Important Exhaust design question,post opinions
02 February 2003, 12:37 PM
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I was always lead to beleive that the angled exit pipe(ie Hiper,Apexi,?Blitz) was best as it deletes the s bend at the rear.This allows a straight exit for the gases which should be more efficient.MSGofaster(Stephen) from Australia in the thread 'USA Turbo expert talks' disagrees with this.He feels that the S bend causes some kind of pulses/waves that 'allow fresh charge capture on cam overlap'.The large mid range advantages of this far outweigh the small BHP increase with deleting the S bend.
What do people think?
I'm changing my exhaust soon(for the last time) and may have a bespoke one made up with all the best features.
He also advocates a splitter incorporated into the DP that extends into the turbo,but not past the flange of the DP.This he feels reduces turbo lag and increases Turbo effiency.Any thoughts on this?
What do people think?
I'm changing my exhaust soon(for the last time) and may have a bespoke one made up with all the best features.
He also advocates a splitter incorporated into the DP that extends into the turbo,but not past the flange of the DP.This he feels reduces turbo lag and increases Turbo effiency.Any thoughts on this?
02 February 2003, 05:05 PM
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The bend in the backbox idea I reckon is pants. How can a pressure wave go from the back of the exhaust, across the turbo, down the manifold & conveniently make it's way up the correct tube at the collector to do this?
As for the splitter... depends on too many things. I don't think there's a vast amount in it myself. I've used both bellmouth & twin-dump (almost 'real' twin-dump where the gases meet 2 foot down the pipe rather than 2 inches - which has a splitter in it to keep the gases seperate) and there is a difference, yes, but nothing to get excitied about. I'd say the twin dump works better, but that's only in my situation!
As for the splitter... depends on too many things. I don't think there's a vast amount in it myself. I've used both bellmouth & twin-dump (almost 'real' twin-dump where the gases meet 2 foot down the pipe rather than 2 inches - which has a splitter in it to keep the gases seperate) and there is a difference, yes, but nothing to get excitied about. I'd say the twin dump works better, but that's only in my situation!
03 February 2003, 05:46 AM
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Deep,
I've done no in depth research on this, but in a dyno run w/my mate's B4 with a Fujitsubo Legalis R type Evolution with the bendy pipe, and my car with a straight Legalis Super R, my mate's car roundly beat my car on the rollers. Both cars were B type B4s, std but for the tail pipes.
On road it felt much more responsive and dare I say powerful than my car until the very top end, when my car would take off (at last
). Over all his car was quicker in the real world and was more responsive to throttle inputs accross the rev range.
Legalis R:
Legalis Super R:
I've done no in depth research on this, but in a dyno run w/my mate's B4 with a Fujitsubo Legalis R type Evolution with the bendy pipe, and my car with a straight Legalis Super R, my mate's car roundly beat my car on the rollers. Both cars were B type B4s, std but for the tail pipes.
On road it felt much more responsive and dare I say powerful than my car until the very top end, when my car would take off (at last
). Over all his car was quicker in the real world and was more responsive to throttle inputs accross the rev range.Legalis R:
Legalis Super R:
03 February 2003, 11:23 AM
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Backpressure to retain fresh charge? Eh? Stopping the exhaust gases from escaping helps the new ones come in?
You want no backpressure post-turbo. Zero. Any more is a compromise, which we all have to put up with through noise levels/emissions, etc, but the idea is to keep it to a minimum.
I believe that back-pressure off-boost will improve driveability. However, again there is a compromise, as top-end won't work as well.
I'd say, on the bend/no-bend thing, if you want a car that's nice to drive in traffic in, it's good. Or, if you're not one for being in the correct 'power gear' then it's a good compromise, as the power is lower but has a wider spread. But if you're willing to adapt to the power being higher up, then straight-through is ultimately better. Of course, it all depends on whether the exhaust is actually the restriction - if you're playing with a little turbo, it's all academic & you'd probably be better off with the bendy pipe as there's no power higher up anyway!
As for the splitter - I think it always improves things - if set up correctly. And the twin-pipe sort of assumes that the splitter is there. If it's not, then it's not really a twin pipe anyway!
