Deck height to compression ratio

[cstreit]

I cannot seem to find my Bruce Anderson Book to look up the formula for calculating compression ratio...

Can someone either give me that formula or let me know how much dropping the deck height by, say, .005" would affect the compression ratio?

Thanks,

[Bill Verburg]

Anderson's formula
cr = (swept volume + deck height volume + cylinder head volume - piston dome volume) / (deck height volume + cylinder head volume - piston dome volume)

Seems like it would be very difficult to measure everything precisely enough to be meaningfull.

[tryan]

hardflx had this link on the 914 board.

i loaned ba's book out also

[cstreit]

Tryan,

You da man... That worked perfectly... Since I didn't have exact head volume, I entered in the specs from the original motor and solved for CC's.... Got just what I expected... Then started playing with the deck height...

So here's what I got.

For every .1mm increase in deck height, the compression ration drops around .1 (allowing for variation in accurate head cc measurements this can vary more or less)

So for my heads, a 1.15 mm deck height would result in about 10:1 but a particular deck height would have to vary by more than about .15mm before the compression ratio changed significantly. I have some slight variance in my deck height from side to side because of the crank varaince (well within factory tolerance) and just wanted to make sure that it wouldn't create off balance compression from opposing cylinders and cause rougher running...

[tryan]

i can take no credit. hardflex listed the link.

solving for x is my only talent. i might get a job someday.

some one also posted that the piston rocking in their bores might 'off' the deck height reading. search for the posting, i think john ' the real man' walker had some insight.

is this for the 930 or the rs?

[cstreit]

Yep... I'm taking the average of the two measurements to account for the piston shifting in the bore... So far I show a slight variance in the heights between adjoining pistons, but no more than the factory tolerances for crank and/or rod variance.

This calculation shows me also that the differance in the compression ratio's between these pistons will not be significant which is ultimately what I was concerened about.