... I was proved wrong, at least on my car.
Finally I would make it clear that I have not tried all the different twin dumps that are available, I can only speak about my experiences with one brand.
Back to downpipes, haveing fitted and removed the two types five times over a period of three months to check on the differing effects I would definately plump for the open kneck design, on every occasion I have fitted a twin dump during the period of experimentation I had to REMOVE fuel to maintain afr's. The point about boost control is as Pete says ... twin dump restricts the "dump" pipe as the wastegate opens forcing air thro the turbo at higher velocity, this causes erratic boost control as the turbo overshoots and struggles to cope, on part throttle it can over speed easily as well. The open kneck does not restrict as the wastegate opens and therefore has a more balanced performance, I had to add fuel whenever I used an open kneck version, I have settled on the 3" open kneck pipe coupled to the HKS Hiper. It has needed a noticeable increase in fuel to maintain afr values.
Just to be clear, I am not talking about running a richer mixture here, every setup was mapped to achieve the same air/fuel ratios. Timing maps were kept the same, boost control settings were optimised with each to get best spool up without overshoot.
One final thing, the wastegate restiction means that there is more pressure build up within the turbo housing, this has the effect of trying to hold the wastegate open when standard actuators are in use, this again aggravates boost control. (Both Motec and Link suffer with this). Also bear in mind that we are talking about a car running at 1.4/1.5 bar boost here as well.
I was very critical during the testing and explored all the differences thoroughly. To be honest, I too expected the twin dump to be better ... I was proved wrong, at least on my car.
Finally I would make it clear that I have not tried all the different twin dumps that are available, I can only speak about my experiences with one brand.
29 November 2001, 10:25 PM
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Originally posted by pat:
Correct me if I'm wrong, but don't the gases exiting the wastegate flow a lot faster than those in the exhaust? In which case creating a venturi using the exhaust gases would have to involve making the exhaust pipe really thin and the wastegate pipe really fat
Also it is not necessary to have the wastegate gases rejoin the main flow at 90 degrees in a twin dump system. I don't know much about the designs of BPM/Scoobymania/Magnex twin dumps, but ultimately the main exhaust has to come down from the engine, turn round a bend and then run straight along the bottom of the car. Surely it is possible to arrange the wastegate dump pipe such that it rejoins the main pipe at the bend at the bottom, so that the wastegate flow is joining the main exhaust flow tangentially?
With regard to the wastegate gasses rejoining the main gas stream, this is bound to cause turbulence, regardless of where it actually happens. The ideal situation, then, is to either a) do not return it to the main gas stream (as in an external wastegate with separate tailpipe) or to create a venturi and to have that literally "suck" the wastegate gas back into the main stream. It is, perhaps, also worth remembering that exhaust gas flow, by its very nature, at the turbine exit is turbulent, NOT laminar. This goes a long way to explain why the ScoobySport downpipe works so well.... rather than forcing the wastegate cas to rejoin the main flow at 90 degrees it allows it to rejoin a turbulent flow in such a way as to minimise any additional disruption to flow.
Also it is not necessary to have the wastegate gases rejoin the main flow at 90 degrees in a twin dump system. I don't know much about the designs of BPM/Scoobymania/Magnex twin dumps, but ultimately the main exhaust has to come down from the engine, turn round a bend and then run straight along the bottom of the car. Surely it is possible to arrange the wastegate dump pipe such that it rejoins the main pipe at the bend at the bottom, so that the wastegate flow is joining the main exhaust flow tangentially?
30 November 2001, 08:52 AM
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OK, thanks to so many people providing so much infromation I have made a decision. I've decided to go for the scoobysport 2.5" downpipe and keep the HKS centre and back box.
Thanks again to everyone.
Ken
Thanks again to everyone.
Ken
