3" Full exhaust without cat
18 January 2003, 10:05 PM
#7
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I've said it before and I'll say it again, *ANY* backpressure behind the turbos is too much back pressure, so whatever can be done to minimise this back pressure will have a beneficial effect on power output.
What some people have observed is that a 3" system causes a reduction in low down torque; this needs to be analysed in two operating regimes, vacuum and supercharged. In vacuum, the 3" system will provide less back pressure and thus reduce reversion, which will lower the dynamic compression ratio and thus the thermal efficiency, hence the car feels less responsive, and fuel economy will suffer. The only way to recover some of this lost torque is to put a vacuum operted butterfly in the exhaust to increase back pressure when the engine is operating in vacuum.
People have also observed that a car with a 3" system will not make boost as early as a 2.5" system. This is more down to a fundamental design flaw of the turbocharging system than the exhaust system. A properly designed turbocharging system would benefit from the increased flow capability and would actually spool slightly quicker. The problem appears to be accentuated with a twin dump downpipe. I believe that I've identified the specific cause of the problem and am working on a solution
Cheers,
Pat.
What some people have observed is that a 3" system causes a reduction in low down torque; this needs to be analysed in two operating regimes, vacuum and supercharged. In vacuum, the 3" system will provide less back pressure and thus reduce reversion, which will lower the dynamic compression ratio and thus the thermal efficiency, hence the car feels less responsive, and fuel economy will suffer. The only way to recover some of this lost torque is to put a vacuum operted butterfly in the exhaust to increase back pressure when the engine is operating in vacuum.
People have also observed that a car with a 3" system will not make boost as early as a 2.5" system. This is more down to a fundamental design flaw of the turbocharging system than the exhaust system. A properly designed turbocharging system would benefit from the increased flow capability and would actually spool slightly quicker. The problem appears to be accentuated with a twin dump downpipe. I believe that I've identified the specific cause of the problem and am working on a solution
Cheers,
Pat.
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19 January 2003, 09:34 AM
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Well put Pat!!!! 
Simon, a thought for you, what about a 3" to 2.5" dowpipe? ie 3" open neck at turbo to 2.5" at flange with centre? (Scoobysport do one I believe)
Is this a compromise? I'm not that technical, maybe Pat would have some idea?
Steve.
[Edited by The TunnelMeister - 1/19/2003 9:36:36 AM]
Simon, a thought for you, what about a 3" to 2.5" dowpipe? ie 3" open neck at turbo to 2.5" at flange with centre? (Scoobysport do one I believe)
Is this a compromise? I'm not that technical, maybe Pat would have some idea?
Steve.
[Edited by The TunnelMeister - 1/19/2003 9:36:36 AM]
22 January 2003, 08:42 AM
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Don,t know if it help,s but we did a back to back with a complete 3" aftermarket system and our 3"-2.5" system and it gave 16 B.H.P more and all round better performance for the road.The 3" system only gained when on full.more info call 01268 726034.
24 January 2003, 11:14 PM
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I've said it before and I'll say it again, the Scoobysport exhaust (made by Hayward and Scott) is without a doubt my favorite one. The thing is that when you are just cruising then you don't really notice it, but at full throttle it screams 
I love the sound, it's somewhere in between, it's not silent and it's not too loud and just sweet as hell.
For example a friend of mine bought the full system from Power Engineering and it was a very sweet sound, a little bit louder than the Hayward and Scott, and he removed the back box because he thought it was too loud.
I sat in my friends car, he had an open 3" exhaust without cats with a Blitz backbox and that car was loud, too loud in my opinion but he loved it. My head was just hurting after one night driving around in his car. But that's just me, that is why I chose the Scoobysport one.
Just choose the exhaust carefully regarding sound. It's not like we have a track here in Iceland so you only have to consider day to day driving And just how loud do you want it to be, everybody has a different oppinion on that
Cheers,
Steven
Subaru Impreza GT MY00
I love the sound, it's somewhere in between, it's not silent and it's not too loud and just sweet as hell. For example a friend of mine bought the full system from Power Engineering and it was a very sweet sound, a little bit louder than the Hayward and Scott, and he removed the back box because he thought it was too loud.
I sat in my friends car, he had an open 3" exhaust without cats with a Blitz backbox and that car was loud, too loud in my opinion but he loved it. My head was just hurting after one night driving around in his car. But that's just me, that is why I chose the Scoobysport one.
