How can i lower compression with a m20b23 motor?

Post Thu Jul 13, 2017 9:49 am

Just wondering how i can lower the compression a 83 323 e30 motor without going thicker head gasket or forged pistons? Will a 885 head bolt straight on and lower compression?

Post Thu Jul 13, 2017 4:22 pm

no- what are you trying to achieve?

Post Thu Jul 13, 2017 7:07 pm

Matto323 wrote:

"Hi all. New to e30's. I have a stock 1983 323 and was wondering if the stock 2.3L motor can be turbocharged? I know compression ratios 9.8:1 which is getting up there but theres pleanty of engines running a similar ratio if not higher with a good computer and tune. Can this be done to the 2.3 m20?

Does anybody know what sort of boost they can handle in stock form? Am i correct that it will have a 731 head? Will a 885 head bolt on and maybe lower my compression ratio and any performance when turbocharging?"

Yes, a 323i should have the 731 head. I would guess you would either need a thicker turbo-specific head gasket, or double/triple up with standard 320i/323i ones stacked on top of each other.

Post Fri Jul 14, 2017 11:40 am

you could fit a later set of low comp 320i pistons would be alot cheaper than forged ones

Post Sat Jul 15, 2017 2:12 pm

So what would the low comp 320i piston bring my compression ratio to? Are they a direct fit to the m20b23 motor?

Also is the only difference between hi and low comp m20b25 motors the pistons? If u were to get a hi comp m20b25 can i just throw in the 8.8:1 factory pistons?

Post Sat Jul 15, 2017 2:16 pm

Some info in the Zone Wiki:

http://www.e30zone.net/e30zonewiki/inde ... 20#Pistons

AFAIK, the difference is in the pistons between the various CRs for the M20B25.

Post Sat Jul 15, 2017 2:39 pm

Just doesnt mention about the 2.0 or 2.3L piston :/. I know the bore size is the same with the 2.0 and 2.3 but thats about it :/

Post Sun Jul 16, 2017 6:52 pm

the 323i is essential a stroked 2litre, using a longer throw crank to increase the cc. Rods are the same- cams slightly different

The early b20 pistons have a small bulge in the centre to increase the comp ratio, with the later pistons having a smaller bulge therefore lowering the ratio.

I think the pistons are otherwise identical and would drop the ratio to 8.8:1 as they did in the late b20

https://en.wikipedia.org/wiki/BMW_M20

or just skim the current pistons lump off