You want no backpressure post-turbo. Zero. Any more is a compromise, which we all have to put up with through noise levels/emissions, etc, but the idea is to keep it to a minimum.
I believe that back-pressure off-boost will improve driveability. However, again there is a compromise, as top-end won't work as well.
I'd say, on the bend/no-bend thing, if you want a car that's nice to drive in traffic in, it's good. Or, if you're not one for being in the correct 'power gear' then it's a good compromise, as the power is lower but has a wider spread. But if you're willing to adapt to the power being higher up, then straight-through is ultimately better. Of course, it all depends on whether the exhaust is actually the restriction - if you're playing with a little turbo, it's all academic & you'd probably be better off with the bendy pipe as there's no power higher up anyway!
As for the splitter - I think it always improves things - if set up correctly. And the twin-pipe sort of assumes that the splitter is there. If it's not, then it's not really a twin pipe anyway!
03 February 2003, 07:27 PM
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Michael,firstly apologies for getting your name wrong,and sorry if I misquoted you.Unfortuately 'making up some pipes and trying them out' is not so easy for me.Each time you ask someone to do these sorts of things you are being chargeded £50/hr labour!! Its not a cheap exercise.I've not heard the S bend theory before and was trying to gauge opinion.
Nom,I would say I have a 'respectable' size turbo(VF30) and am aiming for 350ish BHP.I'm happy to rev the car up top,but living and driving in the city means that low and mid range punch are very important.I need this to make it a good day to day car and then the top end grunt on the days I'm really feeling saucy.Its a balance I understand but if you are only losing 1 at the top but gaining 5 in the midrange this is surely better,no?
Nom,I would say I have a 'respectable' size turbo(VF30) and am aiming for 350ish BHP.I'm happy to rev the car up top,but living and driving in the city means that low and mid range punch are very important.I need this to make it a good day to day car and then the top end grunt on the days I'm really feeling saucy.Its a balance I understand but if you are only losing 1 at the top but gaining 5 in the midrange this is surely better,no?
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03 February 2003, 08:22 PM
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NOM,
Tell me why you think they have varable valve timing ?
The valve over lap is increased at top end to increase scavenging, where at low rpm it is reduced so as to stop the loss of fresh charge out the exhaust valves, the S bend does much the same thing.
I have had this agument before that just make the pipe off the turbo 6" and dump it straight to the ground ya don't need any backpressure blar blar blar.
I have done dyno testing and I can assure you that there is a thing as TO big .
The testing i did with a VF28 on a STI with a straight pipe to one with 2 90 deg bends was the straight pipe made 3kw more at 6500rpm , but it was 16 kw less at 4000rpm and was less till it reached 6000rpm. THIS IS REALL TIME , not a guesstamate that so many seem to be good at. This is why i sent Deep to try it, so someone else might learn the difference.
The way i see it you are trying to justify your money you have spent already or you have been burnt to many times and are over listening to other info. What ever... i am just passing on FREE info and if you don't have the decentcy to go and try it before shooting off that will be your loss.
I am over defending my reports and either you try it or you don't , I don't really care either way.
But next time someone comes along and has more power with the same car/turbo or pulls out and passes you with ease and you want more low to mid grunt instead of making coffee while you wait for the lag to go start with your exhaust.
Cheers
Michael
Tell me why you think they have varable valve timing ?
The valve over lap is increased at top end to increase scavenging, where at low rpm it is reduced so as to stop the loss of fresh charge out the exhaust valves, the S bend does much the same thing.
I have had this agument before that just make the pipe off the turbo 6" and dump it straight to the ground ya don't need any backpressure blar blar blar.
I have done dyno testing and I can assure you that there is a thing as TO big .
The testing i did with a VF28 on a STI with a straight pipe to one with 2 90 deg bends was the straight pipe made 3kw more at 6500rpm , but it was 16 kw less at 4000rpm and was less till it reached 6000rpm. THIS IS REALL TIME , not a guesstamate that so many seem to be good at. This is why i sent Deep to try it, so someone else might learn the difference.
The way i see it you are trying to justify your money you have spent already or you have been burnt to many times and are over listening to other info. What ever... i am just passing on FREE info and if you don't have the decentcy to go and try it before shooting off that will be your loss.