Just choose the exhaust carefully regarding sound. It's not like we have a track here in Iceland so you only have to consider day to day driving
And just how loud do you want it to be, everybody has a different oppinion on that Cheers,
Steven
Subaru Impreza GT MY00
25 January 2003, 03:42 AM
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SPR:yeah mate i might finally listen to you mate Sweet sound and
first they claim that 3-2,5 is better in lower rpm then the Exhaust search is now ower...
Simon
Impreza GT MY00
first they claim that 3-2,5 is better in lower rpm then the Exhaust search is now ower...
Simon
Impreza GT MY00
25 January 2003, 03:49 AM
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My personal feeling is that there should be no reduction in the diameter of the exhaust at any point toward the rear of the car, I prefer to have a low gas speed out of the tail pipe, as this should reduce reflections (if you know about transmission line theory I'm trying to match phase velocities).
However it is undeniable that the Scoobysport / Hayward & Scott downpipe works really well, and the system as a whole flows well... only the other day I managed to get a check engine light on an MY01 I was mapping, it would appear that the ECU thought that we could not possibly be flowing that much air, thus considered the MAF meter malfunctioning!
I'm actually running this type of pipe on my Legacy at the moment, the full system on an older (TD05 equipped car) is quite loud, much more civilised in the later cars though.
Right now I'de say that if you want good all round driveability, go with a 2.5" system, but if you're putting in a hybrid turbo and want to get every last BHP out of it, then go 3"... I'm looking into a few ideas that would allow all of the benefits of the 3" system without the loss of the midrange.
If you're after an extreme flow exhaust system, then watch this space... you won't "need" if you're making less than 400-450 BHP, but it will flow exceptionally well, be quiet and not kill the mid range, so you may want one for those reasons rather than its flow characteristics.
Cheers,
Pat.
However it is undeniable that the Scoobysport / Hayward & Scott downpipe works really well, and the system as a whole flows well... only the other day I managed to get a check engine light on an MY01 I was mapping, it would appear that the ECU thought that we could not possibly be flowing that much air, thus considered the MAF meter malfunctioning!
I'm actually running this type of pipe on my Legacy at the moment, the full system on an older (TD05 equipped car) is quite loud, much more civilised in the later cars though.Right now I'de say that if you want good all round driveability, go with a 2.5" system, but if you're putting in a hybrid turbo and want to get every last BHP out of it, then go 3"... I'm looking into a few ideas that would allow all of the benefits of the 3" system without the loss of the midrange.
If you're after an extreme flow exhaust system, then watch this space... you won't "need" if you're making less than 400-450 BHP, but it will flow exceptionally well, be quiet and not kill the mid range, so you may want one for those reasons rather than its flow characteristics.
Cheers,
Pat.
25 January 2003, 11:04 AM
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Just to back up what Pat has said, the reversion offered by a 2.5" diameter does seem to help the cars off boost and the flow properties of the system offer no back pressure until you reach around 340bhp. That said, the MAF limit problem that Pat was reffering to is on a UK car that is running considerably more than this and the standard Scoobysport exhaust was still performing perfectly.
When we started working with those very clever people at Janspeed, they were able to considerably improve the flow on both the downpipes and rear silencers, with only some fine detail changes. This raised the system rating from around 315bhp to its current levels, whilst keeping the off boost gas at its previous levels.
We were doing some flow testing yesterday with Pat's system and it is very impressive. It won't be suitable for cars using standard turbos (WRX or STi) but will be a "must have" for people with big hybred turbos, looking for big power.
When we started working with those very clever people at Janspeed, they were able to considerably improve the flow on both the downpipes and rear silencers, with only some fine detail changes. This raised the system rating from around 315bhp to its current levels, whilst keeping the off boost gas at its previous levels.
We were doing some flow testing yesterday with Pat's system and it is very impressive. It won't be suitable for cars using standard turbos (WRX or STi) but will be a "must have" for people with big hybred turbos, looking for big power.
27 January 2003, 09:45 AM
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We would just like to put the record straight the my01 car that pat was mapping has infact a Hayward & Scott 4-1 equal length free flow manifold and a full Hayward & Scott turbo back exhaust system & NOT a Janspeed system. The big bore exhaust that Pat has designed for 400 bhp + cars is in fact a hayward & scott exhaust made exclusively for Pat
Ian
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