I think that going 2" to 2.25" will make zero difference. The real restriction to flow in the M42 is the mild camshaft profiles. I am running a 2.1L with higher lift 272/250 cams, larger valves with smaller stems, multi-angle valve seat grind, 12:1 CR and have done a lot of data logging. The exhaust is basically stock, with an aftermarket Magnaflow cat (not for performance, but because the stock one would not pass emissions testing) and a Supersprint cat-back which is more or less the same diameter as stock. My intake is an M30 air box (65% more surface area) and MAF.
Data logs indicate zero air flow increase between the stock M42 box + AFM and the M30 box + MAF. My engine flows about 1.5X as much air as a stock M42. For exhaust, the guy who I worked on tuning with has said that the other 2.1L M42's he has tuned have picked up 5HP, maybe 10HP at most, from larger custom headers and larger main tubing.
Anyway, get the exhaust that produces the sound you want and fits your budget. It effectively poses no restriction to a stock M42, and still not much to a very heavily built one.
Regarding the other mods you listed, here's my opinion since I did many of those things a long time ago when I was younger and less experienced.
- Go back to a stock Mahle or Mann paper filter. The K&N ones do not flow more air and let more particulate crap into the engine. While the much less dense filter fabric in the K&N may seem like it is going to flow better, it has like 1/4 of the surface area of the stock paper ones. It is not 4x more "open" than the paper element, so it is restricting flow while not filtering as well. It will allow for more intake noise, which sounds cool, but isn't an indication of performance. What did you mod on the airbox? I ran an open-element setup a long time ago, but it ended up that it just sucked lots of hot air which was a net performance reducer. Even with the stock air boxes, it is CRITICAL to have the little snorkel tube which connects the box inlet to the baffle behind the headlight. I logged with and without that, and there was up to a 30°F difference in temperature. You definitely want to get the coolest air possible.
- Camshaft sprockets should move back to stock nominal position. While playing with the cam timing will produce very noticeable effects, the car is not going to run optimally. Cam timing interacts VERY strongly with the fuel and ignition timing programming, and unless you got on a dyno to make a custom chip for this configuration, it is lowering overall performance. You effectively make a trade where you get N more power in one RPM range, while losing N+1 power in the other ranges.
- Which chip are you running?
Anyway, don't take the above as me crapping on your efforts. I've "been there done that" with just about every single bolt-on for the M42 over the last 20 years, and honestly it is almost all not worth it.
The ONLY simple bolt-on mods which will really improve an internally stock M42 are:
- Lightweight flywheel (my current recommendation is the RHD 7.4lb flywheel + M20 clutch)
- A chip from a reputable tuner (Sssquid Tuning is my recommendation)
- Wavetrac or Torsen differential. Both are torque-biasing gear-type diffs, not like standard LSDs with fixed bias ratios. A Wavetrac unit has to be custom built and will run more than $2000. A used Torsen unit from a Z3 can usually be found for $300-600. The only drawback with the Torsen is that if you lift a wheel or get a wheel on ice it will behave like an open diff (Wavetrac does not do this as it has a locking mechanism that engages when one wheel loses all traction). The difference in drivabilty and cornering stability is AMAZING compared to an open diff or worn-out LSD.
If you consider cams to be bolt-ons, then you can actually start making a bit more power. I'd guesstimate that you could get to around 160HP at the crank with moderately more aggressive cams and a matching tune. Other than that, the M42 was very well engineered and there really is no easy power to pick up without spending a lot of money on the bottom-end.
Take a look through this catalog. I went with the 2100 Rally build. It's an old PDF and they have improved / changed a lot of things since it was written, but you'll get the idea.
https://metricmechanic.com/wp-content/uploads/2016/07/M42-Engine-Booklet.pdf
Topic: E36 M42 stock exhaust tubing (Read 3350 times)