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 ??
Can someone describe how to do a comp test and exactly what will that tell me. Condition of rings? Valves?
What should I look for on a 74 BB?
I have the compression gauge...just never used it.
Thanks.....Fred
What should I look for on a 74 BB?
I have the compression gauge...just never used it.
Thanks.....Fred
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
Remove all the spark plugs. Wire the throttle open all the way. Put the tester on a cylinder and crank the engine about 5-6 revolutions. Write down the number and then repeat on the other cylinders.
You're looking for consitency, not a magic number. If they're all within 5% you're good. within 10% is acceptable. If you have one or more that are significantly lower, squirt some oil into the plug hole and repeat the test. If the cylinder comes up to normal you have bad rings. If it only comes up a little, bad valves.
You're looking for consitency, not a magic number. If they're all within 5% you're good. within 10% is acceptable. If you have one or more that are significantly lower, squirt some oil into the plug hole and repeat the test. If the cylinder comes up to normal you have bad rings. If it only comes up a little, bad valves.
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
One other thing: Do the comp test on a hot engine. A cold engine will not give you correct numbers and consistency due to clearances not being correct when the engine is cold.
Another test you might want to consider is a leakdown test. A regular comp test will give you comparative data from one cylinder to another, but will not give you absolute data to determine overall condition. A leakdown test gives you actual data to determine each cylinder actual condition, and the results can be better utilized to determine work that may be needed.
Another test you might want to consider is a leakdown test. A regular comp test will give you comparative data from one cylinder to another, but will not give you absolute data to determine overall condition. A leakdown test gives you actual data to determine each cylinder actual condition, and the results can be better utilized to determine work that may be needed.
