Proper way to test compression?
Proper way to test compression? Hey guys, just wondering what the title says? I've heard of people testing their compression and I want to get mine tested but I didn't know how exactly it's done?
Teching In
Joined: Oct 2004
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On the Gen I hardware to which I can claim moderate experience I would take out all the spark plugs, disconnect the coil, screw the compression gauge into the spark plug hole on cylinder one, spin the motor a few times with the starter and write down the result. Repeat seven times.
I am not going to guess at the values you ought to be seeing on an LSx motor, I am sure plenty of regulars here can set you straight on that one.
If one or any or all of the Gen I cylinders seemed to have low compression, the second time around I would put maybe 3cc of motor oil through the spark plug hole into the combustion chamber and record the "wet" values in a column next to the original 'dry' values. If the wet numbers are higher suspect rings, if the wet numbers are the same expect valve seat or valve guide problems. I tend to record the date of the test and the mileage on the motor on the same page, Keep it inthe book with the rest of the motor stuff.
That's the quick and dirty anyway. A cam with a lot of overlap will show less compression than a cam with less overlap. If the dry numbers are good skip the wet test.
I am not going to guess at the values you ought to be seeing on an LSx motor, I am sure plenty of regulars here can set you straight on that one.
If one or any or all of the Gen I cylinders seemed to have low compression, the second time around I would put maybe 3cc of motor oil through the spark plug hole into the combustion chamber and record the "wet" values in a column next to the original 'dry' values. If the wet numbers are higher suspect rings, if the wet numbers are the same expect valve seat or valve guide problems. I tend to record the date of the test and the mileage on the motor on the same page, Keep it inthe book with the rest of the motor stuff.
That's the quick and dirty anyway. A cam with a lot of overlap will show less compression than a cam with less overlap. If the dry numbers are good skip the wet test.
Do not crank until you get the highest pressure on the guage. There is no exhaust scavenging @ 300 RPM to pull the excess charge out, and the reading will be incorrect.
Crank it 4 "puff's", stop cranking, and record that reading.
Crank it 4 "puff's", stop cranking, and record that reading.
Originally Posted by NO-OPTION-2002
Do not crank until you get the highest pressure on the guage. There is no exhaust scavenging @ 300 RPM to pull the excess charge out, and the reading will be incorrect.
Crank it 4 "puff's", stop cranking, and record that reading.
Crank it 4 "puff's", stop cranking, and record that reading.
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EDIT
If you only are checking for a low/bad cylinder, anyway you want to check it is ok as long as you are consistant.
If checking cranking pressure for a performance tuning reference, then the 4 puffs rule applies.
If you only are checking for a low/bad cylinder, anyway you want to check it is ok as long as you are consistant.
If checking cranking pressure for a performance tuning reference, then the 4 puffs rule applies.
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And why is that?
The 4 puff's give a *fairly* good approximation of running engine compression pressure. That's not saying combustion pressure.
Running compression psi can be a good tuning tool, especially if you are on the edge of your octane capability.
Originally Posted by LOnSLO
Actually, IMO, the first "puff", or hit is the most important for checking performance. But others may disagree 
Also, you should check with the engine at operating temp.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
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From: So.Cal.
Originally Posted by bowtieman81
Also, you should check with the engine at operating temp.
SWMN has the basic information you need. I want to add that the throttle should be blocked open (about WOT) during the testing. As a rule of thumb, no cylinder should be more than 10% lower than the average. The engine should be cranked until the pressure maximizes, but this wont take more than about 5 or 6 compression strokes. This type of test is good to determine if a valve isn't seating, or if the compression rings aren't sealing.
A much better type of test is a leak-down test. In this test, a special tester is screwed into the spark plug hole and with the piston at BDC with both valves closed, compressed air is introduced into the cylinder at a regulated pressure and flow rate. In the aviation industry, we use 80 PSI as the regulated pressure. The better the rings and valves are seating, the closer the cylinder assembly will hold the regulated 80 PSI. The pressure test reading is shown as a fraction like 75/80 meaning the cylinder will hold 75 PSI at the controlled flow rate (through a specific diameter orifice in the tester). Most of us, including me don't have access to this type of tester, but this is the more ideal way to do the testing.
I hope that this helps.
Steve
A much better type of test is a leak-down test. In this test, a special tester is screwed into the spark plug hole and with the piston at BDC with both valves closed, compressed air is introduced into the cylinder at a regulated pressure and flow rate. In the aviation industry, we use 80 PSI as the regulated pressure. The better the rings and valves are seating, the closer the cylinder assembly will hold the regulated 80 PSI. The pressure test reading is shown as a fraction like 75/80 meaning the cylinder will hold 75 PSI at the controlled flow rate (through a specific diameter orifice in the tester). Most of us, including me don't have access to this type of tester, but this is the more ideal way to do the testing.
I hope that this helps.
Steve
