M20b23 crank In a stroker? Can I?

[Pdxseth]

My original plan is to build the standard 2.7i
(Seta bottom, 885 head "I" top)

Could I bump up compression using a b23 crank?

From what I've seen you can't use "i" pistons but what about seta pistons?

I'd like to use
Eta block
B23 crank
What rods???
Seta pistons
885 head

I have an m20
B23
B25
B27

All for parts how do I get the most bang id like higher compression because I'd prefer to keep this car n/a


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[Pdxseth]

I'd also like to use a 272 cam


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[Madhatter]

you know you are only going to result in a 100cc increase in capacity over a 2.5L though right? it has less stroke than the eta crank.

[Pdxseth]

M20b23 crank In a stroker? Can I?

To be honest I'm having a hard time understanding this stuff. Math is not my strong point.

My theory is that the b23 crank may help raise the compression wich could result in more power right?

I'm just curious if I can make more n/a power by using the b23 crank instead of the eta crank.


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[Madhatter]

you'll make more power with the eta crank. If you use the 2.3L crank you'll end up with a 2.6L engine.

[Pdxseth]

So the b23 crank won't raise the compression enough to compensate for the 100cc loss in displacement?


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[Pdxseth]

So it's starting to look like 2.7i is a stupid motor.

I've been doing a lot of reading on e30tech and it seams that a b23 crank in an m20b25 is way better.

And a 731 head on a seta is way better. So instead of building on crappy under engineered back yard motor I'm going to build 2 (since I have all the parts) I'll put the torqued 2.7 in my e28 and the r3v happy 2.6 in my e30 :)


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[digger]

So it's starting to look like 2.7i is a stupid motor.

I've been doing a lot of reading on e30tech and it seams that a b23 crank in an m20b25 is way better.

And a 731 head on a seta is way better. So instead of building on crappy under engineered back yard motor I'm going to build 2 (since I have all the parts) I'll put the torqued 2.7 in my e28 and the r3v happy 2.6 in my e30 :)


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dont waste your time with the b23 crank imo.

also seta pistons will probably smash into the 731 head.....the seta piston is similar to the i piston with angled dome for the squish band where as the 731 is for flat top pistons esentially

[Pdxseth]

Alpina and hartge did it with great results!


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[digger]

Alpina and hartge did it with great results!


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using that analogy the 2.7i is great as well since they made 210bhp out of a 2.7L. all you need is the 81mm crank thats it.

you dont need custom piston pistons to the right compression height or ported cylinder heads, bigger camshafts, headers and new exhaust these are overrated

[digger]

enough of me being a smartass so seriously they used special pistons to get it work. they also used modified cylinder heads and camshafts, full exhasust and proper tune etc etc. might aswell do it to get the biggest crank you have to work. less than 2mm extra stroke does little for you

[LJ851]

I remember talking to Pete McHenry about this build way back when. I think it went B25 block, 885 head, B23 crank, B23 rods, and B25 "i" pistons machined down to compensate for the longer stroke.

[digger]

I remember talking to Pete McHenry about this build way back when. I think it went B25 block, 885 head, B23 crank, B23 rods, and B25 "i" pistons machined down to compensate for the longer stroke.

m52 crank will yield much better results. peerless on e30tech did this b23 crank a few years back. the engine never really made anymore power than the B25 motor and same with torque from what i remember. there is any value in the effort IMO unless you have a displacement rule for a race class.

[LJ851]

m52 crank will yield much better results. peerless on e30tech did this b23 crank a few years back. the engine never really made anymore power than the B25 motor and same with torque from what i remember. there is any value in the effort IMO unless you have a displacement rule for a race class.

I agree there is no point in building it.

[Madhatter]

Hartge used the 731 heads on H23's when they built 2.6L motors, but they modified the head a HUGE amount to fit the larger valves, not just the pistons. When you look at the closed chamber shape of the 731 head you can see it needed modification to fit the domed style pistons the 325i used too.

Modified chamber of the 731 looked a lot like this.





Compare it with the shape of the stock 731 head



Quite a bit different and a lot of material has been removed.

For each full point increase in compression ratio, the added power developed is roughly 3-4%, until you get above 12:1 then it's reduced even further. Obviously camshafts make a difference, but all things remaining the same, the increase in comp ratio doesn't result in massive gains in power by itself. As Digger said, you need to take advantage of a whole host of other things to take advantage of it. It kinda sounds like you are trying to do it on a budget, so you are better to stick with known combinations that work.

If you want high comp, best thing would be to use your eta bottom end and find a set of early eta pistons with the 10:1 comp ratio. Anything early, so prior to about 1985 from a 5 series should find a set of pistons with a domed crown in the centre.