I am over defending my reports and either you try it or you don't , I don't really care either way.
But next time someone comes along and has more power with the same car/turbo or pulls out and passes you with ease and you want more low to mid grunt instead of making coffee while you wait for the lag to go start with your exhaust.
Cheers
Michael
03 February 2003, 09:20 PM
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Let's make no mistake here... the best exhaust system, all things being equal, for a turbocharged engine is no exhaust system, pure and simple, the less pressure behind the turbo, the better it will spool. Unfortunately in the real world not all things are equal.
Pulse tuning on a turbocharged engine is a fallacy, it doesn't work, the step change in characteristic impedance at the turbine wheel will screw any pulse tuning you may have had. The only "pulse" tuning you can do is with the headers, and we aren't talking about changing them.
Cam profiles for turbocharged engines are very different to NA engines. For example, the Phase II engines open the intake at 3 degrees BTDC, closes at 33 degrees ABDC, the exhaust opens at 33 degrees BBDC, and closes at 3 degrees ATDC, so there's only 6 degrees of overlap.... not exactly much time for any back pressure to do anything useful. NA engines may run massive overlap.
Comparing two different exhaust systems on two different cars is pointless, if not entirely meaningless. The only way to compare them is on the same car.
Variable cam timing on a turbocharged car has very little to do with the exhaust system. It's no coincidence that it invariably acts on the intake valves. When you consider that the intake valve shuts at 33 degrees ABDC, at low RPM it's actually pushing charge back out the intake port. Advancing the intake cam will close it sooner, reducing the reversion out of the intake port, and thereby improving the cylinder filling, it's all down to dynamic compression ratios. Looking at the STi VII map, I consider it something more than a coincidence that they run up to 35 degrees intake cam advance (ie closing pretty much at BDC). You just cannot take what you know about NA engines and apply it to turbocharged engines....
I am not disputing here that a particular S bend type exhaust on one car worked better than a straight through one on another car. Nor am I questioning the dyno figures. What one does need to do though is compare the two, back to back, on the same car, preferably within an hour or so of eachother.
With regard to downpipes, the design will obviously affect the performance. The 2.5" open neck design will present some back pressure helping keep the wastegate shut. A 3" twin dump will have less back pressure, and would spool better, but because there is no back pressure to keep the wastegate closed it opens sooner and the boost is slugged, but it's not because the exhaust design is "worse", it's because the wastegate design is poor... give it a decent wastegate and the free flowing system would work better. This is why the splitter design is so critical in an open neck... it needs to prevent wastegate gas from interfering with the turbine exit flow, but it shouldn't generate a partial vacuum due to the way it rejoins the main flow (as some twin dump systems would do, which would further explain the lack of performance low down).
The point is that if everything is working properly, then the freer flowing exhaust will always be better, but because of the limitations of the internal wastegate and other issues, in reality an S bend system may work better low down, because the back pressure is covering up an inadequacy elsewhere in the turbocharging system design. Address the inadequacy and you can get the best of both worlds
Cheers,
Pat.
Pulse tuning on a turbocharged engine is a fallacy, it doesn't work, the step change in characteristic impedance at the turbine wheel will screw any pulse tuning you may have had. The only "pulse" tuning you can do is with the headers, and we aren't talking about changing them.
Cam profiles for turbocharged engines are very different to NA engines. For example, the Phase II engines open the intake at 3 degrees BTDC, closes at 33 degrees ABDC, the exhaust opens at 33 degrees BBDC, and closes at 3 degrees ATDC, so there's only 6 degrees of overlap.... not exactly much time for any back pressure to do anything useful. NA engines may run massive overlap.
Comparing two different exhaust systems on two different cars is pointless, if not entirely meaningless. The only way to compare them is on the same car.
Variable cam timing on a turbocharged car has very little to do with the exhaust system. It's no coincidence that it invariably acts on the intake valves. When you consider that the intake valve shuts at 33 degrees ABDC, at low RPM it's actually pushing charge back out the intake port. Advancing the intake cam will close it sooner, reducing the reversion out of the intake port, and thereby improving the cylinder filling, it's all down to dynamic compression ratios. Looking at the STi VII map, I consider it something more than a coincidence that they run up to 35 degrees intake cam advance (ie closing pretty much at BDC). You just cannot take what you know about NA engines and apply it to turbocharged engines....
