At what point power wise is a 2.5" system holding back the power?. Fitted a rotrex to my car which is now running 12psi (can still increase it yet/smaller pulley yet) at the moment so 'should' be between 300-400bhp. Currently has a 2.5" straight through system with 2 silencer boxes, noise isn't a problem anywhere i've run it.

If it would be better with a 3" system, will it be significantly louder?, maybe needs 3 silencer boxes to get the noise back down?.

Is one long silencer box better or worse than 2 short ones for noise or back pressure?.

Searching google suggests it needs to be 3", but i can't find a clear answer on the noise and if i need to make up a new system, i'd prefer to only do it once! (and not after failing on noise somewhere!).

Ian

http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/

Got 3"" on mine with a single box but its a turbo so not to bad noise wise mine is also a short system being on a seven type car hope this is of some help

[Edited on 29/1/15 by Dick]

[Edited on 29/1/15 by Dick]

Noise absorbsion on straight through systems relies on large cans for the exhaust wadding
Multiple cans could cause gas flow issues but with your output it's not an issue, perforated pipes in large cans don't produce much back pressure

I found that the elbow from the turbo was restrictive at circa 350hp on my setup, but not the system. ideally I need a bigger elbow when gasses are hottest and drop to 2.5" under the car to minimise losses. You could see the dyno plot plateauing ue to the elbow design (internal wastegated which didn't help as the WG gasses go down the elbow too). im sure an external WG would allow the use of a 2.5" setup throughout, but I used an external WG on previous racer and they come with their own issues.

ive seen a few hillclimbers on rotrex blowers and they do make a racket, both single seaters on Busas, and a Mk2 escort and corolla. the noise is induction based rather than exhaust though.

as has been said generally a turbos a great silencer in its own right

Are you talking headers or after the collector?

I was talking post turbo, so the elbow and downpipe effectively

Collector back i'm looking at. The manifold primaries are already 2" with a 2.5" collector (that was built for nitrous/rwyb before the supercharger went on).

Quick update. Made up a complete 3" system, 2 silencer boxes and the first one is deliberately a little larger than the old 2.5" system had to try and keep the noise down a bit. Went for a little test run around today, doesn't drone on a motorway, makes a deeper sound but it doesn't seem any louder overall than the old 2.5" system so i'm happy

Ian