M20 build

[E30-M3]

Hey guys looking for a bit of guidance on an M20 build.

How does this look

Rebuilt Stock 885 Head (decked 0.5mm for compression bump)
New Schrick 272/272
New Stock Head Gasket
New Stock Mahle Pistons 84.5mm Bore (+0.5mm)

All this mated to a Stock 2.5L bottom end should give me a compression ratio of 9.21:1 according to the M20 CR Calculator.

on 91 octane pump gas with a MarkD chip for #17 or #19 injectors.

1. Does this all make sense or am I missing something?
2. I know I will have to check P/V clearances physically, however should I be anticipating any issues here?
3. The MarkD chip, is that workable with the bump in compression and the addition of the 272 cam?

Thanks for any info you guys might have.

[redlightpete]

If you're going to the trouble to build an m20, why not make it a stroker? It isn't much more effort for a more significant gain.

[digger]

if you're going to the trouble to build an m20, why not make it a stroker? It isn't much more effort for a more significant gain.

+1

[E30-M3]

Main reason being that I already have two toys in the M3 and M5, what I need out of the 325 is a cheap and reliable daily driver. No forged pistons, no maf or standalone management, just something that gets me from A to B no questions asked. Also I'm in a time crunch and spending time to source the stroker parts is not feasible right now.

Back to the questions, I'm wondering if the 0.5mm decking is going to be too much now, I could go 0.2mm and end up with a compression ratio around stock, yet have a little more piston to valve clearance.

Thoughts?

[digger]

can you get away with just a basic freshen with existing pistons?

[redlightpete]

You could probably round up a good eta crank and rods here on R3V in a week. Then just deck the block to the desired compression ratio. You'd get more piston to valve clearance for a given compression ratio with the longer stroke.

That said, my stroker project is still in process, but that's mainly because I'm wicked slow and I'm still driving the car to work.

[Sirotto]

Or just look for a seta bottom end. That is teh best budget stroker

[Scallywagon]

Your setup sounds fine, its basically stock.

I don't understand why you would spring for those pistons instead of a budget stroker of some sort, like the rest above mentioned.

Digger brings the most clout to this thread.

[E30-M3]

I got a great deal on the pistons thats the reason why I went with them, plus the fact I need something that is rock solid reliable.

I understand it makes sense for most to get a stroker going but it doesn't really fit my goals listed above.

I appreciate the input, especially from Digger, I know of his wealth of m20 knowledge.

[digger]

If you are destined for an overbored M20B25 then the only main issue is how much you can bump the CR by running a thinner gasket or slight skim of the head and still have enough Piston to Valve clearance and piston to head clearance.
If you needed you could deepen the pockets a little there would be plenty of meat in the pistons but it may not be needed with a 272 cam and you may not like the idea of machining the new pistons.
Then set the piston to head squish clearance at 0.040" and be done, it will be what it is.
i wouldn't worry too much about decking the head too much a small change in CR its not gonna make much difference but smashing valves will. the squish clearance is worth getting correct

[Cabriolet]

you are doing a lot lot lot of work for no real benefit.
for a DD car i would get a 272/268 cam, 91 chip and refresh the rest of the engine. new bearings and rings makes more difference than you can imagine. bumping up the compression wont really do anything for you.

[digger]

you are doing a lot lot lot of work for no real benefit.
for a DD car i would get a 272/268 cam, 91 chip and refresh the rest of the engine. new bearings and rings makes more difference than you can imagine. bumping up the compression wont really do anything for you.

seriously there would next to no difference to a 272 and i dont think a split cam is what the M20 needs.

yes its not going to be an earth shattering engine but he made it clear that was not the intended purpose.

[Cabriolet]

seriously there would next to no difference to a 272 and i dont think a split cam is what the M20 needs.

yes its not going to be an earth shattering engine but he made it clear that was not the intended purpose.

the split cam will give it a smooth idle and lower torque band - both these things will make it better for DDing in traffic. i hate my M3 in traffic, having to wind it so high to hit the power bands sucks. and i've driven many rebuilt m20s w/ and w/o cams and for DDing a fully refreshed m20b25 is great fun and plenty of power.

[digger]

then use a 268/268 or a 272/272 advanced slightly both will idle nice enough and have smilar torque.....a split cam does not match the m20 head characteristics stock the exhaust port is not good enough. arguably a stock cam is the best for idle and lowend with such low CR but there has to be a compromise if you want a performance increase at all

[e21jps]

you dont have to worry about the idle with either 268 or 272 cam, it will be fine. I think the combo is perfect for a daily, ive seen basically the same setup put into a stock b23 and it went much better than expected so in a 2.5 it should pull really well.

I wouldnt get carried away with too much messing around like trying to get a tad more compression, it wont make any difference unless you make big changes. Just put a 30 degree back cut on the valves and keep your piston to head clearance down to 040" max, everything else will be perfect. Check valve to piston but should be no problem i would guess.