Exhausts and other stuff.
06-13-2012, 01:15 AM
#31
Drifting
Jeff had a 3.0 done by LR with their 4" exhaust. He took it off and replaced it with the original 3" that came with the car. This car used to be mine when it was a 2.5 with a 3". He noticed better spool, less noise, and no loss in top end on the dyno. He has a 4" exhaust for sale if anyone has interest.
Motorsports in SLC has done some neat large bore 951 motors, and they state all you get is a louder motor with later onset of boost, no matter the turbo.
Motorsports in SLC has done some neat large bore 951 motors, and they state all you get is a louder motor with later onset of boost, no matter the turbo.
06-13-2012, 01:56 AM
#32
Drifting
http://www.modified.com/tech/modp-11...ech/index.html
Interesting read. May shed some insight into what Patrick is asking.
Interesting read. May shed some insight into what Patrick is asking.
06-13-2012, 03:35 AM
#33
Rennlist Member
Thread Starter
Interesting George although that seems to contradict what you stated in post #31?
So I've got a 3.5" d/p now. Wonder if I should just do 3.5" all the way through? Still, the underbody clearance would not be cleaned up much...
I have to say I am still conflicted on this. Quite a few people I respect have made opposite suggestions regarding the exhaust matter. Some have testing to back it up.
With the car a bit lighter since the rebuild I have to say that even though the exhaust may indeed create later spooling, it felt pretty responsive and the noise doesn't bother me. I actually like it to be honest.
However the clearance may be the deciding factor.
So I've got a 3.5" d/p now. Wonder if I should just do 3.5" all the way through? Still, the underbody clearance would not be cleaned up much...
I have to say I am still conflicted on this. Quite a few people I respect have made opposite suggestions regarding the exhaust matter. Some have testing to back it up.
With the car a bit lighter since the rebuild I have to say that even though the exhaust may indeed create later spooling, it felt pretty responsive and the noise doesn't bother me. I actually like it to be honest.
However the clearance may be the deciding factor.
06-13-2012, 03:53 AM
#34
Drifting
Exhaust
It is, if you are only seeking max hp towards redline. Notice the link doesn't state tq curves once on boost. Cooling is vital for our car's intent. Here is a pic of my exhaust where it seems to matter most in our cars.
06-13-2012, 05:39 AM
#37
Nordschleife Master
http://www.modified.com/tech/modp-11...ech/index.html
Interesting read. May shed some insight into what Patrick is asking.
Interesting read. May shed some insight into what Patrick is asking.
The "best exhaust for a turbo is no exhaust" consensus is supported by the dyno charts, the physics and the books. Still this topic comes up from time to time and the claims that a larger exhaust gives later spool is as far as I can see only based on subjective observations.
Here's an easy to read article about the physics behind it
http://www.evans-tuning.com/support/...back-pressure/
06-13-2012, 05:51 AM
#38
Nordschleife Master
Pat, when it comes to clearance I will run a 3" with a separate 1 7/8" WG piping on the race car to get the best clearance. It can be fitted completely up in the trans tunnel (except for around the starter).
If you get rid of that horrible torsion tube you can route it very high in the rear too without anything hanging down
If you get rid of that horrible torsion tube you can route it very high in the rear too without anything hanging down
06-13-2012, 07:40 AM
#39
No exhaust at all behind the turbine being best is an absolute observation that doesn't apply on any given engine because it doesn't consider what's between the compressor outlet and the intake valves - the turbine's function is to propel the compressor, which performance will not only depend on how the turbine itself flows but also on how the intake side is designed, reciprocally in function of the turbine/exhaust (!).
I draw from Rod's feedback (post #19) that the stock intake (manifold + head) quickly became the restriction on his engine when using a free-er flowing turbine than the stock KKK and no exhaust past the rubine instead of some.
I replaced step by step the IC then the exhaust on my 3.0 engine and I can say this is coherent with how the engine behaviour evolved. The engine was working great at low boost with the stock IC and stock exhaust, but as soon as I installed an FMIC the stock exhaust became even more of a restriction - the restriction in the stock IC actually balanced out the restriction in the stock exhaust.
Rod says the "spool" came 500 rpm later - the turbo may have actually been spooling more quickly from whatever level of vacuum to 0psi than with any exhaust tubing - but this particular moment usually lasts less than a blink of an eye, difficult to quantify and not really relevant anyway.
Past this point, if the turbine flows well enough then backpressure will be so low that the engine will have to supply the turbine with 500rpm worth more of exhaust gases to reach the same level of backpressure and get the turbo to build the same boost level as when using some exhaust tubing. This and/or the compressor was surging.
Some people using GT turbos and 3" exhausts have reported compressor surge, so basically the 951 intake must become a restriction at some point, and earlier anyway than the need to apply the "no turbo at all behind the turbine" theory.
I draw from Rod's feedback (post #19) that the stock intake (manifold + head) quickly became the restriction on his engine when using a free-er flowing turbine than the stock KKK and no exhaust past the rubine instead of some.
