i944scania

trying to search for this topic is very difficult.
I am curious to hear what people have to say on here about exhaust sizes as it relates to turbo performance, in my case an LSJ turbo swap.

Here is a popular belief:
-Having a larger downpipe to a smaller catback is ok because the air can cool enough by the time it hits the catback, therefore it is more dense and takes up less space so the restriction becomes less.

-Having a smaller downpipe than catback can allow for the exhaust right out of turbo to be hotter, and hotter air flows better so once it hits the larger catback, it flows with very little restriction.

in my opinion, as an engineer, if your downpipe is short the air from the turbo will NOT cool much at all before it hits the catback. Under load, that air is moving hard and will not see much temp change until further down.
With this in mind, if you ran a 3" downpipe and 2.5" catback i think there would be some serious bottleneck considering the air has not cooled much.


Let me know what you guys think. Im curious because ive got a 2.5" catless custom downpipe to 2.5" catback at the moment. I dont have any room to the trans mount for a 3".
Tomorrow Im getting a 3" catback made all the way up to the subframe area so only about 12" downpipe at 2.5" diameter. Ill let you guys know how it feels.

574

iTrader: (5)

If done right there has to be room for 3" I know it's tight (gigity) but I'm about to make it work on my car.

slapbetcommissioner

iTrader: (3)

I think it really depends on hp goals.
I would say the smoother the flow the more easily it should move down the pipe. Ie 2.5-2.5 may flow better then 3-2.5. If you go too large the air is allowed to become turbulent inside the tube and can loose low end torque.
Allot of people have reported loss of power with a cutout due to the turbulence the exhaust leaving strait out the turbo creates and lack of exhaust vectoring effect a longer pipe creates.

noorj

iTrader: (17)

You want the highest pressure difference across the turbo as possible, thus largest possible volume post turbo 3". Any decrease in diameter downstream (aka 2.5" catback) will increase upstream pressure thus impeding flow. That being said gains from 2.5"-3" are probably only within 5-10%

slapbetcommissioner

iTrader: (3)

With a larger turbo that 5-10% would probably change.

Too big of an opening post turbo can be counter productive if it doesn't help with a smooth exit

i944scania

thanks for the input guys. Well i went with the 3". I gained some spool time and a little bit of power. its nice to have my boost curve shifted down a little! sounds great too.
I feel like the 2.5" downpipe helps keep some torque, and its only about 12" of 2.5" pipe too, then 3" the rest of the way