Piston Characteristics for Overbored M90...

[mincePy 17]

For context - the M90 engine is a 3453cc variant of the M30 from the late 70's to early 80's with the shorter stroke (84mm) and wider bore (93.4mm) than the standard M30.

Trying to ignore what’s going on with gas prices right now …

I have my M90 engine in at the machine shop being rebuilt. The cylinders are worn to about 93.9mm, and the head is cracked (original M90 head). I already have a B35 head that has tested well, so I am considering going to 94mm bore with the B35 head (if I can track down intake, TBs, crank balancer etc.).

I’ve seen plenty of folks talk about doing this – boring the M90 to 94mm, using VW forged pistons or JE customs. Slapping on the B35 head will lower compression if you don’t go custom pistons – I’ve read compression drops from 9.3:1 to anywhere between 9.0:1 – 7.5:1 ish. I’m fairly open but it does seem that using the B35 head, with the larger valves and better flow, is a good option granted I get custom pistons (I want to keep this NA so not interested in turbo).

Even though this has been done many times before, I can’t really find how folks are determining the shape and/or compression height of what customs pistons should be so that compression isn’t lowered too much. Paul Burke used to make pistons for this exact application, but I understand he’s long out of the game now.

Then again – would I even notice the compression drop, especially with the overbore?

Interested in people’s thoughts…

[BM WORLD 1286]

i used cp pistons they make to order , just tell them what comression you want , what cam /head you are using etc

BTW i have some old m88 pistons if of any use

m88 and m90 blocks almost the same 

[mincePy 17]

Cheers Brent! Cam is something I need to decide on, but was leaning toward stock B35 cam..

I'll look at CP. Are the ones in your picture for your M88? Don't think I have a need for old ones.

[BM WORLD 1286]

1 hour ago, yoshie said:

Cheers Brent! Cam is something I need to decide on, but was leaning toward stock B35 cam..

I'll look at CP. Are the ones in your picture for your M88? Don't think I have a need for old ones.

yea give me a call if you like 

 

[mincePy 17]

Looking like I might need to find a B35 piston and send it off to JE or VAC to get the shape to suit the B35 head.

VAC Motorsports list an off-the-shelf 94mm piston to suit an M30B35, so kind of hoping they can just take that shape and change a few measurements around to form a custom M90 piston..

https://store.vacmotorsports.com/cp-carrillo-forged-piston-set-bmw-m30-p1608.aspx