Checking compression

Hi All; Simple question for today. How many compression strokes do you allow to get to the "real" number? I've been using about 10. Too many? Thanks and ride safe, Steve

I generally only go about 5 at the most with all plugs removed and wide open throttle. Any more than that tends to wear on the coils.

"How many licks does it take to get to the center of a Tootsie Pop?"

5 compression pulses as recommended above should be good enough. I thought the reading on first pulse was supposed to mean something? I could be wrong.

Make sure the battery is completely charged, for stronger cranking.

Speaking of compression.

If I know the following:
Bore = 66mm (2.598 inch)
Stroke = 54mm (2.126 inch)
Compression ratio 9.0:1
Total Displacement 738cc, 4 cylinders

is it possible to calculate what the compression should be ?

No problem, just curious.

anywhere from 120 to 140 and you should be in the ball park and once your done that then you should do a leakdown test to find out the real condition of the motor.

also on how many times you should turn the engine over go until the needle quits moving there is no set amount that will occur in, also if your worried about the ignition burning up turn off the ignition and jump the starter or pull the plugs and install them in the plug caps and ground them against the head or cylinder and crank away. you will not hurt anything then, promise

JR wrote:

Speaking of compression.

If I know the following:
Bore = 66mm (2.598 inch)
Stroke = 54mm (2.126 inch)
Compression ratio 9.0:1
Total Displacement 738cc, 4 cylinders

is it possible to calculate what the compression should be ?

No problem, just curious.

I think you have to know the cylinder area volume when the piston is at TDC then compare to the volume when the piston is at the bottom. At bottom when the intake valve is open, it's basically atmospheric pressure in there which is about 14.7 psi. Then when the piston rises it squeezes it into a smaller volume so I think the final PSI is the ratio of the first to final volumes multiplied times the initial 14.7. I could be wrong, it's been 40 years since I took chemistry, but the law is:

P V = n R T

It shows that volume and pressure vary as inverse.

now myhead really hurts!:laugh:

Doesnt start to build compression till the intake closes. Depending on what cam you have. 35* ABDC can make your #'s fall fast. Compresion using the cam timing is dynamic, just cylinder volume @TDC and BDC is static compression.

Thanks guys.
I think then that the answer would be no - I cant use a simple calculation. I forgot to consider valves opening nd closing and shape of piston tops and crowns etc. Not as simple as I thought it might be.

As I mentioned I dont have any worries. My compression is 145 across all cylinders on a cold engine. Sometimes practice (measurement) is preferable to theory

Cheers