TABSTER
Active VIP Member
Compression Test
A compression test is done (obviously) with a compression gauge. This is basically a pressure gauge that has a one-way check valve and an adapter to screw into a spark plug hole. With a good battery and a warm engine, remove all the spark plugs. The compression tester is screwed in place of one spark plug and while holding the throttle plate open, the engine is cranked over. The engine is allowed to rotate a few times to build up pressure and this pressure is recorded. Each of the cylinders are checked in the same way.
In my opinion, all cylinders should be within 4 or 5% of each other. If you have a cylinder that has excessively low compression, try adding a few drops of oil. If the compression comes up, you have a ring seating problem. If it does not, you have a valve or head gasket problem, or some other problem.
Benefits:
Quick and easy
All shops can do it
Gives a general idea of the internal engine condition
Drawbacks:
Doesn't pinpoint problem areas
Unless done on a regular basis, you have little reference to the actual condition
Leak Down Test
A leak down test is more specific in pinpointing the condition of an engine and if there are problems, it can pinpoint the needed area of attention. A leak down test is performed with a special gauge and manifold set. It consists of a calibrated pressure gauge and a pressure regulator and adapters to connect it to the spark plug hole. Compressed air (or nitrogen) is fed into the gauge and the gauge is calibrated against a zero reference (no air leakage). The adapter is then screwed into the number one spark plug hole (with cylinder #1 at TDC compression) and the gauge is attached. As soon as the gauge is attached to the adapter air will begin to fill the cylinder. As this happens, the gauge will begin to indicate the amount of air flowing into the cylinder.
If air is leaking past the rings, or the valves, or the head gasket, you can directly read the amount of leakage and can easily find the leak. If there is air blowing out the crank vent, you have a ring leak, if it's coming out of the exhaust, you have an exhaust valve leak, and so on...
A leak down test is especially good at discovering leaky valves. I use much less pressure (about 60 psi) than is normally in the combustion chamber (130 plus psi). If there is a valve that is slightly off its seat, the actual combustion pressure may have helped close it, but the lower testing pressure won't exert as much force and the leak will be apparent.
This test is time-consuming. To do the test, each cylinder must be tested one at a time, they must be at TDC compression, and the crank must be held in place to prevent rotation. A freshly rebuilt engine should have less that 4% leak down on each cylinder and if leak down is above 10%, engine service should be performed. All cylinders should be within 5 % of each other. NOTE: Stock engines may have as much as 10% right from production.
Benefits:
Direct reading of each cylinder's condition
Pin points any problem areas
Drawbacks:
Difficult to perform
Time consuming
Requires source of steady air pressure
****If you ask a shop to do a leak down test, they may offer to do a compression test or may hesitate or quote a high price for the work. This is not uncommon -- it is not a simple task. If the shop that you use doesn't know what a leak down test is, then you should leave. They do not have the proper diagnostic knowledge to be working on your engine.****
**** Now all that being said, keep in mind that the timing of your valves with Boost will be advanced/retarded making them differant than Stock valve Timing.*****
A compression test is done (obviously) with a compression gauge. This is basically a pressure gauge that has a one-way check valve and an adapter to screw into a spark plug hole. With a good battery and a warm engine, remove all the spark plugs. The compression tester is screwed in place of one spark plug and while holding the throttle plate open, the engine is cranked over. The engine is allowed to rotate a few times to build up pressure and this pressure is recorded. Each of the cylinders are checked in the same way.
In my opinion, all cylinders should be within 4 or 5% of each other. If you have a cylinder that has excessively low compression, try adding a few drops of oil. If the compression comes up, you have a ring seating problem. If it does not, you have a valve or head gasket problem, or some other problem.
Benefits:
Quick and easy
All shops can do it
Gives a general idea of the internal engine condition
Drawbacks:
Doesn't pinpoint problem areas
Unless done on a regular basis, you have little reference to the actual condition
Leak Down Test
A leak down test is more specific in pinpointing the condition of an engine and if there are problems, it can pinpoint the needed area of attention. A leak down test is performed with a special gauge and manifold set. It consists of a calibrated pressure gauge and a pressure regulator and adapters to connect it to the spark plug hole. Compressed air (or nitrogen) is fed into the gauge and the gauge is calibrated against a zero reference (no air leakage). The adapter is then screwed into the number one spark plug hole (with cylinder #1 at TDC compression) and the gauge is attached. As soon as the gauge is attached to the adapter air will begin to fill the cylinder. As this happens, the gauge will begin to indicate the amount of air flowing into the cylinder.
If air is leaking past the rings, or the valves, or the head gasket, you can directly read the amount of leakage and can easily find the leak. If there is air blowing out the crank vent, you have a ring leak, if it's coming out of the exhaust, you have an exhaust valve leak, and so on...
A leak down test is especially good at discovering leaky valves. I use much less pressure (about 60 psi) than is normally in the combustion chamber (130 plus psi). If there is a valve that is slightly off its seat, the actual combustion pressure may have helped close it, but the lower testing pressure won't exert as much force and the leak will be apparent.
This test is time-consuming. To do the test, each cylinder must be tested one at a time, they must be at TDC compression, and the crank must be held in place to prevent rotation. A freshly rebuilt engine should have less that 4% leak down on each cylinder and if leak down is above 10%, engine service should be performed. All cylinders should be within 5 % of each other. NOTE: Stock engines may have as much as 10% right from production.
Benefits:
Direct reading of each cylinder's condition
Pin points any problem areas
Drawbacks:
Difficult to perform
Time consuming
Requires source of steady air pressure
****If you ask a shop to do a leak down test, they may offer to do a compression test or may hesitate or quote a high price for the work. This is not uncommon -- it is not a simple task. If the shop that you use doesn't know what a leak down test is, then you should leave. They do not have the proper diagnostic knowledge to be working on your engine.****
**** Now all that being said, keep in mind that the timing of your valves with Boost will be advanced/retarded making them differant than Stock valve Timing.*****
