Compression test question

[wildcard]

I want to do a compression test on my 300 I-6.
I've looked at a few different sites, including this one, and can't seem to find a definite answer. I would like to know if the test is done with all the plugs removed or just the one I'm testing. Or does it even matter?
Thanks.
Benny

[colnago]

I did mine with the plugs in. Logically, it should not matter, since you're looking at the change between cylinders. I had 100 - 120 for all cylinders (with plugs in), except for 1 and 3, which were noticeably lower (30 - 60). If I do it again, it will be with plugs in.

Joseph

[Ohiotinkerer]

I've always done them it the plugs in which I learned from my dad and grandpa which is how they always did it.......never had anyone tell me any different so I'm guessing that's the way to do it........just my ......

[Busboy]

You want to do it with all the plugs removed in order to maximize engine cranking speed. Also the throttle should be blocked wide open.

[MadJoe]

I'm with the "plugs out" crowd. If for no other reason than to make cranking easier on the battery. Plugs out reduces the effort necessary to get the full 5 rotations to get a reliable check, so by the time you're checking the last 2 cylinders your battery isn't noticeably weaker - which can affect your readings. You're going to take them all out eventually, so why not just start with them all out?

-Joe

[Lee]

Plugs out, throttle open.

[Jacksdad]

Plugs out, throttle open.

The way I've always done it

[Ranchero50]

Plugs out, never noticed a difference with the throttle opened so I stopped doing it. 5-6 pulses to get max pressure. Write it down so I don't forget.

[tqwrench]

Spark plugs out, throttle blocked wide open, power to coil disconnected - you don't want errant spark flying around. The first two puffs should be within 90% of max from that hole and there shouldn't be any hole less than 80% of max compression.

Another way to put it, you should do 5 "puffs" per hole, those first two should come up fast and be close to the last 3. It might go like this 80-100-105-105-110. If it slowly climbs, you might have a problem. I'd cringe if I saw 50-70-90-100-110. Then, the max compression from each hole should be within 80% of the others, so if your top compression reading was 140, you shouldn't have anything under 112.

The question is, if you come up with low compression, what next? Are you prepared to do cylinder leak down testing? Are you chasing a symptom?