Questions about compression test for a FD:

1. Should we use the leading spark plug hole or the trailing hole spark plug hole?
2. Should we do full throttle during test?
3. Should we perform the compression test with cold engine or hot engine?
4. What else do we need to be aware of in order to obtain the most accurate compression test result?

Thanks

 

I can tell you what I did from reading many posts
1. Trailing (Top holes - take both spark plugs out during entire test)
2. Yup full throttle
3. Ideally warm - since cold will be higher
4. Battery performance should be consistent between tests

 

1. trailing
2. it doesnt really matter. the small difference will not make or break the results
3. either is fine. for best results, do both
4. fully charged battery. no need for a jump box. you only need a healthy battery. also make sure the sensor is fully installed. if loose even a little bit, your readings will be very low.

 

Assuming you know to not use a standard compression tester. There is a rotary specific compression tester.

 

while a piston engine compression tester w the valve removed will give you a reasonably good ideas as to differences on the 3 rotor faces if you are looking to get absolute numbers the rpm needs to be at or corrected to be at 250. any variance from 250 will major skew the numbers. barometric pressure should also be considered although it isn't as significant unless you are at the top of Mt Everest.

the best way to get data from an analog tester is to video it and play it back in slo mo.

 

Baro is more important than you might think. Wile certainly not on top of Mt Everest, where I live at 7200ft elev I'll see a typical map reading of 78kpa with key on and engine off. At sea level you'd see 100kpa of course, so in simple terms there is 22% less air density which is very much reflected in compression tests!

Otherwise I agree with everything else said

 

"Baro is more important than you might think"

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