Flange it in a couple places? You mean for easier installation/removal of the exhaust piping?

Sounds like your pushing out some HP with a Cam too. Maybe you won't gain anything down low with a smaller exhaust. Flange it in a couple places and it's going to be trial and error I reckon.

Go according to HP. If you are making 120 HP net, 2" or 2.5" is called for.

Thanks guys, makes sense. My A12 peaks at 8k rpm (a lot of work been done)...wild at high revs but sluggish from idle up 3-4k rpm, therefore trying to find an arrangement which can help on take off.

Yes, there's been lots of dyno testing on this.

For low RPM power, smaller is better. For high RPM power, larger is better. So there is not an overal optimum best. It depends on what you want.

For an A12 that makes max power at 6000 RPM (think weber carb), a 1.5" mandrel bent system is best. A14 at 6000 RPM can use a 1.75" inch, but you loose a little low-RPM power.

What goes after the muffler makes neglible difference to HP. It mostly effects the noise.

Have you tried press bent 2 inch , the bends effectively reduce flow to 1/4 inch smaller. So 1.75 inch

Maybe a tapered exhaust with larger diameter up front tapering down at the end. Do start 2 1/4 then 2 inch from rear,muffler back.

Theory I read on this says as the gasses cool it keeps the gas velocity up. Some of the wrx guys do this 3 inch down to 2.5 from diff back. Slight drop in outright power but better for noise and response

Hi All, I'm wondering if any has trialled what exhaust set up delivers best performance? Modified A12 or A14 engines seem to lose a fair amount of power at low revs with 2 or 2.5 inch exhaust diameter. im thinking of reverting to 1.5 or 1 3/4 and inch but could be a costly experiment. Comments appreciated.