Compression test ?
Thread Starter
Drifting
Joined: May 2005
Posts: 1,909
Likes: 6
From: Lake Arrowhead - Georgia > 72 Base Coupe & 74 BB Roadster
Compression test ?
I just bought a 74 Big Block Vert.....I am just curious as to the integrity of the rings and valves. It runs good and shows good burn on the plugs...(white ash - no oil residue)
Will a compression test tell me the condition?....if yes....what is the proper method to do the test? I have the gauge, but have never done one.
Thanks for any info......
Will a compression test tell me the condition?....if yes....what is the proper method to do the test? I have the gauge, but have never done one.
Thanks for any info......
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
A compression check will give you an idea of the sealing of the cylinders but it won't really tell you where the leak by is if there is a compression leak. I usually do a compression check first, then a leak down test.
IF you do a compression check and the cylinders are within 10% of each other...I wouldn't sweat it...Just drive it.
IF you do a compression check and the cylinders are within 10% of each other...I wouldn't sweat it...Just drive it.
Race Director
Joined: Dec 1999
Posts: 16,945
Likes: 9
and open the throttle all the way! After the 1st check, see if any cyls that are low improve after squirting some oil into them. The compression numbers heavily depend on not only the condition of the engine but also on the static compression and the cam (amongst others)
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
If all cylinders read close to each other, it safe to assume everything is reasonably alright. A compression test will indicate a problem when one or more cylinders is out of the norm compared to the others.
To do a compression test, run the engine until it is warm. Let it cool down to the point where you can remove all the plugs with getting your hands and arms burnt. Open the throttle and wedge something in to keep them open. It best to have a helper or a remote switch to crank over the engine. The first time the piston come up during the compression stroke the gauge should see an immediate pressure increase. The second and third compression strokes adds very little but is needed to get an accurate reading.
There is no defined reading. Engines will vary depending on the components. I would think a stock engine should be in the 150-160 range. I did a compression test just a few days ago to give me an idea as to where I was at with a new engine that was getting some pinging. I was not surprised when I was getting readings around 200.
To do a compression test, run the engine until it is warm. Let it cool down to the point where you can remove all the plugs with getting your hands and arms burnt. Open the throttle and wedge something in to keep them open. It best to have a helper or a remote switch to crank over the engine. The first time the piston come up during the compression stroke the gauge should see an immediate pressure increase. The second and third compression strokes adds very little but is needed to get an accurate reading.
There is no defined reading. Engines will vary depending on the components. I would think a stock engine should be in the 150-160 range. I did a compression test just a few days ago to give me an idea as to where I was at with a new engine that was getting some pinging. I was not surprised when I was getting readings around 200.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Squirt in some oil and check compression again.
If rings are bad a squirt of oil in the cyl will show an increase in comp if rings are worn - but no increase if its the vlvs are leaking by. Don't ask me what if both the rings and vlvs are leaking by.
My guess u would still see increase in comp with a squirt of oil.
Also record the number of comp strokes/eng revolutions to reach peak press - this tells u condition of cam lobes too (i think i recall
) as low lift lobes will take more revolutions.
mandm1200 pretty much nailed the proceedure (and numbers).
But i like the leakdown test as it tells u the cyl condition right now but the test is almost $100 and u need an 80-90psi air source to make it work.
Good luck and post your results. cardo0
My guess u would still see increase in comp with a squirt of oil. Also record the number of comp strokes/eng revolutions to reach peak press - this tells u condition of cam lobes too (i think i recall ) as low lift lobes will take more revolutions. mandm1200 pretty much nailed the proceedure (and numbers).But i like the leakdown test as it tells u the cyl condition right now but the test is almost $100 and u need an 80-90psi air source to make it work.
Good luck and post your results. cardo0
