WOT when testing compression?

OK, so I'm the kind of person who's not satisfied with just the "what", I need to know the "why". I'm trying to get my mind around why compression is tested while holding the throttle full open. The engine's not running (just cranking), so what's the reason behind holding the throttle open when checking compression?

I believe it has to do with the engine being able to spin over freely without any restrictions, such as compression (all plugs out) and air flow (closed throttle blade)?

Post edited by: Bluemeanie, at: 2007/10/16 13:55

So as not to restrict the airflow in any way. It removes another variable from the environment. Instead of holding the throttle open, you can just pull the carbs off (So easy, right?)

How much air are you going to get into the cylinders if the air supply is blocked off? You are not going to have much compression if there is no air to compress


It's like trying to breathe with your nose pinched shut and your mouth closed, it just don't work real well

OK, that makes sense. Even though the plugs are out, intake air has to be there to "compress" so the gauge has something to read. If I understand correctly then having it wide open will take as much restriction as possible out of the incoming side so that you get a truer reading on the compression side. So, if you check compression with a closed throttle, you're compression would be incorrect by showing too low of a number?

I'm sure it would make a differance, next time you do a test try it. I would think the numbers would be a little lower!

Depending on variables, the compression will generally be 30-70 psi lower when testing with the throttles closed.