Need help on calc CR value 2.5 m20b25/m20b28

[ariel]

Hi guys.
I cant get this right.

Stock high comp engine m20b25 with 135mm con rods is CR= 9.8:1

What if i change con rods to 130mm what CR ratio i will get?
or
If i use m52b28 crankshaft with 130mm con rods?
again what Compression Ratio will get.

ANYONE PLEASE HELP

[S4Driver]

This might help> http://www.csgnetwork.com/compcalc.html

or just Google comp ratio calculator

[GeoffBob]

Stock high comp engine m20b25 with 135mm con rods is CR= 9.8:1

9.7:1 I think for the early high CR motor with 1.75mm compressed thickness gasket.

What if i change con rods to 130mm what CR ratio i will get?

Simply swapping out 135mm rods for 130mm rods will drop the static CR to 6.5:1, and given that piston will be riding 5mm lower relative to bore I am not sure that this is recommended (or what purpose a 6.5:1 CR would serve)

If i use m52b28 crankshaft with 130mm con rods?
again what Compression Ratio will get.

However, with an 84mm stroke crank from an M52B28 with 130mm rods the static CR will be raised up to 10.2:1.

Despite the fact that the piston crown sits 0.5mm lower than the M20B25 (5mm longer rod with 2x4.5mm increased stroke) the CR is higher due to the increased volume of the cylinder at BDC. The engines volume is effectively increased to 2800cc.

Basically, what you are proposing is line-6 of this table here: http://www.e30zone.net/e30zonewiki/inde ... roker_Info

Info on the 2800cc M20B25 conversion here: http://www.e30zone.net/e30zonewiki/inde ... ding_a_2.8

A word of caution though, the CR values presented on the Wiki are not as accurate as they could be due to the fact that deck-height and stack-height (on the stock motor) are assumed the same value. They are not in practice.

[HairyScreech]

2.5 and 2.8 low comp pistons and high comp pistons, also a 0.5mm skim accounted for if using 130mm rods and wanting stock piston placement.