The Leakdown Theory

Leakdown TesterMarch - April 1996 Lane Racing And Rodding Article By Jim Kaekel, Jr.

Many racers occasionally experience problems with either sudden or gradual loss of performance and will sometimes go to great (and unnecessary) lengths to get to the source of the problem.

One way to avoid many problems is by simply doing some preventative maintenance. Cylinder "leak testing" is a good way to keep an eye on your engine's condition. A cylinder leak test is done to determine the effectiveness of the piston ring as well as valve and head gasket seal. While it is similar to a compression test, it is much more accurate, and can be used to pinpoint a particular problem.

To perform a "leak test," you would need an adequate supply of compressed air and a leakage tester, such as the dual gauge model (#8015) made by Tavia and popular with many racers. You would begin the test by warming your engine to operating temperature. Next, the number one spark plug is removed and the engine is rotated until the number one cylinder is at top dead center. Thread the leak tester hose into the spark plug hole and attach the other end of the tester to your air supply. Adjust the regulator on the tester so that it reads 100 psi. - the amount of air entering the cylinder. Then, read the opposite gauge which will show the amount of air being sealed in the cylinder. If the regulator gauge shows 100 psi. and the opposite gauge reads 97 psi., this shows a 3% rate of leakage on that cylinder.

To continue, rotate the engine 90 degrees to the next cylinder in the firing order and so on, re­peat­ing the same procedure and recording the information each time. There are several important tests you should perform as you measure leakage in each cylinder.

First, listen at the crankcase breather for any signs of air leakage. If you hear air at this point, it's leaking past the piston rings. Next, remove the radiator cap and watch for bubbles in the coolant. This indicates a possible head gasket failure and a more in-depth check should immediately follow. If you can hear an air leak through an open throt­tle, an intake valve is not sealing properly and could be burned, bent or cracked. If you can hear an air leak at the end of the header collector, you've got a similar prob­lem with an exhaust valve.

It is common to hear a slight leak of air through the crankcase breather or a negligible amount at the valves. Just remember, that the important thing is that all of your readings should show similar numbers. Engines with conventional piston rings will leak anywhere from 8% to 12%. Some serious race engines equipped with low tension rings such as a "Dykes" type top rings may leak as much as 20% because these rings are designed to seal under maximum cylinder pressure. If a gapless type ring is used, such as a Childs and Albert ZGS or a Total Seal, leakage readings of 1% to 3% can be expected.

Keep in mind that just because one particular engine may leak 12 percent and another identical power plant leaks 2 percent does not necessarily mean that the latter will make more horsepower. Many things enter the picture when it comes to proper ring selection. Often a ring may be selected because of less rotational drag on the cylinder, which in turn increases power.

After you gain experience in "leak testing" your engine, you will learn what to expect for a normal reading. Readings will vary if the engine builder decides to change ring types or tensions of the compression rings.

Don't push the panic button if you suddenly have a cylinder that is leaking more than normal. Occasionally, compression rings turn and the end gaps can align with one another to create a con­ve­nient path for combustible gases to follow. Always check a potential problem again at a later date to insure that your suspicions are false. It will save lots of grief in the long run and you will avoid running over your crankshaft and oiling down your favorite track.

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