m20 blocks? + recipe for stroker

[Andre3000 55]

Im contemplating using a low kms b20 block for my engine build, what are the differences between the b20 and the b25 block? I know the bore on the b20 is 80mm, and the b25 is 84.

Atm im thinking of boring either a b25 or b20 to 85mm and using a set of vac's custom pistons, adding some high performance head gear (vac tier 3 performance head) + mls gasket and arp studs (just for peace of mind) + port matched itb intake.

Anyone got any useful info on this setup? And is there anything else i should be looking into?

edit: forgot to mention using a m52 crankshaft and b25 rods

Edited June 8, 2009 by André3000

[CamB 48]

Random thoughts:

1) I got JE Pistons (custom) ordered locally, and I am sure CP is also able to be ordered from NZ. The price was only slightly more expensive (ok, maybe 15-20%, but same price as VAC which isn't exactly cheap) than ordering from the USA, and I liked the idea of having someone local (in my case, the place that prepped the block) source the piston - in case of problems.

2) I don't know which block is best, but unless you are told otherwise I don't see why a B20 block couldn't be used. I do know that if you are getting custom pistons, you have the option of using the longer rods from the M20B25, which is good because it has a better rod ratio than when you use B27 crank and rod. You get the custom pistons with a lower compression height. Shouldn't affect the price going custom?

3) The VAC stage 3 head looks very expensive. I am sure you can have someone build a head that can flow well locally for cheaper. Unless you run a monster cam, I'm not sure the stage 3, with bigger valves, offers any benefits over a stage 2 (or a locally upgraded head). If you want to run a monster cam, and revs to match... you start getting into the territory of needing the billet rockers which are unbelievably expensive. Better to keep the revs down, as the cost goes up exponentially.

ITBs would sound wicked, so not going to try talk you out of them.

Edited June 8, 2009 by CamB

[Driftit 2079]

Why not go 2.8ltr. Using the E36 2.8 crank, m20B25 pistons and m20B20 rods.

I have it all and am thinking of selling.

[Guest FrantiC]

Read up on an MLS for you get one... Seem like more trouble than they are worth TBH.

I would start with a b25 block, Less work boring from 84 to 85mm than from 80 to 85mm..

[Andre3000 55]

Why not go 2.8ltr. Using the E36 2.8 crank, m20B25 pistons and m20B20 rods.

I have it all and am thinking of selling.

You have pm

[|ncary 0]

driftit's recipie is perfect. I was about to do the same set-up a few months ago with a turbo... Decided on suspension though.

Makes a torquey beast ha.

[CamB 48]

Wouldn't that have a crappy compression ratio unless you decked the block etc significantly?

[|ncary 0]

B25 Piston with B20 rod and the 2.8 crank:

Whole thing:

http://www.msportster.co.uk/faq.asp?FAQ=25

[bravo 35]

do a search for 2.7 build here. There's a couple of threads where Gus and I have posted up the entire 2.7 article from Performance BMW mag complete with run downs on the various blocks, heads, cranks, rods and pistons that can be used.

[CamB 48]

Ooops, I did the maths in my head wrong. I thought 1mm down the bore. It's "only" 0.5mm. If my spreadsheet is correct approx 0.3-0.4:1 reduction in c/r.

[e30plz 1]

There's a good sticky'd thread on E30TECH all about M20 Strokers.

[mops 4]

Ooops, I did the maths in my head wrong. I thought 1mm down the bore. It's "only" 0.5mm. If my spreadsheet is correct approx 0.3-0.4:1 reduction in c/r.

actually there's more swept volume, so compression will be higher.

crank is 84mm instead of 75mm stock 325i, which puts piston 4.5mm (9mm/2) higher.

however m20b20 rods=eta rod=130mm, which is 5mm shorter than oem 325i, so all in all piston will be 0.5mm lower then stock. But, the displacement volume till be higher.

and that ends up at ~9.1:1 for oem 8.7 compression piston or 10.2 if you have oem high compression (9.7) pistons.