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Gents,

Im currently at Stage 2 + running stock down pipes with Primary Cats gutted, then 3" MTM system thereafter with no SEC cats. As I'm about to install water meth and get a revised map done my question is do I get my contact to fab up 3" down pipes or go 3.5" on stock turbo's to make the most of my last mod...for now. Is it a case of bigger is better or will it cause back pressure issues etc...????

Thanks in advance for your feedback.

Wow, will 3.5" fit?

Good one, but long one:
http://www.dsmtuners.com/threads/exhaus ... re.168578/

Short answer: If you can fit 3.5" - it won't hurt anyway. Would be interesting to see a dyno run before and after...

3.5 " DP are too big even for hybrids IMO because you will struggle with flow velocity. I've used 3" catless DP for my 9xxps and didn't have a problem with flow. Imagine blowing through a large pipe compared to a straw... the flow velocity through the straw will be much quicker... just my opinion of course.

classba wrote:Imagine blowing through a large pipe compared to a straw... the flow velocity through the straw will be much quicker... just my opinion of course.

What do you think the benefit of that would be? The velocity @ turbine will be the same, and after that makes no difference.
After turbine, in exhaust: Velocity could be/will be higher (if any difference) due to smaller ID - but this would/could cause back pressure - restricting the system. (however miniscule)

(Bottomline: I think it makes no difference 3" -> 3.5")

The higher the exhaust gas velocity the quicker those gases exit the cylinder, which lowers back pressure too, but make the exhaust too small and the exhaust gases can't leave the cylinder quickly enough regardless of its velocity which creates back pressure.

With the 3.5in bigger bore there's less back pressure but also slower gas velocity so the gas leaving the cylinder is slower too. If the gas leaving the cylinder is slower then the turbo spools slower as the gases spin the turbo slower. With a higher gas velocity hitting the turbine blades, the turbo spins faster which spools up faster.

So it's a compromise and the "best" sized DP for good power with minimal lag is 3in as proven by at least 4 900+ cars. Now that 3in width is for the entire length of the DP but there are some with a 3.5in turbo connection which rapidly narrows within 6in from the turbo to 3in for the rest of the DP.

Ian: Gas velocity is created by the engine, used in the turbine, and then of no use in the exhaust. (Other than generating backpressure in poor setups).
A larger ID exhaust will have no negative effects on the turbine spooling. Gas velocity @ turbine is generated by the engine alone. (If you have a restriction after that will create both backpressure and higher velocity, but not where you want it. (Think Venturi effect).)

"With the 3.5in bigger bore there's less back pressure but also slower gas velocity so the gas leaving the cylinder is slower too."
Is incorrect, at least to my knowledge.

Edit: Open thought. Has any of the 900+ cars tried a 3.5" setup? Or 3.25"? With or without dyno.

3.5" FTMFW








You're trying to rid the gas as quickly as possible, and spoolup is not impacted as the inlet to the turbine from the manifold is stock.
However, on stock turbos, no noticable gains (or losses) over 3".

Lots of great feedback - thanks guys...

In addition to maximizing my revised map, my other theory behind 3.5" DP was to future proof IF I decide to go the bigger turbo's route at a later date assuming it had no detrimental effects. Based on all feedback received I think i'll fabricate 3.5" for first 6" and then 3" from there as it seems the best compromise noting comments received.....

Thanks again and ill give some feedback in due course.

The thing i've noticed about overly large diameter exhausts is they sound rubbish. You lose the nice tone and get a plastic drainpipe sound. There was a green RS6 at a recent cars and coffee meet which was loud but it wasnt a nice sound.

A20LEE wrote:The thing i've noticed about overly large diameter exhausts is they sound rubbish. You lose the nice tone and get a plastic drainpipe sound. There was a green RS6 at a recent cars and coffee meet which was loud but it wasnt a nice sound.

Good point.

Sound could be tuned (pun intended) with adjustable valves.

Interesting thoughts re the sound. I bought the MTM system as my research told me that without the array of specialists the UK has to offer, the MTM would be the best option. I must say however that having previously had the stock system with both cats removed, it wasn't much different, albeit still an improvement. Unfortunately we aint never gonna get the howl of our R8 brethren's so prepared to sacrifice noise for sake of a few more ponies.

Lets see....

LOl... theres much more thing to think about in tuning a C6 than the difference of half an inch of the down pipe lol

I'm sorry but you guys made me chuckle

Will be no negative from going bigger. Can only be a + really from much experience, it's a case of bigger is better

This thread is about this exact thing. This is why there is a forum - to discuss in detail.
Agree that there is much more to tuning a C6 than 1/2 an inch....

Only thing chuckleworthy about this thread is people with 900+ cars not knowing how a turbo works, giving incorrect advice... .truth

leehweitt: Dynos before and after would be gold. Although I don't expect any noticeable difference - perhaps there is a measurable one?