Combustion Chamber size B34 vs B35

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Rick Eickhoff
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Combustion Chamber size B34 vs B35

Post by Rick Eickhoff »

Has anyone cc the combustion chamber of the B34 and B35 heads. Was talking with and well respected M30 engine builder about this, because I want to increase the compression ratio without going into the bottom end. The thought process is to use a late model euro head that has a smaller combustion chamber to increase the CR to 9.8:1. It’s my understanding the late model B34 have the same large valves as the B35, but smaller intake and exhaust ports. The head will be ported to align with the B35 intake manifold and exhaust headers. Will have a new 284/280 cam and a complete head rebuild.

Sounds like a good idea to raise the CR - wanted to get the wisdom of the engine guru’s here.
Rick Eickhoff
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1988 635csi
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Re: Combustion Chamber size B34 vs B35

Post by GRNSHRK »

Rick, you might want to try talking to the guys at Metric Mechanic, long time engine builders that have tried all sorts of tricks :-"

https://metricmechanic.com/

Sounds like you're getting quite deep into this, careful with combustion chamber and valve sizes :shock:
:mrgreen:
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Re: Combustion Chamber size B34 vs B35

Post by bpoliakoff »

What about milling the head? If that can be done and give you the needed valve clearances use the B35 head. Your engine builder can figure that out. Back in the day, building engines in our driveways, we used to put clay on a piston , put the head on with a few bolts and turn the motor over.We could then see if enough valve interference was there. If it wasn't we used to use a fly cuter, like a giant drill bit, inserted from the combustion chamber through the valve guides and turn it with a drill until we cut enough relief in the piston for clearance. Built plenty of nasty motors that way in back yards and they went plenty strong and stayed together!
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
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Rick Eickhoff
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Location: Philadelphia

Re: Combustion Chamber size B34 vs B35

Post by Rick Eickhoff »

Found some additional information off the web - by Chris Graff

B34 combustion chamber volume: 58 cubic centimeters
B35 combustion chamber volume: 65 cubic centimeters
B35 camshaft: 264 degree. Same as the B34 high compression (euro) engine.
B35 inlet valve is 47mm instead of the B34's 46mm.
Inlet port size is much larger on the B35, with better line of sight.

Hence, bolting the B34 head on the B35 lower block with raise the compression ratio to 9:7:1

With some porting of the intake and exhaust ports to the B34 head it should be a good combination to raise compression of the B35 in addition to the 284/280 cam.

Will be interesting... Will give you some updates as it project moves along.
Rick Eickhoff
SE Pennsylvania
1988 635csi
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Re: Combustion Chamber size B34 vs B35

Post by bpoliakoff »

Since the valves are that close in size, get the heads to a specialist and let him do his magic on it. The guys that are very sharp on headwork will match cc and flow rates so if that is the way you want to go pick up as much as you can from the head.There still should be some room to mill the head and let you pick up a little CR there. When you do the headwork I am sure the head will be milled so pick up some CR that way. Mill as much as you can as I am sure you will not be doing another valve job on the car and if so heads are cheap
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.
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