7M Comp Ratio Measurements

[BJ91T]

I just picked up a 7M that was rebuilt by the previous owner, (yes I know the guy he sent it to a good shop etc..). now my only problem is the motor had a 3mm custom cometic MHG on it when I got it the previous owner said he did that to run pretty crazy boost levels ... I have the Head and block seperated but I cannot measure the block for deck height with the main caps installed. Is there another way to figure out comp ratio without measuring from the main bearing saddle to the deck. I can however measure head surface to cam cover surface.



ahhh Im lost

Thanks in advance
BJ

 

[IJ.]

There's a bunch of online calculators like this one where you just punch in the numbers that will give an approximation BJ.

All you need are the base numbers Bore/stroke/Combustion chamber volume/Piston dish/Piston protrusion/gasket thickness and bore size.

 

[BJ91T]

thanks IJ, now how do I measure combustion chamber volume lol

 

[SevenMKIII]

http://www.merkurxr4ti.com/chambervolume.html

 

[IJ.]

BJ: Stock from memory is 40cc (jump in with a correction if I'm wrong guys it's been awhile since I cc'd mine)

 

[isnms]

IJ - That's what you told me in reply to a pm a few years ago

Hey IJ,

I'm using, in inches:
Bore: 3.27 83
Stroke: 3.58 91
HG bore: 3.31 84
HG thick: .0787 1.37
Deck Height: .2 -.2
Piston volume: -19 -19
Combustion Chamber: 40cc 40

http://www.csgnetwork.com/compcalc.html
http://www.rbracing-rsr.com/compstaticcalc.html

Thanks

Hi...I work in Metric sorry for the confusion!

The above in bold are Metric stock specs and the first Calc gives me 8.51:1 which is near enough.

Try -0.007" for Deck Height and it's important that it's - to denote above the Deck!

 

[IJ.]

Many thanks for posting that I'm gettin oldNdoddery so don't trust the memory 100% :: IJ. ::