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Old 11-27-2012, 02:28 PM   #1
kronus
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M90 and M30 hybrid options

(crosspost from here)

I'm taking the early L-Jetronic M90 out of my euro 635csi for a refresh of the internals. I've also got a completely torn-apart M30B35 and a complete B35 harness from an E34 535i in the shop, and have been wondering about clever ways to combine the two to make an engine that is more powerful than either. I don't want to use a B35 crank, though, as the one I have is damaged (the reason the B35 is apart)

From some reading and research I've come up with three alternatives that I'm now deciding between:

B35 cam, front cover and intake manifold, Motronic 1.3 electronics conversion
This would let me keep the M90 head and not lose compression, which is nice, but I don't get the advantages of the B35 head's bigger valves and better port shapes (I'll be using the B35's exhaust manifolds either way, but M90 head + B35 intake manifold = port mismatch)

complete B35 head on M90 block, Motronic 1.3
The main problem with this approach is lost compression, because the B35 head has bigger combustion chambers than the M90 head, as it's designed for domed pistons (B35) rather than flat (M90). However, I get the head's other advantages, including Motronic 1.3, bigger intake valves, better port matching and flow. It is possible to skim the head to bump the compression back up, but that retards the cam timing and needs an adjustable cam gear to fix.

M90 cam and front cover in B35 head, keep everything L-Jetronic
Some people here have mentioned that L-Jet is way more responsive and revvy than Motronic 1.3. I am not sure if that's true or not, but this option is attractive if it's concluded that the conversion to Motronic is not beneficial. I lose compression, but gain valve size and flow. This is not really a great option.


Any thoughts on any of these alternatives are appreciated. Which would you do?
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Old 11-27-2012, 04:11 PM   #2
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Is yours a small or big port M90?

I wouldn't use a B35 manifold on a M90 head, ports are way smaller on M90 head. Use a B34 intake if you keep the M90 head for electronic injectors.
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Old 11-27-2012, 04:29 PM   #3
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It's a '78, so I assume it's small-port, and it already has electronic injectors in the bananafold, it's not carbed. They are old-style injectors, however.
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Old 11-27-2012, 04:39 PM   #4
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This is relevant to my interests.
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Old 11-27-2012, 05:45 PM   #5
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There is nothing wrong with L jet in my book if it runs well. Motronic would improve your fuel mileage but i have never drooled over motronic throttle response on any engine.

The M90 intake works very well and you can substitute E21 bananas for more volume/flow if you desire. The B35 cam is about the best stock cam and i think you can still buy the conversion nut to drive the distributor from MM.

I would try to keep the compression up in the high 9's or so if you can. A standalone ecu controlled M90 would be ideal for your situation.
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Old 11-27-2012, 05:50 PM   #6
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I thought the B35 cam and stock M90 cam were pretty equal, and the B32 cam was a bit hotter?
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Old 11-27-2012, 05:56 PM   #7
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Originally Posted by kronus View Post
I thought the B35 cam and stock M90 cam were pretty equal, and the B32 cam was a bit hotter?


I think you would be spitting hairs with the best of the stock M30 cams. Aftermarket is the way to go for more power. I think L jet is more forgiving of a more aggressive cam than motronic btw. Overlap puts motronics panties in a bunch.
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Old 11-27-2012, 05:59 PM   #8
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Also, the main advantage of the B35 manifold would be the more modern injectors and electronics. I think I'll end up using whatever management came stock for the head casting I end up using, so I guess the concrete question is, stock M90 head + L-Jet or shaved M30B35 head + adjustable cam gear + Motronic 1.3?

I'd venture that a lot of the throttle response comes from the much lighter flywheel. All B35s came with bigass dual-mass flywheels that weigh 20+ lbs, but I think the M90 came with something a lot smaller and more reasonable.
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Old 11-27-2012, 06:07 PM   #9
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Can you cut the B35 head enough to get the cc down to M90 numbers and retain the compression ratio? Remember to check valve to piston clearance, the valves are getting closer to the piston by the same amount you cut (plus the larger B35 valve).

My M90 flys are pretty light, 14 lbs iirc.

I think you are on the right path, i would go all B35 if it will clear and you can get the compression back up. More potential for a better tune.
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Old 11-27-2012, 06:20 PM   #10
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Can you cut the B35 head enough to get the cc down to M90 numbers and retain the compression ratio? Remember to check valve to piston clearance, the valves are getting closer to the piston by the same amount you cut (plus the larger B35 valve).
That's a good question. How would I measure that? Some sort of bolt-it-up-with-clay-and-do-a-rotation to check the clearance, but not sure how to figure out how much to deck the head, and I haven't found anyone who's done it.
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Old 11-27-2012, 06:31 PM   #11
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That's a good question. How would I measure that?
I would work with the heads loose. You will need a burette with some light oil and a small piece of glass or plexiglass. Its pretty simple to do, i'm sure you can find a writeup on cc ing a head online.

Once you know the difference you can calculate how much you will need to cut to remove the unwanted cc's. On a head with a chamber that is basically round this is pretty simple as it's easy calculate the volume of the cylinder of space removed.
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Old 11-27-2012, 06:35 PM   #12
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Great, thanks for the info.
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Old 11-27-2012, 06:48 PM   #13
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awesome: http://www.mye28.com/viewtopic.php?t...ion+calculator
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Old 11-28-2012, 05:12 AM   #14
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Quote:
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Sounds cool, cant seem to open it on my computer?
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Old 11-28-2012, 06:02 AM   #15
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What about the b35 block with the m90 head, m1.3, runners from an e21 and bananafold for cool factor? That will give you higher compression than anything else I believe.

http://www.firstfives.org/faq/e21_in...0_runners.html

EDIT: Ah. I just saw that you said your b35 crank is damaged. Carry on.
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