I was thinking the same thing.

I'm surprised this issue causes so much contention.

A Porsche mechanic told me that the inside diameter of the stock downpipe is just over 2" and any increase in exhaust pipe size over this is therefore a waste of time.

I think this is a gross oversimplification. The higher the engine speed, the longer the exhaust tube post turbo and/or the tighter the bends and greater their number, the more susceptible the system will be to backpressure. If backpressure is a potential issue, surely an increase in tube diameter can help - it would be analogous to shortening the length of pipe. There would be a point at which backpressure is no longer an issue - once that point is reached, i imagine there would be no benefit in increasing size. No doubt turbo sizing would play a part here and quite likely the "matching" of the hot and cold sides.

Wrt engine speed and length of exhaust tube, Dr J C Morrison did extensive research on this at the University of Glasgow. He even designed and constructed a testing apparatus. No, he didn't specifically test the 951 - it was well before the 951s conception.

If anyone is interested, some of the results are reproduced in: "Scientific Design of Exhaust and Intake Systems" by Smith & Morrison published by Bentley.

 

i heard of some people having ground clearance issue on the street when the car is lowered with a 4in system

 

Mine is instsalled but the car is not running yet and I'm yet to do suspension mods. It doesn't look like there will be any clearance issues, but it would be good to have some feedback from others with 4".

 

I had the Lindsey 4 inch system on my old car (lowered 1.5), and didn't have any clearance issues at all, none. And isn't it ironic that the 944T Cup Cars all ran 4 inch system too? And that is a fact, period.

 

And they did this even though they only used the tiny K26/8 turbo.

 

That's right -->

 

Amazes me how people continue to argue so vehemently against this.

 

Well isn't there a point of diminishing returns? I'm not saying it's 4" but surely it's not a case of the bigger the better? I was always told that turbo's needed a certain amount (?) of backpressure to spoolup properly?
We (Sean/JET951 and I ) are going with a full 3" system as shown in his thread, and then at a later date weld up a 4" back from a certain point. We hope to data log the differences.

 

I think this is essentially what the 4" systems are (I know mine is)... 3" to the cat and then 4" thereafter.

 

I find it odd that Porsche would invest money into R&D and production to put them on the Cup Cars back into lower BHP cars, don't you? If you have tons and tons of hard-core empiracal data, with all variables being equal for both the 3 and 4 inch system, please share it with us.

 

Sorry, last post messed up.

Of course there is... the point at which exhaust back pressure is no longer an issue. LR say there is still an issue with 4"... so, they are working on 5" which they believe will fix it - at least for the 2.5l. I'm thinking I may even install a gauge on my car to measure backpressure.

wrt spoolup, my understanding was that exhaust velocity and coordinating the exhaust pulses is what was important. I believe Porsche used exhaust port inserts to aid spoolup. Whilst a smaller pipe to the turbo can aid exhaust gas velocity and hence spoolup I thought that once the exhaust gases had entered the turbine, you'd want them to escape as easily and quickly as possible, ie minimal backpressure, because if the gases are backing up, it would slow down the turbine.

 

Exhaust diameter really doesn't matter unless you really do have a huge HP rating. Dfastest has a stock sized 2.5" on his 560Hp monster.

 

Sorry, I don't quite understand the point you are making here.

 

Thank you for saving my keyboard from excess typing! That paragraph says it all. The purpose of everything post-turbine is to get as much exhaust gas past the turbo as quickly as possible, at least in terms of acheiving maximum performance (in otherwords minimize restriction/backpressure). Pre-turbine exhaust flow is a different thing entirely and can have have positive or negative effects on many factors as a result of changes to size or shape of crossover, header, or exhaust housing components. In short, there is no real performance downside to running the largest exhaust possible (space and weight constraints aside).



I was hoping someone would catch on to that. How much so really depends on each individual's specific turbo setup but, for higher power levels even the #8 housing is getting to the "grape-through-a-straw" territory. Simply looking at a typical Garrett T3 .82a/r turbine housing makes it pretty obvious how small the #8 is.


Billindenver, I would highly recommend a straight 3" if you really are running a stock/gutted 2.5" on your car. Making 347whp @ 16psi with a gutted 2.5", that has to be a generously sized turbo. There's no doubt in my mind you'd notice a definite improvement in power and spool. You'd also loose a decent amount of weight as the stock cat and especially the stock cat-back muffler section are extremely heavy. Don't take my word for it though as I don't know what I'm talking about : Link This alone should make it pretty much a non-issue for everyone: Link

 

I'd love to run a 4" on our "track only" car. The only concern I have is db level at restricted tracks. In most cases a turbo racecar has little issues with noise. However I have read several threads referencing how loud the 4" exhaust is on the 951. I plan to run a Borla XR-1 and would love to hear if anyone has experience running a 4" at a track with sound meters.