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Old 08-03-2012, 03:51 PM
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Compression test

Just wondering went on a dynamic compression calculator an it said my v/p was 205 when just turning the motor over doesnt this go up when the motor is running if so how much

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Old 08-04-2012, 06:04 AM
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From what i have read dynamic compression is lower than static. There is a lot of info on the net about this. Basically summarizing the blog below, the faster the piston travels the less time the charge has to be drawn in.
For examplei,If you fill your lungs with a litre of air breathing in through your mouth in 2 secs, try and fill your lungs with the same litre of air in only one second. You wont be alble to suck in that litre in one sec , maybe 3/4 of a litre.

( high vacuum when carb butterfly valve nearly closed and low vacuum in cylinders when throlltle wide open under load.)
This is from Rich
The intake valve doesnt magically snap shut when the piston hits the bottom of the bore. It stays open a good bit of time longer. So the piston is already part way up the bore as the valve is closing. Lets just say the engine has a 4" stroke. Now lets say that the camshaft card says that the intake valve closes at 70 degrees after the piston hits the bottom of the bore. Typically this would mean that the piston would be about 3/4" up the bore when the valve is nearly closed. How far up the piston actually is will vary depending on that crank stroke to connecting rod ratio,but the variation is slight, unless you build an engine with either extremly short or long rods. So basically you need to recalculate the compression ratio formula using 3 1/4" as the stroke since the first 3/4 " of piston travel is lost due to the valve being open. So dynamic compression ratio is always lower then static compression.

Last edited by 67Mustang Al.; 08-04-2012 at 06:31 AM.
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