2.25" vs 2.5" exhaust on M42?

[jaredmac11]

Im looking at buttoning up my e21 M42 exhaust setup. The collector runs 2.25" and I believe the E30 was 2.25" all the way back to the exhaust.


I know quite a few folks go for 2.5" exhaust, but has anyone tried both systems and have concrete feedback?


For me, I want to run a cat, resonator and dynomax super turbo muffler.. just trying to figure out 2.25 or 2.5" piping.


My fear is losing mid-range torque from insufficient backpressure by going to 2.5". Then again, plenty of folks have said that they went to 2.5" without noticing any losses, so who knows.


Regardless Im excited to try the M42 with 2.25 or 2.5 over my M10 1.75".

[roguetoaster]

Needing backpressure is baloney. Scavenging is important, but beyond the header/manifold it probably has little impact on performance.

Run the pipe size you need, and nothing more. In theory, too large of a pipe decreases exhaust velocity, which can be bad. IMO 2.25" is perfect for a fairly stock M42. A seriously built N/A M42 would probably want 2.5" pipe all the way.

[jaredmac11]

Needing backpressure is baloney. Scavenging is important, but beyond the header/manifold it probably has little impact on performance.

Run the pipe size you need, and nothing more. In theory, too large of a pipe decreases exhaust velocity, which can be bad. IMO 2.25" is perfect for a fairly stock M42. A seriously built N/A M42 would probably want 2.5" pipe all the way.

Im thinking 2.25 would work as well as that is stock. This engine is mostly stock with very minor improvements. Coil on plugs, single mass flywheel, e-fan, chip with higher flow injectors. I can imagine going turbo later.

[ibatic1]

I think 2.5. You are going to want to add boost anyways. That car deserves only the best in my own opinion. Gonna have to put more time in the garage than in PubG though lmao.

[jaredmac11]

Any recommendations on a resonator?

[roguetoaster]

Anything 14-16" to keep it quiet.