"Disconnecting" the coil wire leaves the secondary winding of the coil to generate maximum voltage...with nowhere to go. This provides maximum stress on the coil-winding insulation; leading to failed insulation, and defective ignition coils.
In the old days, we'd GROUND the distributor end of the coil wire, so that the "spark" would drain harmlessly away at near-zero volts.
But disconnecting the ignition module would accomplish almost the same thing--the coil would never fire, therefore cannot generate high voltage. And you'd eliminate the fuel injector spray, too.
Otherwise, you could pull the appropriate injector/ignition fuses.
I don't care about having the engine warmed-up. Compression and leakdown testing is not hugely affected by engine temperature. There's a heap of variables that can affect the indicated pressure on the gauge. Hopefully, they affect all cylinders equally, so all the cylinders show about the same pressure. Example: Altitude (normal air pressure) has a big effect on compression pressure. The higher the altitude, the lower the indicated compression pressure.
The throttle doesn't have to be WFO. On the older, carbureted engines, I'd flip the fast idle cam to the top step. As long as the throttle is "somewhat" open, you're fine.
Some guys make a point of cranking the engine a certain number of revolutions, and make a big deal out of the "first hit"; the initial jump of the compression gauge from "0" to wherever it lands. I crank the engine until the needle stabilizes; and I found out the...interesting...way that the "first hit" is pretty meaningless because nobody looks at where the crank started in relation to the compression stroke of that cylinder. If the piston was half-way up the bore on the compression stroke when you crank the engine, the "first hit" is gonna be pretty moderate. OTOH, if the piston was on the exhaust stroke, the crank builds up lots of speed before the compression stroke. The "first hit" will be significant.
You DO want a fully-charged battery so the engine cranks as fast for the 8th cylinder compression test as it does for the first.
It's nice if you have recent experience with that compression tester on at least one "known-good" engine; so you don't get freaked-out by an inaccurate pressure gauge. I test my compression gauges against each other and the regulator gauge on my air compressor a couple times a year. MOST (not all) compression testers that have a quick-coupler in the hose, use Industrial Interchange couplers. At the time these photos were taken, my compressed-air system used "M" or "Industrial Interchange" fittings for my air tools...which is what gave me the idea to test my compression tester against the gauge on the regulator. My compressed-air system is now using Milton "V" or "European High Flow" couplers.
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Last time I looked, GM spec for compression testing results was 100 psi minimum, with the lowest pressure at least 75% of the highest. And that's probably fine for Grandpa. It's not what I want to see on MY engine. Anything under 140 psi, or beyond 10% difference gets my attention. (But that could change if I was at a higher elevation.)
