M20 Stroker FAQ for newbs such as myself?

[SA E30]

There is no point in revving 2 8k if u power is falling off b4 then... as a M20 head doesn't flow well enough at those RPM's

[digger]

There is no point in revving 2 8k if u power is falling off b4 then... as a M20 head doesn't flow well enough at those RPM's

Yes, but i don't think it is the head alone! The intake manifold / head combo is not condussive for such rpms.

I haven't seen an M20 dyno plot with peak power over 7k (stock is around 6k) and the power drops off pretty quick at such rpms after the peak. Even the alpina and hartge are around 5800. I am sure it can be done but low performance and driveability will suffer somewhat.

[SA E30]

It is 2 an extent the manifold aswell, but more so the head.

I mean, u look at a M20 with ITB's ... they still not making power up 2 7000rpm, the power falls of at like 6800rpm or so... well this is with E34 M5 ITB's.

[Brian D]

most people who go with custom pistons go with the 85mm bore (who knows why, when 86 costs the same).

both 85mm and 86mm bores require custom pistons and cost about the same, so I don't get why everyone in the states is so hot for the 85mm overbore.

Nando, did your machine shop check the cylinder wall thickness of your block after the overbore to 86mm? I'm new to m20's, so I don't know if a 86mm bore makes the walls a little thin. If the thickness is good this will be the way I go as well. Thanks!

[StereoInstaller1]

It is 2 an extent the manifold aswell, but more so the head.

I mean, u look at a M20 with ITB's ... they still not making power up 2 7000rpm, the power falls of at like 6800rpm or so... well this is with E34 M5 ITB's.

I think most of the issue is the age of the technology that the stuff we are talking about happened. Think about it, most of this is like 1988 stuff, or even earlier.

Much has been learned in the last 20 years, and we are talking about the "cast offs" from way back.

Piston ring technology was crap in those days, and BMW was looking to make power down low, where piston speeds were within spec on the available rings and bearings of the day.

Plus, I don't know about you guys, but I never want to have to spin my motor up to 9,000 RPM to make power.

[Brian D]

In the end I think the most important thing 2 remember when choosing ur combination is to make sure your rod ratio is within limits (between 1.6 and 1.8, with 1.6 geared more towards a performance engine... )

Actually the higher the ratio the "better", although the differences for a street engine are almost negligible, IMO. There's a huge amount of debate about rod/stroke ratio on the intarweb, but no one seems to be able to consistently show an improvement on a dyno. 1.6:1 is perfectly acceptable.

[Brian D]

Plus, I don't know about you guys, but I never want to have to spin my motor up to 9,000 RPM to make power.

Amen, brother! I'll take a nice fat curve from 3000 to 6000 any day over an engine that lays flat until 7000-for a street car anyways!

[nando]

Nando, did your machine shop check the cylinder wall thickness of your block after the overbore to 86mm? I'm new to m20's, so I don't know if a 86mm bore makes the walls a little thin. If the thickness is good this will be the way I go as well. Thanks!

no, I didn't worry about it. there's plenty of material for an 86mm bore.

[DrMcDave]

that MM 3.2 is so tempting- anyone heard or driven one of those? 260hp from an NA m20 w/ no standalone and no ITB's is pretty much crazy.

nando did u dyno that thing yet?

[Adrian_Visser]

can you imagine what your piston speeds would be though? i cant imagine that engine lasting very long before a rebuild. still that sounds pretty sweet. does that use the 86mm pistons then?

[SA E30]

Yeah, I agree, Dyno queens live for that nice big peak power... real race cars have a nice big area under the torgue curve (lol, did that make sense ? )

AFAIK, the biggest u can go on the cylinde is 86.2...

I'm actually in the process of rebuilding my motor (well not me this time, I didn't have inclination 2 do it, but changing the rods to M40B18 ones which r 138mm over 135mm... So that I can bump up the CR some more, on the S50B30Euro pistons... the tops will be cut down now right 2a flat top piston.

Almost every single stroker I know of here in South Africa, they run 86mm pistons. Weather it be on 81mm, 84mm, or 86mm throw cranks.

The new trend here is to chuck in a M54B30 crank, M20B20 rods, 86mm pistons...

Does anyone know what length a M20B20 and M20B23 rod length, and also M20B32 stroke length is ?

[digger]

can you imagine what your piston speeds would be though? i cant imagine that engine lasting very long before a rebuild. still that sounds pretty sweet. does that use the 86mm pistons then?

no offense but i think they know a bit more than you about M20's and how to make them reliable. You should take a look at their website

[pdx 528e]

no offense but i think they know a bit more than you about M20's and how to make them reliable. You should take a look at their website

Has anyone verified those metric mechanic HP numbers on a dyno? I'm guessing they are a bit optimistic.

[DrMcDave]

i dunno they do seem to be very high. I nearly build a 3.2 last year but the cost kept going up and up and it eventually went out of my price range at the time. Whether they are right or not, they are very high quality engines and i'd take one over a lot of other options.

[nando]

no dyno yet.. will do it eventually, but right now I'm just having fun driving my car. I don't want to work on it again for a very long time. ;)

Has anyone verified those metric mechanic HP numbers on a dyno? I'm guessing they are a bit optimistic.

I don't think they are that optimistic.. keep in mind their headwork and piston design is unique to their motors. they CNC the ports on their head and have done extensive flowbenching to get them to flow better than stock. also, that 260hp is probably with the 11.5:1 compression "rallye" pistons. they only quote 205hp at the crank on the 2900, that's not unreasonable at all for a 2.8l M20 - jordan had almost that much at the wheels with his 2.8, and he didn't have the benefits of the 86mm bore / high sillicone content pistons.

as far as piston speeds, they'd be no different than any of the other larger BMW motors. both the S52 and S54 use 135mm rods with that huge stroke of theirs, and they both work fine.. keep in mind the forged, lightweight components that were designed to work together for those particular motors.

I think if you want a 3.2 and you have the money, buying an MM engine outright is the way to go.