1991 318is exhaust recommendation?

[Boris3]

The Bosal catback the PO had put on the car has gone from annoying buzzy to a more annoying baffle rattle/resonance. I know there are some current and former e30 318is owners on the forum. Any recommendations for an exhaust?

[B C]

In my experiences, Magnaflow and Vibrant mufflers sound very good on 4 cylinder cars and are less prone to rasp, more-so when paired with a good resonator. 

Does a straight-through design matter to you?

[Boris3]

Thanks. Rasp does seem to be the 4 cylinder challenge.

Pros/cons of offset versus straight through? Flow, easier fit?

Suggestions on model #s for the Magnaflow and Vibrant mufflers and a resonator in a single 2.5 in and out.

Ideally looking for a deeper tone (which is tough on a 4 cylinder), daily driver friendly, and low on the LEO radar set up preferred. Stealth but some attitude when under load.

[B C]

http://vibrantperformance.com/catalog/product_info.php?cPath=1022_1031_1110&products_id=31  It also comes in a centered outlet. 

+ a tasteful welded on tip

+ a 12-18" resonator  (vibrant ultra quiet resonators are also very good) 

Bassboy has the centered version of this muffler on his M10 and it is surprisingly deep.

The offset outlet will mostly allow for better flexibility of installation if you need it. 

I have a Vibrant muffler and resonator on my car if you are curious about hearing what sort of change in tone it gives an M20. Otherwise I cant really find any good examples of an M42 online

[Earl]

I too am curious as to what exhaust I should have made up for the compact. I was thinking of going the vibrant route with at least one 18" resonator. I just don't know whether 2.25" or 2.5" would be better since it is so small.

I remember on my Rabbit that if you went over 2.25" you were losing too much back pressure and harming the already small amount of power.

[B C]

Backpressure in the complete exhaust stream mostly a consideration on a car with forced induction when the ECU cannot adapt/is not tunable for an increase in boost pressure due to the turbine spinning higher RPM from less exhaust restriction. 

Tubing size is important for gas velocity and matters mostly for reversion waves and evacuating the exhaust gasses from the cylinder adequately  (in other words the header, or any piping upstream of an X or Y pipe) It is also a consideration on a turbocharged car but in both scenarios, once the exhaust gas is past the collector/turbocharger,  the main goal is to get the exhaust out the back of the car as effortlessly as possible.

Yes you want to keep the piping to a smaller diameter to aid in exhaust gas velocity, but going too small and creating excessive pressure drops will be more detrimental than going too large and losing a bit of exhaust gas velocity. 

Since finding the perfect tubing diameter that has highest velocity with minimal pressure drop is unrealistic, err on the larger side rather than smaller side.