I am not disputing here that a particular S bend type exhaust on one car worked better than a straight through one on another car. Nor am I questioning the dyno figures. What one does need to do though is compare the two, back to back, on the same car, preferably within an hour or so of eachother.
With regard to downpipes, the design will obviously affect the performance. The 2.5" open neck design will present some back pressure helping keep the wastegate shut. A 3" twin dump will have less back pressure, and would spool better, but because there is no back pressure to keep the wastegate closed it opens sooner and the boost is slugged, but it's not because the exhaust design is "worse", it's because the wastegate design is poor... give it a decent wastegate and the free flowing system would work better. This is why the splitter design is so critical in an open neck... it needs to prevent wastegate gas from interfering with the turbine exit flow, but it shouldn't generate a partial vacuum due to the way it rejoins the main flow (as some twin dump systems would do, which would further explain the lack of performance low down).
The point is that if everything is working properly, then the freer flowing exhaust will always be better, but because of the limitations of the internal wastegate and other issues, in reality an S bend system may work better low down, because the back pressure is covering up an inadequacy elsewhere in the turbocharging system design. Address the inadequacy and you can get the best of both worlds
Cheers,
Pat.
04 February 2003, 02:15 PM
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Deep, we've done a bit of a test...
The LREvo was consistently better than the Super R - and yes, we did put it down to the exhaust from the word go (esp. since my car also had an LREvo before the Super R - a supposed "upgrade" ). We knew from experience here that too much flow actually makes our cars slower (small turbos and all that). I think the message is that scaling everything to work at its best is better than just chucking alot of stuff *****-nilly at an issue. And yes, I have been in a couple of Imprezas that were slower after "one size fits all" mods. And I've been in some that are bloody quick after some judicious modding too 
Back to the exhaust tests: we also ran an HKS Hyper (about as straight as you can get!) and it was pathetic - slower than a std car!! And it was all because of the lag/too much flow (something usually quite limited by the TT system on the Legacy). And then there was the fuel usage, which was, in short, attrocious. It took heaps more throttle to get the car off the line, and was so hopeless the owner got a LREvo! That one was binned early
Basically, we found that the LREvo had a better step off perf, faster spool, much bigger mid range than standard (so much so that it got to high revs much, much faster than anything else), and was still good up top (red line 7500rpm). With the result was that it was the best compromise for those looking for a good performance gain (in the real world), without the minuses of lag.
Cheers
(oh, and each to his own on tuning I'm just sharing my little group's experiences. Peace)
The LREvo was consistently better than the Super R - and yes, we did put it down to the exhaust from the word go (esp. since my car also had an LREvo before the Super R - a supposed "upgrade"
). We knew from experience here that too much flow actually makes our cars slower (small turbos and all that). I think the message is that scaling everything to work at its best is better than just chucking alot of stuff *****-nilly at an issue. And yes, I have been in a couple of Imprezas that were slower after "one size fits all" mods. And I've been in some that are bloody quick after some judicious modding too Back to the exhaust tests: we also ran an HKS Hyper (about as straight as you can get!) and it was pathetic - slower than a std car!! And it was all because of the lag/too much flow (something usually quite limited by the TT system on the Legacy). And then there was the fuel usage, which was, in short, attrocious. It took heaps more throttle to get the car off the line, and was so hopeless the owner got a LREvo! That one was binned early
Basically, we found that the LREvo had a better step off perf, faster spool, much bigger mid range than standard (so much so that it got to high revs much, much faster than anything else), and was still good up top (red line 7500rpm). With the result was that it was the best compromise for those looking for a good performance gain (in the real world), without the minuses of lag.
Cheers
(oh, and each to his own on tuning
I'm just sharing my little group's experiences. Peace)
04 February 2003, 03:06 PM
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Another spanner for the works...
