Proper Compression Testing

[BLACK'98DSM]

Is it required to open the throttle when compression testing?

If you don't, will the numbers be too high or too low?

 

[Vegas Smith]

I've never actually opened the throttle on any of my tests but it's the recommended method.

 

[BLACK'98DSM]

I've never actually opened the throttle on any of my tests

Neither have I, that's why I'm asking

 

[silvreclips]

I think overall you will get very similar numbers. But it may take more cranks to get it there

 

[AWD-Tony]

Open the throttle, you'll get there faster.

 

[FIBA M - EC17PSE]

When i did a comp test i just cranked the engine and the TB was closed, it will always take a few cycles to rise anyway, never tried it open on the TB so i cannot say if its a good or bad thing to do

 

[stillrunning]

WOT
You are trying to fill the cylinder with air which will be compressed by the piston.
Best way to achieve that is to eliminate the restriction create by the throttle blade and let the cylinder breath as naturally/easily as possible.

 

[rabenne]

Both ways work so who cares.The old rule of thumb is to block the throttle fully open. That is also what it says in "the books". For the reasons stated above.

Do either one, I think the more important thing is to do the same procedure on each cylinder to minimize differences you introduce into the test. Typically you are supposed to crank the engine through the same number of compression strokes while testing each cylinder. This is a more accurate measure of cylinder compression balance when compared with today's standard advice "crank it until it stops going up".

If it takes 10 compression strokes to reach 175 psi on the gauge for cylinder 1 and 5 compression strokes on cylinder 2, you should probably repeat the test after dropping a bit of oil in the cylinders (wet compression test).

That all assumes consistent cranking speed as well. If possible, hook up a strong battery charger to ensure the battery isnt getting weak by the last cylinder.

Best of luck!

 

[BLACK'98DSM]

I've already compression tested. I was just wondering why people said to leave the throttle open.

Thanks again guys!