The 383 in my ‘68 Road Runner is lightly modified with a 750cfm Holley carburetor, headers, Edelbrock RPM dual plane intake, Mopar electronic ignition, and a mild hydraulic cam.

I’ve had it since 1998 and I’ve put about 28,000 miles on the car. The engine runs good but it uses about a quart of oil in 900 to 1,000 miles. It does not really leak. The engine does have some “blow by” coming out the valve cover breather when it’s warm.

I have a good compression tester, and have heard about leakdown testers. Will those tests tell me if it needs valve work or new pistons/rings? I’ve heard that many times it’s more economical to get new aluminum heads than to rebuild my existing “906” heads? But if it needs new rings/pistons/bearings, I will plan a full rebuild.

Will the compression and leak down tests determine which way to proceed? Thanks so much for your advice.

J.P.

The short answer is I think a compression test will tell you more than a leakdown test. For those who may not know what a leakdown test is, this involves a tool that pumps air pressure into a cylinder with both valves closed and displays on a gauge the amount of leakage that is expressed as a percentage.

Most mild street engines will probably test at 10 to perhaps 20 percent leakage. This is not an unusually high number. Race engines generally shoot for much lower numbers of less than five percent, but street engines rarely achieve this rather low percentage. Of course, if you find one or two cylinders with exceptionally high leakdown rates, that would indicate a problem in those cylinders. However, it does not sound like that is your problem. A leakdown test unfortunately will not necessarily indicate a problem involving high oil usage. That’s why I think a compression test is a better overall indicator.

For the compression test, look for all eight cylinders to be within 10 percent of each other but also you will be looking at the actual pressure reading. This should be around 150 to 180 psi. A higher number means more cylinder pressure which is a good thing. It’s best to test each cylinder the exact same way. If you discover one cylinder at 185 psi for example but another at only 155 psi, then that cylinder clearly is under-performing but may not necessarily indicate oil usage. 

Let’s say that you find a weak cylinder and there’s more oil on that spark plug than on the other plugs. That would be a clear indicator of a problem. But frankly I’m not sure that 1,000 miles for a quart of oil is really all that bad and you might just be chasing a problem that isn’t necessarily excessive.

If the cranking pressure test comes in even at 175 to 185 psi for example, this test would not point to a problem. If the engine smokes slightly on deceleration then it might just be valve guide seals. There are some really good valve guide seals that will probably require machine work to install on your heads that might be one way to reduce the amount of oil the engine uses.

Do you know what kind of valve guide seals are on the heads now? I’m not really conversant on big block Mopars but I believe these engines use an umbrella seal on the intake valves. These seals can become brittle with time and heat cycles and easily fail. If so, then this is a good indicator of an oil consumption problem. Intake valves with poor valve guide seals can easily pull oil through the valve guide and into the combustion chamber.

It might be worth the effort to pull a valve cover and look if you can see which seals are used over both the intake and exhaust valves. Umbrella seals get their name because they merely push over the valve stem and run up and down with the valve acting like a rain umbrella to control the amount of oil that runs past the intake valve guide to lubricate the valve stem. Positive type seals are a far better design that are intended to be pressed over and adhere to the valve guide. This positive seal is a far better way to control oil but will require machine work on the guide. This means you will have to remove the heads to perform this process. 

Sometimes substituting a higher viscosity oil may improve oil mileage. Assuming the current package is something like a 10w30, you could try a 10w40 oil and measure its mileage performance. Sometimes just changing brand of oil can make a difference.

Another possibility is that an overly aggressive PCV valve can pull quite a bit of oil out of the engine and yet not be displayed with oil smoke out the exhaust. One way to investigate this possibility is to install a vapor separator between the PCV valve and the intake manifold or carburetor supplying the vacuum source. Drive the engine for 500 miles and then inspect the reservoir for the amount of oil that was prevented from entering the intake manifold. It’s possible that this could further reduce your oil usage or at least point you in the right direction in terms of improving oil mileage.

This is a leakdown tester. The gauge on the left displays inlet air pressure while the gauge on the right reveals the amount of leakage. This test is on a near-dead small block with 40 percent leakage which is a serious problem. A well sealed but used engine should be 10 percent or less. (Image/Jeff Smith)
This is a positive seal positioned on a small block Chevy cylinder head but illustrates the style of seal. These can be easily installed on virtually any cylinder head. (Image/Jeff Smith)
Keywords
compression test, leakdown test, oil consumption
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.