We made a rear silencer that wasn't, last year. By wasn't I mean it was just a length of pipe, the same length as the it would have been, but with no perforated tube and box around it.
It killed the performance of the car. Off boost it was the same, but it was sluggish to build boost and killed the torque once boost was achieved.
We decided that it was the reflection of the sound wave off the end of the pipe. There can be no explanation for this other than some pulse tuning does exist and the bare pipe allowed the pulses to reflect back up the flow, to the exhaust ports.
This would also mean that the pipe length is still critical on a turbo engine and may explain why AJ saw a loss of performance with the shorter, straight version.
The bare pipe was very loud
We made a rear silencer that wasn't, last year. By wasn't I mean it was just a length of pipe, the same length as the it would have been, but with no perforated tube and box around it.
It killed the performance of the car. Off boost it was the same, but it was sluggish to build boost and killed the torque once boost was achieved.
We decided that it was the reflection of the sound wave off the end of the pipe. There can be no explanation for this other than some pulse tuning does exist and the bare pipe allowed the pulses to reflect back up the flow, to the exhaust ports.
This would also mean that the pipe length is still critical on a turbo engine and may explain why AJ saw a loss of performance with the shorter, straight version.
The bare pipe was very loud
04 February 2003, 03:16 PM
#17
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Hi Pete,
On the (I think H & S ScoobySport zorsts sold mid last year) SS zorts that I bought for my JDM STI-7 did you guys test S bends vs. 90 degree bends, splitter vs. non splitter, 2.5 vs. 3" etc ?
Just wondering if the SS zorst was tested for peak low down torque as my car seems definately more torquey down low etc..just wondering if by adding a splitter if more gains will be had.
Cheers,
Ray
PS: Still mulling the mail-in ecu in my head
On the (I think H & S ScoobySport zorsts sold mid last year) SS zorts that I bought for my JDM STI-7 did you guys test S bends vs. 90 degree bends, splitter vs. non splitter, 2.5 vs. 3" etc ?
Just wondering if the SS zorst was tested for peak low down torque as my car seems definately more torquey down low etc..just wondering if by adding a splitter if more gains will be had.
Cheers,
Ray
PS: Still mulling the mail-in ecu in my head
02 March 2003, 09:26 PM
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Sorry, started writing this before the Pat documentary was posted, so going in circles possibly!
A few things I thought of -
The 'straight through' ones aren't very good on rough roads (read 'London') as they can catch on speed bumps without too much difficulty. Sort of removes them from being a sensible choice if that's where you are! Certainly did it for me
Back to the discussion... I'd be suprised if the exhaust - if it was any reasonable no-cat affair - would be the restriction on a VF28, bends or no bends. It would be the turbo! I'm only thinking of the bigger turbos here - VF22+ sort of thing. And I'm pretty convinced that as far as exhaust gases leaving the cylinder are concerned, life changes quite dramatically at the turbo. Before it there are pulses, big heat & being sucked back & forth. Then the fumes hit the fan and life slows down considerably & smooths out; certainly the pulsing & back & forth has gone. The barrier of the 'fan' has been crossed & there's no return... (sorry about the silly explanation - I'm just coming to the end of a very long day & Mr Brain needs to keep himself awake somehow).
Next! And a bit by-the-by... I've found that the larger turbos can actually give a very comfortable 'pottering' type of feel to them, presumeably because the exhaust side of the turbo is large/free-flowing enough to not 'get in the way' at off-boost speeds. No, there's no grunt, but it also feels as though the banana has been removed from the exhaust at these lower revs. I have found that boost (TD04) often inconveniently decides to assert itself just before the traffic flow speed, so tries to majestically slam the car into the back of the pottering 205 in front. Not helpful.
Just IMHO!
