(Image/Wayne Scraba)

If you build engines or if you campaign a race car (or any street-strip car for that matter), a leakdown tester is pretty much a go-to tool.

Why Do a Leakdown Test Instead of a Compression Test?

Simple. There are too many variables involved in a compression test. One good example is engine cranking speed. Too slow and it can have an effect upon the test. Valve lash can have an effect. So can air density. And those are just a few.

When performing a compression test, the idea is to compare the test numbers to a factory specification. If the engine in question is modified (for example, more compression ratio), there is no factory spec to compare it to. Additionally, if a cylinder or two (or all of them) are down, you’ll know there’s a problem. But it becomes a bit more difficult to actually pinpoint that problem.

How to Perform an Engine Leakdown Test

In order to perform a leakdown test, the spark plugs are removed and the engine is rotated so that the cylinder you’re testing is at Top Dead Center. Next, the leakdown tester hose is screwed into the spark plug hole for the test cylinder. Connect the other end of the leakdown test to a compressed air source (typically, 100 or more PSI works). Set the regulator on the leakdown tester to 100 PSI on the first gauge. The second gauge on the leakdown tester indicates the amount of pressure actually in the cylinder. This number is subtracted from the first reading of 100. This allows you to come up with a percentage of leakdown in the cylinder.

Due to the fact the air supply is completely regulated, a leakdown test is much more consistent than a compression test.

It also means the leakdown test can tell you when your high performance engine has said “Enough! It’s time for a rebuild.”

The bellwether is this: With a fresh engine (and with all of the cylinders having close to the same leakdown figures), the engine shouldn’t leak more than 3%. If the engine leaks down 5%, its most likely still healthy. Once it approaches or exceeds 10-15% leakage, it’s time to tear it down.

But That’s Not All A Leakdown Test Can Tell You

You can use a leakdown test to pinpoint other issues. Since a sizeable volume of air is introduced into a cylinder, it obviously has to escape somewhere—even if the amount of leakage is only 3%.

Air leaking from the carburetor or throttle body typically means there’s an intake valve issue. It could be bent or leaking. Air escaping from the exhaust indicates there’s a similar issue with the exhaust valve. If air is leaking from both the carburetor and the exhaust, you have a serious valve issue on both the intake and exhaust (often multiple bent valves).

Air leakage through the carb or throttle body usually takes the form of a whistle. Ditto with air escaping from the exhaust, although headers can change the sound. Air leaking from the oil filler cap (or breathers) indicates bad rings or a damaged piston. This usually sounds like a whistle. Air leaking into the radiator coolant (bubbles) indicates a blown head gasket (or worse—it could indicate a cracked cylinder head or block).

Some Leakdown Tester Basics

As you can see, a leakdown tester can prove to be an extremely versatile tool. There are a ton of different leakdown testers on the market, so we’ll highlight some below and give you the basics of a dual gauge system too. Check it out:

This is the writer’s ancient (and now discontinued) Manley dual gauge leakdown tester. It’s been in my toolbox for 40 years. (Image/Wayne Scraba)
The tool is hooked to an air compressor on this end. (Image/Wayne Scraba)
The other end screws into a spark plug hole. (Image/Wayne Scraba)
The regulator is set at 100 PSI on the gauge closest to the regulator. Meanwhile the second gauge measures the air in the cylinder. The difference between the two is the cylinder leakdown. (Image/Wayne Scraba)
This dual gauge leakdown tester from Longacre Racing Products comes with an easy-to-use regulator and the gauges are protected by rubber bumpers. (Image/Summit Racing)
Here’s a leakdown test kit from OTC that includes 10mm, 12mm, 14mm, 18mm spark plug adapters. (Image/Summit Racing)
This Longacre leakdown test setup includes high end digital gauges. It’s capable of determining cylinder leakdown to half of 1%. The gauges are battery powered and they’re also backlit for excellent readability. (Image/Summit Racing)
Here’s a Moroso single gauge leakdown tester. Included are 14mm and 18mm spark plug adapters. In operation, it’s first hooked to an air supply (80 to 100 PSI). Zero by turning the knob on the regulator. Screw the plug adapter into the cylinder and then read the percentage of air leaking from the cylinder. (Image/Summit Racing)
This popular dual gauge leakdown tester from Total Seal features an adjustable left hand gauge coupled with a precision regulator and a large easy-to-read right hand gauge. With an adapter they can also be used to set injection barrel valves. 14mm plug setups are standard; 18mm spark plug thread hose assemblies are available separately. (Image/Summit Racing)

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Author: Wayne Scraba

Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4,500 tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.