I'm not sure if my old heads were 76cc or 64cc (they were crappy 193 swirl ports). I want to find out about what my compression is. With all calculations it should be either 8.3 or 9.2 with the heads gaskets, and such.

When I do the compression test do I take all the plugs out? Or just the plug of the cylinder I'm testing?

About what psi should I see for 8.3 or 9.2?

thanks

 

Cap'n,

Take out all the plugs. Disable the ignition. Hold the throttle open fully. Make sure the battery is fully charged. Crank the engine at least 6-7 times to generate the maximum readings.

So many factors affect the actual compresion pressure that it's difficult to guess. Piston top design, cam timing and duration, and even temperature will affect the final readings. With an average cam, flat pistons, and 64cc heads you should see at least 150 PSI on a tight engine. It is more important to have each cylinder's result to be within 10% of all other cylinders. Dynamic compresion can be a lot higher than cranking compression might imply. A good intake and cam lobe design can really raise the cylinder pressure.

 

K. The engine is real tight still. Only 20k miles on the shortblock. The cam is retarded 2*, which I hear can affect cranking pressure.

I'll go try cyl #1 and see what I get.

thanks

 

Factors that affect the actual compresion pressure could be.

Bad rings
Cracked pistons
Cracked cylinder (bore block)
Irregular block surface
Damaged head gasket
Irregular head surface
Damaged valve seats
Bad valves
Missadjusted valves
Cracked head
Damaged spark plug hole! (mine)
Engine temperature at compresion test
Piston rings lubrication (cylinder with or without oil)
Cranking speed? (full or dead battery)


Others.......?


Regards,

Denis V.