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Engine Mechanical TopicsDiscussion for motor builds, cams, head work, stripped bolts and other engine related issues. The good and the bad. If it goes round and around or up and down, post it here.
Mike i use the big boyz calculator, but the factor missing in your calc is the cam, usually input intake valve close value, for true figures & ccp
but inputting what you got i'm getting 11.56 with -1.2 piston
Last edited by yobtaf103; Jun 27, 2020 at 10:27 AM.
Wouldn’t there also be some conversion using CCP with ACR disconnected?
For example, the M8 has a nominal CCP spec of 175 Psi. So, if the cylinder is compressing nominal ambient barometric pressure of 14.7 psi, then (175/14.7)= 11.9:1 compression ratio...but...We know that’s not correct....why not?
Mike i use the big boyz calculator, but the factor missing in your calc is the cam, usually input intake valve close value, for true figures & ccp
but inputting what you got i'm getting 11.56 with -1.2 piston
But as far as the using this calculator I'm correct, right?
I'll check out the Big Boyz one, I'm just doing some comparing now.
Wouldnt there also be some conversion using CCP with ACR disconnected?
For example, the M8 has a nominal CCP spec of 175 Psi. So, if the cylinder is compressing nominal ambient barometric pressure of 14.7 psi, then (175/14.7)= 11.9:1 compression ratio...but...We know thats not correct....why not?
I'm just looking at and trying to compare to the advertised compression given for a 128" cylinder kit.
Mike i use the big boyz calculator, but the factor missing in your calc is the cam, usually input intake valve close value, for true figures & ccp
but inputting what you got i'm getting 11.56 with -1.2 piston
Are you saying if I plugged in a cam calculation my numbers would match the S&S 128" kit at 11:1 comp?
The problem with most all of these calculators is they are skipping the area from the top ring to the deck. I also see you place the deck height at 0 which is not typically where any HD engine comes in at. A typical M8 107 is .010 - .014 down and the SE big bore kit typically come in at 0.016 - 0.020 down. You would need to know all this to get a proper compression static ratio. As far as trying to calculate a running compression ratio the calculators get event further off since the valve opening and closing points given on a cam card are not correct. The points on a card are when the lifter has already moved .053" for a twin cam and .050" for a M8! Using the stock HD rocker ratio of 1.65:1 that means the valve is at 0.087 lift for the twincam and 0,083 for the M8. So as you see the cam card doesn't give you where the vale opens or closes at all. At best the various calculators get you in the ball park but none of them are truly correct.
Wouldnt there also be some conversion using CCP with ACR disconnected?
For example, the M8 has a nominal CCP spec of 175 Psi. So, if the cylinder is compressing nominal ambient barometric pressure of 14.7 psi, then (175/14.7)= 11.9:1 compression ratio...but...We know thats not correct....why not?
See this is the rub, my shovel runs 220-225psi cold per hole but rated compression is 11.7-1.
The problem with most all of these calculators is they are skipping the area from the top ring to the deck. I also see you place the deck height at 0 which is not typically where any HD engine comes in at. A typical M8 107 is .010 - .014 down and the SE big bore kit typically come in at 0.016 - 0.020 down. You would need to know all this to get a proper compression static ratio. As far as trying to calculate a running compression ratio the calculators get event further off since the valve opening and closing points given on a cam card are not correct. The points on a card are when the lifter has already moved .053" for a twin cam and .050" for a M8! Using the stock HD rocker ratio of 1.65:1 that means the valve is at 0.087 lift for the twincam and 0,083 for the M8. So as you see the cam card doesn't give you where the vale opens or closes at all. At best the various calculators get you in the ball park but none of them are truly correct.
Wouldnt there also be some conversion using CCP with ACR disconnected?
For example, the M8 has a nominal CCP spec of 175 Psi. So, if the cylinder is compressing nominal ambient barometric pressure of 14.7 psi, then (175/14.7)= 11.9:1 compression ratio...but...We know thats not correct....why not?
Because when you compress air quickly it self heats.. See Adiabatic Process.
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Compression Ratio Calculation
