A Proper Compression Test
09-08-2008, 12:34 AM
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A Proper Compression Test
A discussion today with my son prompts these tips for reading compression and what it means.
There are 2 types of tests, dynamic and static. Dynamic is spinning the engine with a gauge in the plug hole. It will give you the # of pounds the piston pumps up. A hot engine will usually give better compression than a cold engine because the pistons and rings are expanded with heat. I personally have little use for dynamic testing because of getting fairly good readings with it verses bad readings with static testing on the same engine.
Static testing is setting a piston at tdc, valves closed, and using a compressor to build up a pressure in the cylinder thru a device with 2 gauges that are on the line that goes into the plug hole. Observing the difference of the gauge pressure going into the cylinder and the gauge that shows the pressure held in the cylinder chamber, gives you the information on the conditions.
I use an aircraft gauge with 80 pounds of air, some may use 100 pounds. I like a cold engine; it gives a lower reading and does not burn me. So what is a good reading—many cars ads say the compression is within 5%. The trick is 5% of what—80 pounds, 70 pounds, 60 pounds? Such as 80, 80, 80, 80, 80, 76 pounds. Or say 60,60,60,60,60,57 pounds. All are within 5% but one engine that is newish and one worn. GET THE ACTUAL NUMBERS OF EACH CYLINDER COLD. 70 thru 75 will work for me, but you need to establish the numbers that are best for you. All 80s with a 55 or so means trouble, OR does it? Could be a valve adjusted too tight. Might be a piece of carbon under the valve keeping it from sealing. With a cold engine, checking the valve clearance is easy. Staking a valve with a carbon chip has worked for me. Put the piston down; with the cylinder pressurized, tap the intake and exhaust valves with a brass hammer to bounce them off the seat. This will sometimes blow out the debris and make the valve seat tight to get a better reading. Taping on the loosened rocker may not be a good idea; however I have done it once without breaking anything.
Assume that this is a car you own and you have gotten a reading that is not good. How do you identify the problem causing the low pressure, ruling out a loose head?
Easy------------
1. A hiss thru the exhaust pipe is an exhaust valve.
2. A hiss thru the intake air box is an intake valve.
3. A hiss thru the oil filler tube is worn rings, or broken ring/piston.
There are 2 types of tests, dynamic and static. Dynamic is spinning the engine with a gauge in the plug hole. It will give you the # of pounds the piston pumps up. A hot engine will usually give better compression than a cold engine because the pistons and rings are expanded with heat. I personally have little use for dynamic testing because of getting fairly good readings with it verses bad readings with static testing on the same engine.
Static testing is setting a piston at tdc, valves closed, and using a compressor to build up a pressure in the cylinder thru a device with 2 gauges that are on the line that goes into the plug hole. Observing the difference of the gauge pressure going into the cylinder and the gauge that shows the pressure held in the cylinder chamber, gives you the information on the conditions.
I use an aircraft gauge with 80 pounds of air, some may use 100 pounds. I like a cold engine; it gives a lower reading and does not burn me. So what is a good reading—many cars ads say the compression is within 5%. The trick is 5% of what—80 pounds, 70 pounds, 60 pounds? Such as 80, 80, 80, 80, 80, 76 pounds. Or say 60,60,60,60,60,57 pounds. All are within 5% but one engine that is newish and one worn. GET THE ACTUAL NUMBERS OF EACH CYLINDER COLD. 70 thru 75 will work for me, but you need to establish the numbers that are best for you. All 80s with a 55 or so means trouble, OR does it? Could be a valve adjusted too tight. Might be a piece of carbon under the valve keeping it from sealing. With a cold engine, checking the valve clearance is easy. Staking a valve with a carbon chip has worked for me. Put the piston down; with the cylinder pressurized, tap the intake and exhaust valves with a brass hammer to bounce them off the seat. This will sometimes blow out the debris and make the valve seat tight to get a better reading. Taping on the loosened rocker may not be a good idea; however I have done it once without breaking anything.
Assume that this is a car you own and you have gotten a reading that is not good. How do you identify the problem causing the low pressure, ruling out a loose head?
Easy------------
1. A hiss thru the exhaust pipe is an exhaust valve.
2. A hiss thru the intake air box is an intake valve.
3. A hiss thru the oil filler tube is worn rings, or broken ring/piston.
