Firstfives.org

Next question unraveling the mystery of my e12:

Which exhaust system performs better? 633CSi [6-into-1] or 528i/535i [3-into-1, x2]?

It turns out I have the 633 version (?): 6-into-1 manifold, single 2" pipe back to the Cat, which has 2 outlets, thru a resonator, the muffler & out thru dual tips. Everything downstream from the manifolds looks pretty tired, so I'm open to options, as I prepare to re-assmemble. Basic spec: rebuilt 635 CSi head, euro pistons, e21 intake runners, staying with the L-jet FI. I want to make sure I have an adequate [but not crazy $$$] exhaust system to match. I'm tempted by some Supersprint-knockoff headers on eBay, but that means everything downstream has to change. One possibility would be to cut in a new 2+" cat and stay single pipe all the way out... Thoughts?

79-81 528i and 80-81 633CSi used identical exhausts. Especially if you have a 2.8 I don't think the 2 unconnected manifolds as used on earlier and later M30s are going to get you anything performance wise. IMO the only downside to the one manifold bolted to the other manifold with a single downpipe is the rear manifold has been know to crack. Too much heat I guess. Not common, but not rare either. I have had semi plugged cats cause all sorts of problems, but I see more importance in the intake than the exhaust, so keep that perspective in mind.

Ebay headers. Be careful they're not E28 application. They'll bolt up, but a stock E28 exhaust will hit the idler arm. On headers all bets are off, they might be just fine or unusable.

Hmm. Okay. I was kinda hoping I’d found another interesting anomaly. I’ll check for cracks, but will likely stay the course. If I don't spend all my budget on the engine, there’s certainly more I can fix!

I just read that WA was eliminating tailpipe emissions testing in 2020, so if you wait a year you can delete the cat. This allows for a freer flowing muffler and larger resonator (which I found necessary to moderate the M30 drone).

WA already doesn’t require emissions testing for vehicles older than 25 yrs, so no need to wait!