Compression test
 

I was thinking of doing a compression test on my motor just for fun to see what my cranking compression is. Not having done one before I'm just wondering about the procedure. So I would pull all the plugs and probably disconnect power to the distributor (HEI), although with the wires disconnected and the plugs pulled may not need to pull the power to the HEI? Then for the fuel supply, just disconnect the fuel supply line at the pump inlet and plug it? Then the primary throttle blades need to be open?

Thanks.

Yes , open the throttle blades... many people do not do this.
please post your results.,,, just to help others.

Do not just unplug the wires from the plugs!

You need to kill the power to the HEI other wise when cranking the motor it’s still producing a spark with no where to ground out to so then it can arc across it’s coil and harm that.

This is the same issue that can take place with any system with a coil, even a points system.

Thanks for that. I would have probably done that anyway because just because it's so easy and it would be the safest way to proceed. But, it's good to know there's a sound rationale for it as well.

All four barrels wide open.
I don't like dumping all that fuel in the engine so I usually just pull the carb off and do the test.

Thanks for all the advice. It'll be at least a couple weeks until I get to it but I report back when done.

Warm engine. Throttle blocked open (primaries at least). Ignition and fuel disabled. 5 to 7 revolutions per cylinder. All cyls should be within about 25% of each other or less.

Can a compression test be done on a cold engine; sans manifold and carbs? The main reason would be to ck consistency between cylinders.

Sure. Warm or cold, doesn't make a lot of difference. Cold tends to produce slightly lower pressures. Yes, absolutely disconnect power from the HEI for the reasons described previously.

Some guys go wild about the pressure produced during the "First Hit". I quit caring about that a long time ago, when I figured-out that there's likely to be a big difference in pressure depending on where the crank stopped in relation to the compression stroke of the piston/cylinder being tested.

Some guys have a defined number of compression strokes they allow for testing. I crank until the pressure doesn't increase any more...which is generally about six compression strokes, more-or-less.

I don't care about having the throttles blocked "wide open", but I do want the throttles set to fast idle at least. More doesn't really make a difference, but less-open can.

Altitude and weather (barometric pressure) make a big difference in the pressures produced during a cranking compression test. So does battery state-of-charge, as the battery loses voltage the starter motor cranks the engine slower, which reduces pressure indicated on the gauge. There's other variables, but opening the throttle some, and keeping the battery voltage up using a battery charger as needed are about the only ones that can be controlled.

I make sure my engine is warmed up ,pull all the plugs,open the throttle fully. One thing that I do on all my Pontiac engines is have an underhood connection on the keyed starter wire that I disconnect and add another wire that I simply tap on the battery to bump the engine over.

There no reason to disconnect the fuel when doing a compression test cranking the engine does not pull fuel out of the carb as long as the choke is open and the throttle is held open. I use a remote starter switch I hook up to do the cranking so it's done with the ignition off. I do it with the engine fully warmed up wire the carb open and crank until pressures stops increasing. You can make your own remote starter switch with some wire a couple alligator clips and a spring loaded switch. That way you can watch what's going on under the hood and you don't have to have your wife cranking the engine over for you.

Sometimes I forget what a great resource the factory service manual is. According to it, with a warm engine, pull the plugs, lock the throttle and choke wide open and then do the test. No mention of the fuel supply. or coil power. The Hayes manual does mention disconnecting power to the coil but no fuel supply comments.

And I forgot to mention, yes, I will be disconnecting the power to the HEI. Is that an error in the service manual? It is for a 70 with points ignition, but from what I've read you're still supposed to disconnect the power to the coil?? If anyone has a manual for a HEI model year I wonder what it says?

I just did a compression test on my 77 400. I pulled the BAT wire from the HEI. I do have a 78 Pontiac Service manual - but , I did not check the manual :(
HEI started in..1974/75? So, I'd guess later manuals might discuss.
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For me, I tested cold
(i) removed all spark plugs
(ii) disconnected HEI BAT terminal
(iii) held open throttle with twist tie (and held open qjet choke blade and secondary flap)
(iv) screw gauge into spark plug hole (#7 cyl hardest for me)
(v) cranked until pressure did not go up

Yes, any coil should be unpowered when the plugs are all disconnected, even modern engines with with coil on plug set up’s.

Of course with some set ups you can yank the fuse to the ignition box and accomplish the same thing.

I was under the understanding that all plugs should be in, since you are also plugging the one cylinder you are testing. That way all cylinders are plugged. This is what I was told from a co-worker who builds race engines as well as daily drivers. Also, cranking revolutions should be the same on all cylinders (5-6 revs) even if it is still building pressure. Or, if it is still building pressure, go back to the other tested cylinders to test at that higher number of revs (7-8?).

Take all your plugs out. It will spin faster, last longer, and you can use the sounds to help you diagnose.


After all these years in the shop, I haven`t found holding the throttle wide open to make any difference on the readings. Just saying.