Thread: Exhaust sizing...

so technically you were running a open downpipe?

Yes, open downpipe. 3" outlet on the turbo, with a 3" downpipe. Wastegate was still recirculated. I would imagine a divorced wastegate would make even more power.

http://www.querycat.com/faq/df21a24b8c9a0baf4a6822fb0c42f4fe

actually this says better what i was trying to say.

What diameter exhaust piping should I use? Is bigger tubing always better?
Frequently Asked Questions - Bassani.com
Contrary to what you may have heard, bigger tubing Is not always better! While big fat tubing look aggressive and allow for a high volume of exhaust flow, they can actually hurt your engine performance by reducing exhaust gas velocity, which is critical to maintain low-end/mid-range torque and horsepower. Bassani aft-cat exhaust systems are tuned and designed for optimum flow rate for each specific application. This provides power through a broad RPM range, adds fuel economy and quality sound.

^^^ true, but it only works on NA setups. It is all about pulse tuning on NA. Each exhaust pulse has a frequency for resonance based on rpm and pipe diameter. When that frequency is hit, the VE is improved and more power is made.

A turbo is based around pressure, not resonance. The larger the difference in pressure between the manifold and after the turbo, the better VE you get. If there is any backpressure in the exhaust system, it increases backpressure in the turbo manifold by usually a 2:1 ratio. Higher backpressure in the manifold not only results in pumping losses from the engine, but the turbine as well.

Even exhaust gas velocity considered, the turbo will make more bottom-end power with no exhaust at all. Look at my graph. I made 10whp more without the exhaust @ 2500rpm and more than 50whp at 4500rpm. Top end power would have been more if the car were running better on this particular day, but it still made more power throughout the powerband even with 4* less timing.

If you pick too big of an exhaust on an NA engine, you will not hit a resonant area until very high rpm. This will cause a drop in low-end power. A smaller diameter pipe will resonate at lower rpm and create more low-end, but will most likely be past its resonance area in high rpm.

This is how Honda gets over 100% VE in the K20. At around 6500rpm the K20 is consuming a lil more than 2.1L of air. For a 2.0L engine this is impossible, unless several components of the engine are working together. Honda spends a lot of money in developing systems like this. With the intake, manifolds, and exhaust tuned to resonate at the same frequency you can make the engine perform better than is normally possible.

GT2871RBLUBIRD, as Coheed has said, that information applies to N/A (or supercharged) cars only, not turbocharged cars. Not after the turbine anyway.

What he says about size still holds for turbocharged cars, but only to the manifold, not after the turbine. Giant pipes pre-turbo would be a bad idea, yes.

^^^ of course haha. Yes, big pipes before the turbo would not be good.

i completely agree with you,^^ im running open 3'' downpipe and 3'' side exit, so i know the benifits of a open setup, i get great spool up i greatly think its atributed to the open downpipe. even turbo applications have pulsing larger pulsing tho...completely agree tho

Cody started a 4" setup for me, but it was a pain in the ass by the sway bar I had up front and the over axle in the rear. We dropped it and stuck with 3". 3.5" would install easier, but the cost is more as it is more rare. Anyway, the best is as big as you can go, no matter what turbo. People say you don't want big for highway on turbo, well I went from 2.25 to 3" and my mileage is up! So, yeah, bigger you can the better.

Brent

Sweet, I'm getting 26mpg on average with this setup, which just wowed me, so I'll just keep it and go cutout on the track.

26mpg! Dude, Highway I am getting 34-35mpg with the turbo. You need tweakage.

Brent