Could anyone tell me my CR from these numbers
Head CC'd at 44cc's
deck looks to be .140" down
bored 30 over.
I tried with my virtual engine calculator butr I dont know what "Cbr cc's" means
Thanks Phil

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9.58 by my math, assuming your 44cc and your 0.14" are accurate. Sounds like you've milled the head. Don't forget to factor in whatever head gasket you're using.

Thanks Dan, I forgot to add the thick Fel-Pro at .39 compressed.
and yes I asked for 0.100" to be taken off the head at the machine shop.

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I would expect "Cbr cc's" is short for "Chamber cc's", I.E. combustion chamber volume; in your case 44 cc's.

With the .039" head gasket I come up with 9.75:1.

-S/6

One of us is making an error in our calculations. I came up with 9.58 without the gasket's 0.039" thickness; you came up with 9.75 with the gasket.

Rerunning my math to include the gasket, I come up with...

9.78, which is within rounding technique differences of your 9.75. Error looks to have been in my original calculation!

Next time I won't try to calculate CR and watch Star Trek at the same time.

That's why I 'programmed' Excel to do the math for me...I am the king of fat-finger error on every calculator ever made!

What you really needed was one of these:

You and ST, scary!

MJ

The guys over at moparts gave me a couple of links that have CR calculators http://www.straightline-perf.com/ . Would my bore be 3.43 with the 0.30" over bore? if so they came up with 9.797:1.

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Yes, your bore is 3.43.

The math isn't very complex. Works like this:

(bore ÷ 2)² × π × (stroke) = piston swept volume

(bore ÷ 2)² × π × (negative deck height + head gasket compressed thickness) + (combustion chamber volume) = TDC volume

(piston swept volume) + (TDC volume) = BDC volume

Therefore:

(BDC volume) ÷ (TDC volume) = compression ratio.

Don't forget to keep your units consistent; work in inches (linear and cubic) or in centimetres (linear and cubic).

the nice thing about the calculator on the link I posted is that you do not have to do any standard to metric conversions. you input the bore, stroke, gasket thickness, deck height, and gasket bore in inches and chamber volume in cc's. then it tells you the displacement of the engine in cubic inches(228.69), litres(3.75), and static compression ratio(9.797:1).

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