Exhaust: 633CSi v 528i

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Sharky1980
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Joined: Thu Mar 28, 2019 1:46 pm
Location: Seattle, WA

Exhaust: 633CSi v 528i

Post by Sharky1980 » Tue Jul 16, 2019 8:03 pm

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?
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- 1980 528i
- 1983 944

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Mike W.
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Location: Sonoma County

Re: Exhaust: 633CSi v 528i

Post by Mike W. » Tue Jul 16, 2019 11:49 pm

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.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

Sharky1980
Posts: 10
Joined: Thu Mar 28, 2019 1:46 pm
Location: Seattle, WA

Re: Exhaust: 633CSi v 528i

Post by Sharky1980 » Wed Jul 17, 2019 12:39 am

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!
- 1980 528i
- 1983 944

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528i-1981
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Location: Boston North Shore, MA

Re: Exhaust: 633CSi v 528i

Post by 528i-1981 » Wed Jul 17, 2019 4:41 am

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).
(oo=00=oo) Eric
1981 528i Manual

Sharky1980
Posts: 10
Joined: Thu Mar 28, 2019 1:46 pm
Location: Seattle, WA

Re: Exhaust: 633CSi v 528i

Post by Sharky1980 » Wed Jul 17, 2019 1:45 pm

WA already doesn’t require emissions testing for vehicles older than 25 yrs, so no need to wait! :D
- 1980 528i
- 1983 944

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