When considering the purchase of a new racecar or evaluating the health of the engine in the racecar you already own, compression and leakdown tests are a decent way to assess its condition.
We use the word “decent” because the Society of Automotive Engineers published a paper in 2011 saying it would like to see testing variables reduced. We didn’t read the entire paper, which was behind a paywall, but we were able to read the executive summary, below:
“Compression and leak-down tests are frequently used to identify and diagnose failed engine power cylinders. It is also often desirable in research and testing programs to use these tests to monitor incremental changes in cylinder leakage. This paper investigates whether these tests are adequate in their present form to monitor incremental changes in cylinder leakage. Results are presented from two vehicle fleets at two test sites.
“Compression and leak-down tests were conducted on these fleets periodically during a mileage accumulation study. The results were used to establish the variability inherent in the compression and leak-down test processes. Comparisons between the results at the initial mileage test for the study vehicles with those at the final mileage test are shown to be largely within the uncertainty established for repeat assessments. The relative overlap in the magnitude of leakage changes and measurement uncertainty shows that the leak-down and compression tests are not useful in their present forms for monitoring incremental changes in engine leakage. Recommendations for reducing the variability of the leak-down test, perhaps making it useful for future studies, are presented.”
It is important to note the use of the words “incremental changes,” which was the purpose of the study. That means the margin of error is significant enough to mean the tests are not well suited to measuring small changes over time, but as tools for evaluating an engine’s health, compression and leakdown tests are the most effective methods we have to work with. The good news is that they are not terribly difficult.
Compression tests, like the one we did in the April 2024 issue of Speed News, demonstrate a cylinder’s ability to build pressure. A leakdown test measures its ability to hold pressure, and it provides excellent insight where the leakage is occurring. Think of a compression test as a snapshot, and a leakdown test as a panoramic image.
This is the 2.5 engine we swapped into a 2006 Mazda MX-5 in April 2023. At that time the engine only had 20,000 miles on it, and after one season of NASA Time Trial, we were hoping it would still be in good shape.
As with a compression test, it is important to warm the engine to operating temperature so that the piston rings are expanded, the oil is fully circulated and filling in the ring grooves and lands, and serving its purpose as a lubricant and a gasket. If you have an engine with gapless piston rings, leakage figures will be less than traditional rings with a thermal expansion gap.
To ready the car, you will need to bring the cylinder you are measuring up to top dead center on the compression stroke. There are a number of ways to do that, depending on the engine, using its timing marks on the crankshaft pulley and timing cover, and a zip tie in the cylinder to determine the top of the piston’s stroke. Because you are going to be testing all cylinders, it’s generally easiest to test them in firing order.
We were 360 degrees off the first time we pressurized the cylinder and it was easy to tell because the tailpipe was hissing like a feral cat.
Understand that highly compressed air can actually spin the rotating assembly, so you should always remove the wrench or socket from the crankshaft pulley, and have a way to keep the engine from spinning. With the engine in the car, we were able to put the car in gear and set the parking brake.
For the actual test, the tool differs from that used for a compression test. A leakdown test uses a differential pressure gauge to measure the delta between air going in and air escaping. The methods can vary a bit by tool, but for the most part, you need about 100 pounds of compressed air connected to the differential pressure gauge. In addition to the gauges, a mechanic’s stethoscope is a good tool to have when doing a leakdown test. It will help you hear the minor leaks.
Start by threading the hose and fitting into the spark plug hole. With the test gauges hooked up to the compressor, “zero” the gauge. The gauge we used had a zone marked “set,” and we were able to use the adjuster knob to get the needle to rest right on zero.
Once you have zeroed the gauge, connect the gauge to the hose that is threaded into the spark plug hole, and the cylinder leakage gauge will spring to life and indicate the health of that particular cylinder.
To make sense of the numbers, figure zero to 15 percent leakage is an excellent result. From 16 to 25 percent is still good. Readings between 26 to 40 percent are fair and are usually accompanied by reduced performance. Anything from 41 percent on up means you have problems that need to be diagnosed and repaired for the engine to perform as it should.
For example, a leakdown test can help determine whether an exhaust valve or valve seat is leaking because you will actually be able to hear air escaping the cylinder at the tailpipe or header collector. A leaking intake valve will reveal itself via noise coming through the intake or throttle body. Faulty piston rings, worn cylinders or damaged pistons will allow air to escape into the crankcase, which you can often detect by listening to the opening of the oil dipstick tube or the oil filler cap in the valve cover. Finally, you also can detect a faulty head gasket by removing the radiator or expansion tank cap and checking for bubbles or movement in the fluid.
A compression test does none of those things, which is why a leakdown provides a more complete picture. When performing the tests on all cylinders, you want uniformity among them, and ideally they should fall within 4 to 5 percent of one another.
