More M42, this time exhaust.

[OSAH racing]

In the planning stage as my 318i gets a m42 swap by Jack Fuhina. My car has a Montana registration. I will be getting a California registration. The swap will include all the California emissions equipment and stock cat for the inspection. The inspection will include a CHP inspection because of the non original engine M42 and out of state ( originally a California car) registration.

At some point after all that I want to optimize the exhaust and I have a custom exhaust guy here in The Inland Empire. I picked up on here the stock exhaust header is great. I know I can run a high flow cat that is California legal. My car is Low. 5" to the pinch welds with AST coilers for track days. I do intend to raise it up an inch. With The m50 that was in their the 2 3/4" exhaust with v bands were a ridiculous 2 1/2" off the ground at the front controls arms and the fuel tank.

What I am interested in is light and free flowing. what is the optimal tube size? single or double? any pictures of a custom exhaust would be great to see.

[bmwman91]

How much power do you expect to be making with the M42? The stock exhaust is a little small for a 200bhp NA M42, but it isn't the main impedance to flow. Really, the head is where most of the restriction is, and a lot of gains can come from hotter cams and some mild/moderate work in the ports and valves.

Anyway, if you stick a SMOG compliant high flow cat in there, that takes care of the bulk of the restriction in the exhaust. Not having the stock resonator helps a little too. IIRC the stock exhaust is like 60mm (~2.25") diameter. I am still running a stock header and pipes on my 2.1L, but the guy who tuned mine (and several other built M42s) did think that there were some gains to be had with slightly larger tubing and a custom built header with larger runners. How much the gains would be is TBD I guess! I will be interested to hear about where you decide to go with yours.

[OSAH racing]

The engine refresh is just bearings, gaskets, runners, oil and water pump, etc, ect, so stock. My goal right now is a Ca registration. You have convinced me that there are modest gains in HP to be had without a full rebuild with cams/stroker crank. The squid chip and exhaust I want to do right after the Registration process. I will eventually get a build thread going and show what is being done. I will consult with the exhaust guy but I am shooting for a lighter, better flowing system placed up as high as possible.