Compression And Vacuum Tests
Apr. 21-22, 2007
 Jeanne assists Lynn with the first round of compression tests. |
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Finding The Time
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This venerable compression tester has been used by Lynn for over 40 years (left).
After performing one flawed test with the old gauge, Mr. Highpockets (aka Lynn)
springs for a new tester (right) which also allows for easy one-man testing.
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Lynn's been meaning to make compression and vacuum measurements on Penny since he bought the car last October. But she runs fairly well and there always seemed to be more pressing needs, like brakes and lights.
Finally on Apr. 21 Lynn finds that he has the time to do a test. He drags out the compression tester that he's been using since the mid 1960s and persuades Jeanne to sit behind the wheel and press the starter switch for him.
After taking and analyzing the data, Lynn realizes that he didn't perform the test with an open throttle. Also, Lynn has to work hard to get the old tester to seal well in the spark plug hole. Maybe it's time for him to buy a new tester.
Lynn locates a nice tester at a local auto parts store. In addition to the 14mm adapter he needs for Penny, it also has an 18mm adapter that he needs to test Bugsby. The tester also allows for one-man testing, so he doesn't have to wait for someone else to help him with the test.
Compression Test Results
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Compression test results.
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The results of the second compression test on Penny are displayed in the preceding table. Lynn performs two trials, the second one after he's squirted some engine oil in the cylinders. Big differences between the dry and oil trials would indicate issues with the rings.
From Lynn's perspective everything looks good. Compression varies by 7 PSI (6-7%) across all the cylinders. Compression is increased by 3-10 PSI by adding oil to the cylinders, indicating some leakage past the rings, but not an excessive amount. The test confirms Lynn's seat-of-the-pants reading that the engine is running well.
According to a formula at the bottom of the preceding table, Lynn would have expected to obtain compression readings of about 124 PSI, somewhat higher than the 110 PSI readings he measured. Lynn thinks that he should do a valve adjustment to make sure that they have sufficient lash to fully close. He will definitely be looking forward to taking the head off the engine at some time in the future to have a closer look at how well the valves are seating.
Vacuum Test Results
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The vacuum test on Penny shows a rock-solid, healthy 21.5"Hg of vacuum in the intake manifold.
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Lynn also measures the intake manifold vacuum at idle and at various engine speeds. He observes a rock steady meter reading of 21.5"Hg.
This behavior in the vacuum indicates that valves, valve springs, valve guides and rings are moving and sealing well, and that the timing is approximately correct. The measurement reassures Lynn that the engine is performing well and there is no need for immediate repair actions.
Ignition Tests, Yet To Be Done
Additional tests that Lynn should do is measure the timing of the ignition and the dwell angle of the ignition points. Maybe next week.
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with your comments or questionsCopyright © 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Lynn Kissel
Last updated: May 24, 2009