I'd expect a cold engine to crank slower due to the thicker oil drag. Slower cranking speed might result in relative lower readings. But as mentioned before, consistency is what you're looking for. As a rule, a cylinder needs to measure about 100PSI cranking compression to fire. What you're attempting to measure with cranking compression test is a cylinders ability to seal.
To keep the test consistent for all cylinders, it's best to remove all plugs, have the throttle wide open, and a charger going on the battery.
For a running compression test, what you're attempting to measure is the ability to fill the cylinder - or volumetric efficiency. For that, here is my list of instruction that I keep handy:
Step 1. Preform a cranking compression test with all plugs
out and the throttle open.
Step 2. Record your cranking compression readings. Make
sure the charging system was up to snuff so all cylinders
were tested under the same conditions.
Step 3. Install all plugs but one. Install a standard
compression guage in the remaining hole with the shrader
valve in place.
Step 4. Start the engine. Yes, it will run but will be
missing on the cylinder with the compression guage. Burp
the guage and allow the reading to stabilize.
It will be bouncing around at about 50 to 60 psi. (the throttle is
causing a restriction and the piston is moving fast
compared to cranking speed causing only a partial fill of
the combustion chamber).
Step 5. Snap the throttle wide open and return to idle.
The guage will hold at the peak reading. This reading is higher
than idle because the peak comes at the instant the
throttle restriction is removed and the piston speed is
relatively slow.
Step 6. Record your readings for running snap compression
for all cylinders. The running compression reading should
be approx. 80% of cranking compression. (example: cranking
compression = 150psi running compression should = about
120psi)
If your running reading is below 80% of cranking
compression, the intake system is causing a restriction. If
the running reading is above 80% of cranking compression,
the exhaust system is causing a restriction. If the problem
is in one cylinder you either have a problem with a worn
camshaft, broken spring/rocker, carbon build up, etc. If
the problem is on all cylinders look for a restricted
intake system or clogged muffler.
The 80% spec is a ballpark spec and should be used as such. Remember that
cranking compression is a measure of cylinder seal while
running compression is a measure of volumetric efficiency
or the cylinders ability to draw the air/fuel mixture into
the combustion chamber.
Hope you find this useful. :moped:
