m20 rod/crank/piston/head combinations for stroker

[Mops]

hello dudes.
Can somebody confirm that I'm on the right track ?

CC = combustion chamber volume
CR=compression ratio
sweep = bore*stroke

all calculations for single cylinder.

now, on stock m20b25 euro with 9.7:1 compression,

CC = sweep/CR-1 = 47.774 cc's

because the crank has extra (6mm/2) = 3mm throw, piston will be 3mm higher on TDC, BUT I'll use eta rods which are 5mm shorter therefore effectively it will be 2mm lower than stock @ TDC, effectively incerasing CC by 2mm*(84/2)^2*PI = 11.083cc's to the total volume of 58.8574cc's

with new crank the sweep volume will be (bore/2)^2*stroke*PI = 448.883 cc's

therefore :
stock euro 9.7 piston with eta rod and eta crank will give CR of

CR = sweep+cc/cc = 8.627

according to my calculations same thing with m20m25 US 8.8 compression piston is CR = 7.9735


so the question is: whether m20b25 euro style piston is better for boosted application (matches the combusion chamber better with it offset dome and stuff) and stock eta piston ?

AFAIK there are three types of (us and uk), all different compression
11:0 (they have 'convex' dome on top), 10.2:1(nearly flat but a little concave dome) and i think 8.8:1 (visible concave dome)

http://www.bmwccn.no/rogaland/Mahle_m20.html

never the less, I'd be very keen to try 8.6:1 in a boosted application... just wondering whether it still will be able to run 1 bar wihtout detonating... i piston has a big domed edge and valve pockets, while eta piston is pretty smooth.... (except for small valve pockets)...

problem is that it is hard to say how big the compression chamber is for eta piston+i head combination... because of different shapes... i know people commonly say it's 7.9:1, but is that a calculated value (how) or measured (how again) ?

bottom line is that I do know that eta pistons work (as prooven by many stroker builds) , BUT the compression is pretty low, and they are not matched to the i head chambers.... I'm also concerned why nobody did that before... if my calculations are right, 9.7:1 i piston seems much better choice for a budget stroker than eta piston.. especially if either m50 84mm crank would be used or decking the head by 1mm...

what do you guys think ?

[Jhonno]

Hi, the c/r figures you've ended up with seem a little low they have been worked out before

[Ant]

i pistons ( 8.8:1 )
Eta crank
Eta/323/320 rods ( same )
885 cylinder head

this will leave the pistons 1.35mm down the bores compared to stock, ( the eta block is 0.5mm taller so add that to calcs if thats the block you want to use )

so given your calcs, as a rough guide the above mix of parts would reult in around 7.7-7.9:1 C/R perfection for boost up and beyond a bar, the stock 8.8:1 is pushing it a bit @ a sustained bar, 0.7 is about the max, otherwise you have to retard the timing so much its almost counter productive.

Using the 2-5 pistons maintains the stock CC shape, always a good plan.


HTH

[fowler]

oi oi me old fruit
whta about using a combination of
m44 pistons
eta 323/320 rods
eta crank
885 cylinder head
what boost roughly ???

[e21Jason]

Hi

I would look at m40/43 pistons as they are 2 valve so the shape is better, the m40 are dished a lot more compared to the m20, I haven't stripped a m43 yet to check the shape

Jason

[Mops]

Thanks fo input guys.
I never realized that eta block is 0.5mm higher...

the seem to be differences between UK and US edition of m20 engines. here in New Zealand you can find both types...

so essentially i got an early 9.7:1 m20 engine, so i'll just rebuild it with eta rods and crank for 8.6:1 (thats i head, i block, eta crank/rods) or on eta block it would come down to: 8.3:1 according to my calculations.... I'm looking for sustainable 1 bar definitely.. but it will be a good setup with big intercooler, efficent turbo 98 octane gas, fewwfolwing exhaust....

is there a rtegister of different engine blocks and their markings ?

[Mops]

i pistons ( 8.8:1 )
Eta crank
Eta/323/320 rods ( same )
885 cylinder head

this will leave the pistons 1.35mm down the bores compared to stock, ( the eta block is 0.5mm taller so add that to calcs if thats the block you want to use )
Using the 2-5 pistons maintains the stock CC shape, always a good plan.
HTH

i got an early i with 9.7:1 compression pistons.
how did you calculated "1.35mm" ?
Damn, if I could buy JUST eta crank around here it would be great... 320 rods are all over junkyards but eta's are hard to come by...
as i said crank i 3mm taller but rod is 5mm shorter therefore piston sits 2mm lower than stock +0.5 if eta block used...

[Ant]

the 1.35mm is from the last 2.7 hybrid I had apart, late 8.8:1 pistons 320 rods, 81mm crank and 2.5 block

[Mops]

the 1.35mm is from the last 2.7 hybrid I had apart, late 8.8:1 pistons 320 rods, 81mm crank and 2.5 block

hello.
I appreciate your input, and i do not want to be rude or argue, but i just do not understand the 1.35mm you mentioned. you say that on the 2.7 stroker (as described abouve) the pistons sit 1.35mm lower than stock... according to my calculations it's 2mm lower than stock.. possibly the block was decked ? how did you measured/calculated it ? I'm not saying that i'm right, but it is simple mathematics and i do not understand that how the calculated vaule is different than measured one ?

clearly, 3mm (extra crank throw) - 5mm (shorter rods) = -2mm (i.e. 2mm lower than stock for any same type of cylinder... but 1.35mm ?

[Ant]

the block may have been decked, TBH all I had the car in for was a new H/G and away she went, but as a N/A engine it was a bit of a low comp slug TBH.

agreed , the Math cant be wrong, but there are variables when dealing with mix and match bottom ends, the main one being the block heights change, and/or decking and head skimming which you are not always aware of.

I only quoted the measured figure as a matter of interest, I didn't understand it @ the time either