I replaced step by step the IC then the exhaust on my 3.0 engine and I can say this is coherent with how the engine behaviour evolved. The engine was working great at low boost with the stock IC and stock exhaust, but as soon as I installed an FMIC the stock exhaust became even more of a restriction - the restriction in the stock IC actually balanced out the restriction in the stock exhaust.
Rod says the "spool" came 500 rpm later - the turbo may have actually been spooling more quickly from whatever level of vacuum to 0psi than with any exhaust tubing - but this particular moment usually lasts less than a blink of an eye, difficult to quantify and not really relevant anyway.
Past this point, if the turbine flows well enough then backpressure will be so low that the engine will have to supply the turbine with 500rpm worth more of exhaust gases to reach the same level of backpressure and get the turbo to build the same boost level as when using some exhaust tubing. This and/or the compressor was surging.
Some people using GT turbos and 3" exhausts have reported compressor surge, so basically the 951 intake must become a restriction at some point, and earlier anyway than the need to apply the "no turbo at all behind the turbine" theory.
Last edited by Thom; 06-13-2012 at 07:56 AM.
06-13-2012, 08:15 AM
#40
Nordschleife Master
No exhaust at all behind the turbine being best is an absolute observation that doesn't apply on any given engine because it doesn't consider what's between the compressor outlet and the intake valves - the turbine's function is to propel the compressor, which performance will not only depend on how the turbine itself flows but also on how the intake side is designed, reciprocally in function of the turbine/exhaust (!).
I draw from Rod's feedback (post #19) that the stock intake (manifold + head) quickly became the restriction on his engine when using a free-er flowing turbine than the stock KKK and no exhaust past the rubine instead of some.
I replaced step by step the IC then the exhaust on my 3.0 engine and I can say this is coherent with how the engine behaviour evolved. The engine was working great at low boost with the stock IC and stock exhaust, but as soon as I installed an FMIC the stock exhaust became even more of a restriction - the restriction in the stock IC actually balanced out the restriction in the stock exhaust.
Rod says the "spool" came 500 rpm later - the turbo may have actually been spooling more quickly from whatever level of vacuum to 0psi than with any exhaust tubing - but this particular moment usually lasts less than a blink of an eye, difficult to quantify and not really relevant anyway.
Past this point, if the turbine flows well enough then backpressure will be so low that the engine will have to supply the turbine with 500rpm worth more of exhaust gases to reach the same level of backpressure and get the turbo to build the same boost level as when using some exhaust tubing. This and/or the compressor was surging.
Some people using GT turbos and 3" exhausts have reported compressor surge, so basically the 951 intake must become a restriction at some point, and earlier anyway than the need to apply the "no turbo at all behind the turbine" theory.
I draw from Rod's feedback (post #19) that the stock intake (manifold + head) quickly became the restriction on his engine when using a free-er flowing turbine than the stock KKK and no exhaust past the rubine instead of some.
I replaced step by step the IC then the exhaust on my 3.0 engine and I can say this is coherent with how the engine behaviour evolved. The engine was working great at low boost with the stock IC and stock exhaust, but as soon as I installed an FMIC the stock exhaust became even more of a restriction - the restriction in the stock IC actually balanced out the restriction in the stock exhaust.
Rod says the "spool" came 500 rpm later - the turbo may have actually been spooling more quickly from whatever level of vacuum to 0psi than with any exhaust tubing - but this particular moment usually lasts less than a blink of an eye, difficult to quantify and not really relevant anyway.
Past this point, if the turbine flows well enough then backpressure will be so low that the engine will have to supply the turbine with 500rpm worth more of exhaust gases to reach the same level of backpressure and get the turbo to build the same boost level as when using some exhaust tubing. This and/or the compressor was surging.
Some people using GT turbos and 3" exhausts have reported compressor surge, so basically the 951 intake must become a restriction at some point, and earlier anyway than the need to apply the "no turbo at all behind the turbine" theory.
Reducing VE and putting a restrictor after the compressor outlet would indicate a high boost level but there would be very little flow and very little power. Point is, actual boost level is not of interest in this discussion.
What we want is to extract maximum energy from the turbine and the turbine works of the pressure differential before/after the turbine. And the greater the differential the more energy we can harvest. If you would have higher post turbine pressure than pre turbine you are in trouble.
What I think this discussion comes down to is this:
At some point a restrictive exhaust can increase the pre turbine back pressure. That in combination with that so very few of these cars are actually tuned could give bit earlier spool.
But spool and boost vs rpm does not necessarily correlate with actual power or acceleration.
Added backpressure will always decrease VE and if your car wasn't properly tuned for the increasaed VE of a low backpressure setup then it's not a valid comparison.
With low backpressure you can add a bit of extra cam overlap and tuned for it you will have extra gains.
