E30 Zone

i was just idly wondering about this today (as a substitute for working)...

the bore x stroke of a 2.0 litre m50 engine is 80x66 mm

the dimensions of the 2.5 version are 84x75 mm (all same values as m20)

so, if i wanted to build a small, short-stroke engine, is it feasible to use a 2.5 block and pistons with a 2 litre crank to give 2195cc? which rods would be needed, and would the block/pistons need machining?

i reckon based on the 24v M50 this could be really nice with some bike TBs, a decent exhaust manifold, a high comp ratio, MegaSquirt and a sky high rev limit. not looking for huge power or a replacement for the 335, just something that sounds cool and revs really high.

am i dreaming?

thought about this before with a shorter throw crank in a 325 block with 325 pistons i think your gona need 140mm rods, stroke will be alot less then bore so good for revving, need to find a link to that US guys BMW engine chart to see if you can find some M50 stylr 140 mm rods and then you have very solid bottom end, add a block girdle in there, some billet rockers, 294 cam, uprated springs and some weldimg over the top of cam and your good for 8000 rpm+ i reakon

didnt read that you meant doing this to a m50, i would speak to PPF about this, not sure what m50 valve train and head is good for, will no doubt be alot better then what stock m20 head can do, are they similar to s50's?

yeah i was thinking 24v, i don't think it's worth doing to an m20 due to inadequate breathing.

i have a feeling m20/50 rods are interchangeable which might open up some options

yeah bearings in m20 and m50 rods are the same but you get 130, 135 and 140 mm rods in both types, m50 rods are slightly lighter but also abit stronger. i think m20's valves are fine if you look at whats been acheived with them

WillG wrote:i think m20's valves are fine if you look at whats been acheived with them

i'm thinking about a relatively cheap NA build, and in my opinion there's no comparison between m20 and m50


260bhp from an m20 2.8 (Scott's) with absolutely everything done and a fortune spent, mostly on valve and port work, vs around 220-230bhp from a standard 328i lump with just the inlet mani changed for a 2.5 one.

You'd be stuffed for longer rods as the 2 litre M50 already uses these! I stripped a brand new M52 a couple of years ago - I bought it from an autojumble for twenty quid. The block was marked '28', i.e a 2.8 litre.

WOOHOO!!

I wondered what the four small and rather crude cut outs around the top of each cylinder liner were. I measured the bore. 80mm. The penny dropped.

It was a 2 litre wrongly marked up as a 2.8. The BMW mechanic doing a Nikasil block swap bolted on the 2.8 head, fired it up to the sound of 24 valves clouting the bores.

I robbed a few bits from it and threw it away is disgust.

ok

so i take it that means custom rods (i.e. i can't be bothered)?

or is it feasible to lop some off the top of the block and use a vernier pulley?

maybe some other m20/m50/m52 pistons will have a taller crown. all the larger sizes share the 2.5 bore size.

your mind works in strange ways mr unna!

no i was wrong i think you just use crank and 140 mm rods from 320 and rest b25 and that works, you dont need longer rods then 140mm, for 2.0 to 2.2 is just from 80mm bore to 84

m42 could donate ome 140mm rods, but the rod/stroke ratio would end up weird

best oddball rods are the M21, same as 320/323/eta but the wristpin holes are 2x the size, offset bronze bushes and the worlds your oyster

The 2 litre already has 140mm rods! For some reason it has mega long rods and weird pistons with most of the skirt cut away and the wrist pin right at the top under the rings. You can't shave an M50 block because the timing chain tensioner doesn't have aenough travel. So you'd be better off keeping it 1990cc.

You may be better with an M20 tbh - you can get 180+ bhp from a 2 litre Pinto and that's an old 2v boat anchor albeit with very big cylinders (and thus room for big bin lid valves. You could get some big valves in there and a racy cam as well as your TB's.
I'll measure up some 320i/525i pistons next week. It may be that you could use a 320i crank with 135mm rods and flat top 525e slugs and machine the valve cut outs nice and deep so you can use a maniac cam and really shave the head. 100 bhp per litre??

I doubt it but I'm sure you'd get close. I think the works E46 320i racers were doing 250 bhp using a 2 litre M54.

the valves are already pretty big the intake valve seat is something like 2mm from edge of combustion chamber anyway, the valves have flowed enuff for 800hp on a PPF built e21, that same engine had a high rev limit aswell not sure exactly what but it was the one with all the block girdle, welded head, billet rockers, custom intake with massive TB etc, dont think they used bigger valves. isnt the 4 valves per cylinder more about getting a more even burn then increasing flow? i mean m50 ports arent much bigger anyway are they?

I don't think that a stock M50 hydrolyc lifters could handle 8K+ RPM.