Proper compression test procedure...

[Cliff Beer]

Quick question here for the engine builders. I learned many years ago that to perform a compression test properly on an engine with induction installed (either carb or FI), the throttle valve should be opened. This is how I have run compression test myself over the last 30 years....and how I've seen professional mechanics do the same.

Without opening the throttle valve there's a possibility of not fully drawing in the intake volume completely, thereby making the test results erroneous.

Thoughts?

 

[Bob Putnam]

That's correct, but on a V8 (with all the spark plugs removed), there's enough valve overlap and pulse reversion that opening the throttle probably won't have a significant effect. Interesting to test that effect though...

 

[Jac Mac]

Correct, but I see compression testing as only a quick confirmation that something is amiss, Leakdown testing helps to isolate the source, but sometimes you have to use that at both TDC & BDC to help pinpoint the problem. Inlet cam lobe worn and valve seat recession can really throw compression test results a curve ball. Those mini-TV cameras are a great thing, I had one client who must just about be Dow Cornings favourite customer, or was until we stuck the camera probe down thru the intake port, hopefully he learnt he lesson, not much point in buying high $$$ heads if your gonna fill the port with RTV.

 

[Cliff Beer]

Got it. Thanks, that's what I thought.

Yup, a leak down would be good information too....probably better, as you say JM.

Thanks

 

[Randy V]

Both tests are best done at operating temperature or close. The LeakDown test should be done with the cylinder being tested at TDC compression stroke and crankshaft locked in place.
I typically perform both tests cold and log them right after the engine has been broken in and dynoed.
That way I can take comparitive measurements with the engine cold later on...

 

[Robert May Eng.]

You would be a long time doing leakdown tests at bdc with the sorts of cams most of us use. A few degrees off maybe or back off the rockers.

Bob

 

[Ron Earp]

Compression test: Engine hot, all plugs out, throttle open, crank and measure. Leak down is better as mentioned.