M20 B20 race engine

[flyingfarrier]

Hi Guys and Girls

Anyone have any cheap mods to extract power from an M20 B20, want to do a couple of races and this is the only engine i have available to use. The regs are open apart from running a single throttle body, and displacement must stay 2.0 or below.

I cannot fit a b25 so don't mention it :0)

I don't want to break the bank and have access to lots of old BMW engines so any interchangeable parts would be welcome.

Many thanks

[Ant]

completely refurbish the head, skim it for more compression, 3 angle valve seats on stock valve sizes, bored out intake with B25 TB( if allowed ? ) and a decent remap should see you a few more HP than stock, the B20 is far from a bad engine design.

given you're location and the heat during the day, watch out for any pre-ignition with a higher CR , it may not be an issue but worth a mention.

remove the head , use plasticicine on one piston crown, fit head with old gasket and belt again and tighten down to 30NM, turn the engine over a couple of times and remove the head again, this will give you some idea of the clearances you have to work with, add a safe margin for heat expansion though !

HTH

[Jhonno]

Headwork/cam/mapping/2.5 throttle body enlarged/325i inlet manifold/decent exhaust manifold/exhaust..

[flyingfarrier]

Spot on cheers guys, am i mistaken in thinking a 325 head would fit?

[ross2009]

No it wouldn't different pistons mate

[HairyScreech]

completely refurbish the head, skim it for more compression, 3 angle valve seats on stock valve sizes, bored out intake with B25 TB( if allowed ? ) and a decent remap should see you a few more HP than stock, the B20 is far from a bad engine design.

given you're location and the heat during the day, watch out for any pre-ignition with a higher CR , it may not be an issue but worth a mention.

remove the head , use plasticicine on one piston crown, fit head with old gasket and belt again and tighten down to 30NM, turn the engine over a couple of times and remove the head again, this will give you some idea of the clearances you have to work with, add a safe margin for heat expansion though !

HTH

pretty much this.

only thing i would add is to use a split rod with 400grit emery cloth to produce a suitable surface finish on the ports. the effects of boundry turbulence can be significant.

and when skimming a head or block make sure to account for rod stretch at high rpm, there's a lot of force going on in an engine and it can see the rods act like a spring and stretch quite a bit.

[Ross-E30MTECHII]

Does the cylinder head switch work the other way round.
a 731 head fit with a 2.5 block, just cause ive been offered a 2.5 block and wondering if i could change it for the 2, in my 320i?

[ross2009]

Does the cylinder head switch work the other way round.
a 731 head fit with a 2.5 block, just cause ive been offered a 2.5 block and wondering if i could change it for the 2, in my 320i?

the 2.5 heads are much better designed you might aswell get a 885 head

[Ross-E30MTECHII]

yer i know, but the head off the engine was fucked so was selling it really cheap,
just wondering if it would work.

[ross2009]

im not sure as i think the 2.0 pistons are different to the 2.5 so i think in theroy no it wont

[Ross-E30MTECHII]

my only worry is the clearance to the valves at TDC?

[vokietis]

just make 2.7 engine using 2.0 block and u r the winner

[Ross-E30MTECHII]

what bore it out and put 2.7 crank and piston in?

[reggid]

An efficient 731/200 head will out do a large port 885 for a 2L

[HairyScreech]

agreed, the 885 head has ports that are borderline too big for a 2.5/2.8, for a 2l its massive.

also the combustion chamber would overhang the bore and lead to some odd geometry and probably gasket sealing issues.

best bet is a properly worked 731 head, even one with multi angle seats and 885 valves and stock ports would be a lot better than a stock 885.

[Ross-E30MTECHII]

Is that setup better because of the small combustion chamber increasing te compression ratio?

[reggid]

Is that setup better because of the small combustion chamber increasing te compression ratio?

CR difference is not a huge difference TBH. Just make sure pistons match head for proper squish

[reggid]

agreed, the 885 head has ports that are borderline too big for a 2.5/2.8, for a 2l its massive.

also the combustion chamber would overhang the bore and lead to some odd geometry and probably gasket sealing issues.

best bet is a properly worked 731 head, even one with multi angle seats and 885 valves and stock ports would be a lot better than a stock 885.

i wouldnt say it's borderline, i would say a definite. The stock 885 will do an est. 250-260hp based on flow numbers alone. It is not that efficient with known issues so with a more efficient shape you can make the flow numbers you need on a much smaller port. There is some neat software that tells you want cross sectional areas you need to reach certain power levels based on a bunch of spec for the motor. Use this for a B25 engine and its ports are way oversized. TBH most havent got funds to develop a proper head so a basic port job with multi angle seat and B25 valves would work pretty good on a 731.

[Silverfang]

Don't forget about the bottom end, a 6kg lightened flywheel will help with revs, being the B20 is an oversquare engine if i recall.

[Ross-E30MTECHII]

What do you mean by "over square" ?

[e301988325i]

over square is piston diameter > stroke
square is piston dia = stroke
under sqaure is piston dia < stroke

[flyingfarrier]

Canned the idea, waiting till the summer and will maybe buy a honda to race, i'm close to finishing my E30 LS1 race car so will concentrate on that for now.


thanks for the info guys. but just wont make anywhere near the power to chase the little V-tec buggers around the track

[leeparkes]

If you had to stay under 2.0 id be swayed into fitting an M42 and tune that.

[mrLEE30]

Canned the idea, waiting till the summer and will maybe buy a honda to race

A honda???!!!! Noooooooooooooooo! Time to move out of Hamed Town mate, or have you been on L's Calpol?

[Tracktoy]

I am in the process of building an m20b20 engine for the production bmw championship. The engine needs to remain stock so no porting, no 325 throttle bodies or inlet valves etc
Boring the engine to a piston size of 80.48 is allowed, I have taken into account a very helpful comment above reffering to rod length etc etc... Luckily I have a few engines so I am hoping to find a good matching set.

If someone could please explain the difference in the m20b20 heads between the years, some say that early 84 (1984) heads perform better in terms of torque etc.

Bullet nosed valve guides......any benefit?

Expansion : given the fact that we are running at high revs especially when downshifting, what would the optimum compression ratio be (without skimming the block...not allowed) be before valve to piston contact?

All comments and thoughts appreciated,

Fine tuning in a one make series that offers little modification is a real challenge

No clever ecu mods allowed unfort Speedtouch

[Steven320]

My m20 engine started his life as a m20b20 but changed it to a b23 high comp with some mods. maby an idea?