I have been studying a turbo book, and it talks a lot about static compression. Can anyone shed some light as what the difference is between normal and static compression?

 

Static compression is where you pull the coil wire and crank the car repeatedly without starting it get a single cylinder compression rating, then move to the next cylinder, etc. Dynamic compression testing is where you do the compression test by actually starting the engine and letting it run (obviously down one cylinder since you will be testing one at a time) to get a reading on that cylinder.

 

Often you could have proper readings on a static compression and a running/dynamic compression is necessary to identify a problem. When performing a running compression, you check 2 readings usually. Idle and a wot throttle snap. Its pretty easy to calculate what numbers you should be getting when doing a running compression test:

For idle: let's say idle vacuum is 18 in.hg and atmospheric pressure is 30 in.hg. The difference of the 2 numbers is what the engine is sucking in. so 30-18=12 in.hg . In absolute pressure , thats about 6psi. If your compression ratio is 10:1, you should get 10x6=60 psi of idle running compression.

For throttle snap, you should get about 80% of static compression.