Also, if I remember properly, engine should be warm (at least). Compression losses can be higher when cold, but the engine is going to run hot anyway
.In a warm engine valves are going to open sooner too (metal rod expansion). Don't know how much difference that can cause.
And after checking some old threads...
http://slantsix.org/forum/viewtopic.php ... test#35932
you should remove the air filter. I'll check that in the service manual when at home.