this cant be right , right
05-25-2015, 05:22 PM
#1
Racer
Thread Starter
this cant be right , right
when I tested my compression on my car with all the plugs out I had from 218 to 227 lbs of compression .found a formula for checking compression ratio . it said divide compression by 14.7 . that gives me just under 15 to 1 . I have 64cc heads and the domed double eyebrow pistons raise above the block which is decked .40 is that high of compression possible or is the formula wrong ?
05-25-2015, 06:41 PM
#2
Melting Slicks
Member Since: Apr 2009
Location: Great Plains Iowa
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when I tested my compression on my car with all the plugs out I had from 218 to 227 lbs of compression .found a formula for checking compression ratio . it said divide compression by 14.7 . that gives me just under 15 to 1 . I have 64cc heads and the domed double eyebrow pistons raise above the block which is decked .40 is that high of compression possible or is the formula wrong ?
There is no "formula" that I know of and I've been building performance engines for almost 50 years. The camshaft duration drastically affects the cranking pressure. The longer the duration is the less the compression pressure is and vice-versa.
05-25-2015, 06:56 PM
#3
Race Director
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Yeah, this came up in another thread and it seems the way it is that there no way to get the compression ration of a built engine.
I really doubt you have 15:1
05-25-2015, 09:26 PM
#4
Team Owner
Were the throttle plates on the carb completely opened when you did that 'compression test'? {probably not}
05-26-2015, 08:36 AM
#5
Racer
Thread Starter
couldn't tell you . but I did do the test the same way on my evinrude 140 and it came within specs at 105 . im guessing in real world my cars somewhere around 11 to 1
05-27-2015, 08:16 PM
#6
Melting Slicks
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An engine with a stock camshaft can show 200 psi but with a 245 degree duration camshaft it would show around 150 psi because of the additional overlap of the lobes. So there is no correlation between the two whatsoever and certainly no "formula" that could be in any way accurate.
05-28-2015, 01:17 PM
#8
Le Mans Master
While you can't back into CR from cold cranking compression, given the pressures you're seeing you might want to consider a step larger cam than whatever is in there now. My $.02
05-28-2015, 04:41 PM
#9
Racer
Thread Starter
it was 25 years ago I put it together . they had a 350 350 hp bebuild kit I put in . then I did it again and went to domed pistons and was told I was getting dart racing heads but I think there world heads . if I remember right the cam was either a 480 or something in the 505 area . have to look and see if I still have all the paper work
