Compression Test. Hot or Cold?

It's been about 20 years since I last did a compression test and I can't remember if it should be done with a cold engine or a hot one, or doesn't it matter.

Thanks.

SU/s ?

cos compression is supposed to be done hot with throttle open.

Do SU pistons lift at cranking speed?

I voted cold and here's why. If it's good cold it will be good hot. If it's not so good cold, it's quick and easy to warm it up and retest to see if it healed. BTW, how hot is hot? Also you should always test with the throttle held open and battery fully charged. For consistent results!

Both.

http://www.automedia.com/Engine_Compression_Test/ccr20050801cc/1

http://www.youtube.com/watch?v=9U8iNb21arw

I voted HOT as it seems like after I have done a cold compression test I always have to do a hot one too,Dang curiosity anyhows.:thumbsup:

Would it make any difference if you did the cold compression test without the SUs and with just the iron manifold installed?

Ken

Thanks for the answers, folks.

I hadn't though about SUs opening, Pooch. I'll have to check on that.

Dave, when I said hot, I meant full operating temperature.

All I have to do now is find out where I put the compression tester 20 yaers ago.

The reason I do it on a hot engine, is that sometimes head gaskets will seem o.k. when cold, but start leaking at operating temperatures...

I initially ran my compression test with the engine cold and didn't take into consideration that the throttles should be open. After reading that the throttle should be open I ran it again (still cold engine) and it didn't seem to make any difference. Tests were within just a few pounds of each other. Is that common or does it usually make a "big" difference when the throttles are open as opposed to closed?

Compression tests should be done with the engine hot to allow dissimilar metals to reach operating temperature and expansion rates. The carbs should be wide open to allow for full air intake. To do the cold method will not give you as accurate a measurement.

i just checked the compression on my '72 BGT and got some very interesting figures

First I checked it cold

#1 135

#2 110

#3 115

34 130

Not too good so I squirted some oil down the cylinders and the readings went up about 10#


So i thought what the hell and dumped a can of seafoam in the crankcase and let it run for a bit.

The next test was hot

#1 160

#2 145

#3 155

#4 160

Now is there usually that much difference between hot and cold? Or did the seafoam do it's magic?

The car was and still is running fine but the difference between hot and cold really surprised me.
Pat

That looks like the difference I'd expect to see with throttles closed versus open. Normally its 25-40 psi difference when open/closed. It could just be that you were getting blowby at the rigns and the Seafoam worked. Test it again cold tomorrow to find out. :D

The springs still push the pistons down.

Engine stopped prior to cranking throttles open=== ambient pressure in manifold and it dont suck air anytime... pressures trying to equalize.outside hi press going in..
Engine running pressure of autside (ambient) is trying to overcome the Reduced pressure in intake manifold
as throttle is a restriction ..Physics ,,dont like a vacuum.
Dampers on top of vacuum chamber act a damper to sudden movement of piston ,,Example is if throttle
opened suddenly while engine running the damper will resist movement due to oil in damper tube resulting
in a a considerable Richer mixture to help acceleration,,,thats why SUs dont need an acceleration pump.
Hi velocity air across bridge causes a low pressure wich results in outside pressure in float bowl to push fuel
out into low press area helping vaporization of fuel /air mixture and taper of needle regulates fuel flow for
correct mixture.