mperformance

Has anybody ran any tests with a 3" or larger diameter exhaust systems on a n/a stock, or streetported rotary engine?

I was reading this thread 3" exhaust

and I found it interesting that no restriction is good for mostly turbos etc. I know noise is a factor with n/a engines, but I wanted to know if anybody runs a custom made exhaust over 3" in diameter with any success or better performance than the usual 2.5" or smaller commercially available exhaust systems.

TIA

REVHED

Firstly, we should clear up that the accepted primary diameter for most all n/a engines is 2". The diameter for the rest of the system then comes down to the length of the primaries and the level of porting.

If it's a short primary system then anything above 2.5" will be too big as you'll start to lose exhaust gas velocity. On the other hand, if it's a long primary system then I don't think it will make much difference whether you run 2.5" or 3" over the diff as it's quite a short distance. Though, there's probably nothing to be gained by going to 3" unless you have a 13B bridgey or something like that...

Manntis

...and a 13B 6 port requires exhaust gas backpressure to open #5 and #6 ports, does it not?

peejay

The problem is velocity.

Turbo engines don't really care about the velocity post-turbine, because there's "significant" pressure pre-turbine, and there are for all intents and purposes no gains to be had by keeping the velocity up post-turbine. the best exhaust is something just long enough (and shaped properly) to straighten the exhaust flow and that's about it! i used to run a turbo car with no downpipe whatsoever, nothing hanging off the turbo except the small 45deg albow that the DP bolted to. turbo response went from "good" to "unreal"! (this being a low boost low powered 2-liter boinger)

N/As are nothing like this. They don't use the exhaust pulses for anything except scavenging, which is why you need the proper size pipe and not just the biggest one you can fit under the car.

Call me crazy but I like 2" singles for something that has to be driven on the street. Not as small as the factory stuff (1 3/4"!) but not so big as to cause annoying resonances.

REVHED

2" single would be a bit on the small side wouldn't it? Escpecially for any sort of 'performance' application.

Resonances can be annoying but they can be eliminated with the right choice of mufflers. Mine doesn't resonate at any rpm.

peejay

Remember this is for a purely street setup. Bigger is boomier, and I've been in cars that would rattle your fillings out at the right RPM.

It does help to put a muffler in the middle of the pipe, and to use thick tubing - this is RB's approach, they put a hefty muffler on the end of the header (the "presilencer") and they make the whole exhaust system out of 1/8" (~3mm) thick pipe.

I want to try an expansion chamber instead of a typical muffler-presilencer. Have to rework one of the header collectors anyway. Summit lists some cone 3"-2.5" reducers that I could turn around backwards - cut off the existing 2.5" collector, weld on the cone, then reduce that down to 2" or whatever I determine is best (this not being for a strictly street operation) and then see how it works.

DriftinJim

iTrader: (24)

You need 2-2 1/4" on an NA b/c an NA needs exhaust velocity, while a turbo just needs all out flow. Check what RB sells, they've done their research. BTW, yes, the 6 ports are backpressure operated, tho you can make a vacuum switch of sorts that opens them at a certain rpm, independent of backpressure.

mperformance

ok so it's all about exhaust velocity in N/A applications. Cool thanks...


I'm running a collected 2.5" exhuast for a stockport.