Hypothetical best possible setup on 87 octane

[ELVA164]

Alright experts,

I want to see what you guys can come up with for this one!

I don't know what made me think of it, but a thought popped into my head recently: What is the best possible setup for an M20 that could run on a stock-type ECU and 87 octane?

"Best" is obviously subjective, so split it into A. Power and B. Drivability. I was thinking along the lines of the "usual" parts used to build different strokers and things, but custom parts would be allowed too. By stock-type ECU, I mean a chip in the stock housing-no standalone. I wouldn't expect there to be a huge amount of lost potential, but I know BMW had to have left something on the table! What say you all? :D

[ELVA164]

You're no fun :(

[TobyB]

Nope. Because I was gonna say

"stock"

I have an entirely stock engine/computer in the PRO3 car, and it
makes best power on 87. But it also shows just a bit of det on a warmer day.
So I run a mix of 87 and 89, and then don't det.

So my wild- assed guess based on a sample of 1 is that the factory tuned the thing
pretty close to the limit for 87 on the race track.

See?

No fun.

t

[ELVA164]

That WAS no fun, haha.

There's obviously more potential there, though.

[TobyB]

My dets don't agree wit yo' fun...

...I haven't done much afr vs power for the M20, but the M10 made best power in
the low 13:1's, and that's where my stock stuff keeps it through most of the power band... I say mine, because others
have had varying experiences with 'I swear it's all stock' motors....

yeah, there might be some 'area under the curve', especially with a bit more cam,
especially overlap (which we can't do)
but 87 octane is pretty prone to detonate... which limits timing and compression,
in turn limiting what we can squeeze out of it.

Again, just a sample of 1.

t

[ELVA164]

Very true.


So what if you increase displacement a bit without hugely changing compression ratio? Like more bore than stroke? How much room does an M20 have for overbore? I'd imagine that would make a difference on its own.

[digger]

Very true.


So what if you increase displacement a bit without hugely changing compression ratio? Like more bore than stroke? How much room does an M20 have for overbore? I'd imagine that would make a difference on its own.

you can overbore the m20 up to 87 mm (so i've heard but 86 mm is more typical maximum) but increasing the bore is bad for detonation as the flame front has further to travel. its a small difference like 1 mm but every little bit helps/hurts...

id think a smaller engine with more revs would be better than a larger one to limit peak cylinder pressures. there are various strategies used to limit risk of detonation with combustion chamber, piston design, swirl etc most builds dont follow these practices but with poor fuel theyd be must do's

id probably runa colder thermostat to keep the head a little cooler and make sure the cooling system is in top shape not sure what temp motorcnic goes into closed loop but youd think an OEM 75C would be fine

[ELVA164]

Interesting. So stay at or near 2.5L, but perhaps switch to heavier-duty internals so the engine lives better at high rpm? Stroking obviously won't facilitate a "revvy" nature. The thermostat change makes sense.

[digger]

Doesnt need to be but if err on the smaller side perhaps 2.7/2.8. Would be my guess. Cam timing plays a role so an adjustable cam gear and modified valve Events can bring power and bleed off compression. This is the MM method and not desirable on an engine with good fuel