What happens if i put a b25 crank in a b23?

Mon Jul 31, 2017 2:16 am

Hey all im not the greatest when working out compression ratios and cc's etc but im looking at lowering compression on a m20b23 motor. I noticed the b25 motor has a slightly shorter stroke then the b23. Im not sure if its in the pistons, rods or crank but i was wondering what the cc's and compression ratio would be if i used a stock b25 crank in a stock b23. Does anybody know?

Mon Jul 31, 2017 9:24 am

The 325i crank would give a cubic capacity of 2.262 litres (2262cc) in an M20B20/23 block, compared to the 2316cc of an M20B23 (i.e., 54cc less).

Not sure what the CR would be - somewhat lower than the M20B23 one, obviously, seeing as the M20B25 crank stroke is 75mm compared to the M20B23's 76.8mm, a 1.8mm shorter stroke.

Mon Jul 31, 2017 9:30 am

2262cc, approx 8.8:1 CR

Mon Jul 31, 2017 9:31 am

what you really want is the b23 crank in the b25 block, gives a cc of 2554 like the early hartge h26 & early alpina c1 (iirc).

Mon Jul 31, 2017 9:53 am

Yes, but the OP is aiming for a turbo conversion on his M20B23.

Mon Jul 31, 2017 10:02 am

Speedtouch wrote:Yes, but the OP is aiming for a turbo conversion on his M20B23.

oops didn't see that bit

will teach me not to skim read

Mon Jul 31, 2017 10:18 am

just tune it properly or use a thicker gasket