06-13-2012, 08:19 AM
#41
Rennlist Member
Thread Starter
Well apart from all this conjecture over the post turbine exhaust goes, we still haven't covered much on the intake. Naturally this is because there isn't enough quantifiable information on this integral part of our motors. Just to add to the mix of all this. Neil Harvey who put the motor together in the crate said he came up against some real anomalies with the EGTs at idle and low throttle positions. 2 of the 4 cylinders were waaay hotter than the others. Now on a race motor this may not be extremely important but he wanted to make sure that even under a potential Yellow flag, I wouldn't run into possible problems. He chased these problems for some time. Changing and testing everything he could think of. Even running it in n/a config. Eventually he felt that the large headers were doing something unusual. As these headers have been on other race motors without incident it was decided that perhaps the headers and the race head were working in conjunction to create issues with the modified intake. He was going to test a stock intake and stock headers when I called a halt to the build. As I was progressing with Motec down here I decided that there were too many reasons not to continue with the build in the U.S.
I brought this point up with the engineer down here and as mentioned before, he felt that the stock intake 'should' be very well engineered by the factory. Hell, why wouldn't it be? I suspect that as they never planned on these motors running 25psi with much larger turbos and all the rest of the mods, that the intake was probably fine under stock or somewhat modified conditions. He also had a take on EGTs being a very fickle way of tuning in his experience. Fewer people tune via EGTs anymore with O2 sensors being much better and hardier nowdays.
The continuing tune of this motor shall be interesting....
I brought this point up with the engineer down here and as mentioned before, he felt that the stock intake 'should' be very well engineered by the factory. Hell, why wouldn't it be? I suspect that as they never planned on these motors running 25psi with much larger turbos and all the rest of the mods, that the intake was probably fine under stock or somewhat modified conditions. He also had a take on EGTs being a very fickle way of tuning in his experience. Fewer people tune via EGTs anymore with O2 sensors being much better and hardier nowdays.
The continuing tune of this motor shall be interesting....
06-13-2012, 08:21 AM
#42
Rennlist Member
I have not found too much connection with intake/exhaust myself as yet, I still have more back pressure in the crossover than I would like, but I don't think there is anything to gain by going bigger than 3" with the exhaust.
06-13-2012, 09:07 AM
#43
Restriction on the compressor side will vary the boost level produced but that has nothing to do with the actual flow or power the engine will make.
Reducing VE and putting a restrictor after the compressor outlet would indicate a high boost level but there would be very little flow and very little power. Point is, actual boost level is not of interest in this discussion.
What we want is to extract maximum energy from the turbine and the turbine works of the pressure differential before/after the turbine. And the greater the differential the more energy we can harvest. If you would have higher post turbine pressure than pre turbine you are in trouble.
What I think this discussion comes down to is this:
At some point a restrictive exhaust can increase the pre turbine back pressure. That in combination with that so very few of these cars are actually tuned could give bit earlier spool.
But spool and boost vs rpm does not necessarily correlate with actual power or acceleration.
Added backpressure will always decrease VE and if your car wasn't properly tuned for the increasaed VE of a low backpressure setup then it's not a valid comparison.
With low backpressure you can add a bit of extra cam overlap and tuned for it you will have extra gains.
Reducing VE and putting a restrictor after the compressor outlet would indicate a high boost level but there would be very little flow and very little power. Point is, actual boost level is not of interest in this discussion.
What we want is to extract maximum energy from the turbine and the turbine works of the pressure differential before/after the turbine. And the greater the differential the more energy we can harvest. If you would have higher post turbine pressure than pre turbine you are in trouble.
What I think this discussion comes down to is this:
At some point a restrictive exhaust can increase the pre turbine back pressure. That in combination with that so very few of these cars are actually tuned could give bit earlier spool.
But spool and boost vs rpm does not necessarily correlate with actual power or acceleration.
Added backpressure will always decrease VE and if your car wasn't properly tuned for the increasaed VE of a low backpressure setup then it's not a valid comparison.
With low backpressure you can add a bit of extra cam overlap and tuned for it you will have extra gains.
We just can't stick a fat turbo, throw away the exhaust past the turbine and reasonably expect a highly increased VE without working on the intake as well.
06-13-2012, 09:27 AM
#44
Nordschleife Master
Yes, but the bottom line, for me at least, is that VE will remain strongly dependant on how the whole intake line is designed after restrictions on the exhaust have been removed.
We just can't stick a fat turbo, throw away the exhaust past the turbine and reasonably expect a highly increased VE without working on the intake as well.
We just can't stick a fat turbo, throw away the exhaust past the turbine and reasonably expect a highly increased VE without working on the intake as well.
My firm opinion about the exhaust topic is in short
Use the largest, least restrictive, exhaust you can and choose a turbo suitable for your engine and goals.
06-13-2012, 09:33 AM
#45
Rennlist Member
Thread Starter
There's probably less than a handful of builders/shops that have done much comparative testing at the upper levels. Milledge would have to be the top guy I suspect? A few of us have done limited testing but much of that probably falls within a 'subjective' grouping. Still, at least these threads are a bit more interesting than "How wide of a tire can I fit on my 944" etc...