[Edited by nom - 2/3/2003 9:28:04 PM]
A few things I thought of -
The 'straight through' ones aren't very good on rough roads (read 'London'
) as they can catch on speed bumps without too much difficulty. Sort of removes them from being a sensible choice if that's where you are! Certainly did it for me Back to the discussion... I'd be suprised if the exhaust - if it was any reasonable no-cat affair - would be the restriction on a VF28, bends or no bends. It would be the turbo! I'm only thinking of the bigger turbos here - VF22+ sort of thing. And I'm pretty convinced that as far as exhaust gases leaving the cylinder are concerned, life changes quite dramatically at the turbo. Before it there are pulses, big heat & being sucked back & forth. Then the fumes hit the fan
and life slows down considerably & smooths out; certainly the pulsing & back & forth has gone. The barrier of the 'fan' has been crossed & there's no return... (sorry about the silly explanation - I'm just coming to the end of a very long day & Mr Brain needs to keep himself awake somehow).Next! And a bit by-the-by... I've found that the larger turbos can actually give a very comfortable 'pottering' type of feel to them, presumeably because the exhaust side of the turbo is large/free-flowing enough to not 'get in the way' at off-boost speeds. No, there's no grunt, but it also feels as though the banana has been removed from the exhaust at these lower revs. I have found that boost (TD04) often inconveniently decides to assert itself just before the traffic flow speed, so tries to majestically slam the car into the back of the pottering 205 in front. Not helpful.
Just IMHO!
[Edited by nom - 2/3/2003 9:28:04 PM]
02 March 2003, 09:36 PM
#21
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I hear you Pat, and yes the IHI and TDO turbo has a shet house design for a waste gate, the garret has the pivot turned 90 deg and there is a cast divider used between the two. Have you see the turbo they use on the WRCar ? it is just this!
The cams on the 99-00 STI and all new age wrx/sti are much much bigger than the wrx PH2 cams , over lap has been increased by 23deg on the 01wrx and the STI are bigger again! So my results hold water on this as my testing has been with V5/6sti & 01/02 wrx/sti with regards to the S bend. The dump pipe has prooven a winner on any model with VF and TDO turbo's.
Some have gone to external WG off the up pipe and 4" and haven't got the gains they thought they would.
This bit doesn't work>>
With regard to downpipes, the design will obviously affect the performance. The 2.5" open neck design will present some back pressure helping keep the wastegate shut. A 3" twin dump will have less back pressure, and would spool better, but because there is no back pressure to keep the wastegate closed it opens sooner and the boost is slugged,<<<<<
The spring keeps the WG shut and a small down pipe just reduces flow and power overall, It doesn't help to keep the WG shut other than it has less VE and pressure to push it open from within, I have tried and put an external spring on the waste gate that took 20psi of pressure to push it off it's seat , then under full load it still made 20 psi at 3000rpm & 17 @ 2600rpm with or without the extra spring on it ( this was as high as i would go with rpm due to much over boost) Point was the EBC and preset acuator spring tension was bring boost on as fast as posible. the fact that i used a 4" collector has made the biggest increase in low rpm boost over a 3" or twin pipe.
Michael
[Edited by msgofaster - 2/3/2003 9:50:59 PM]
The cams on the 99-00 STI and all new age wrx/sti are much much bigger than the wrx PH2 cams , over lap has been increased by 23deg on the 01wrx and the STI are bigger again! So my results hold water on this as my testing has been with V5/6sti & 01/02 wrx/sti with regards to the S bend. The dump pipe has prooven a winner on any model with VF and TDO turbo's.
Some have gone to external WG off the up pipe and 4" and haven't got the gains they thought they would.
This bit doesn't work>>
With regard to downpipes, the design will obviously affect the performance. The 2.5" open neck design will present some back pressure helping keep the wastegate shut. A 3" twin dump will have less back pressure, and would spool better, but because there is no back pressure to keep the wastegate closed it opens sooner and the boost is slugged,<<<<<
The spring keeps the WG shut and a small down pipe just reduces flow and power overall, It doesn't help to keep the WG shut other than it has less VE and pressure to push it open from within, I have tried and put an external spring on the waste gate that took 20psi of pressure to push it off it's seat , then under full load it still made 20 psi at 3000rpm & 17 @ 2600rpm with or without the extra spring on it ( this was as high as i would go with rpm due to much over boost) Point was the EBC and preset acuator spring tension was bring boost on as fast as posible. the fact that i used a 4" collector has made the biggest increase in low rpm boost over a 3" or twin pipe.
Michael
[Edited by msgofaster - 2/3/2003 9:50:59 PM]
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