Compression and Detonation Testing, #2


Here is part #2 of the series. Part #1 can be read here: Compression and Detonation Testing, #1

I am currently assembling the test motor, but a builder wrote in with a valid question: He asked why was I so sure that the motor would make flight power with very low differential compression? The answer is this: I have 28 years of experience on this, and there have been plenty of chances to see builders’ engines running after they made a mistake that blew out a head gasket. Below are two examples pulled directly off my traditional website, long ago documented. However, after our current tests, we will have sharp comparative numbers on a back to back test. For today, some older evidence that provides the basis of my understanding.


Above is a cylinder pictured at Corvair College #20. This is the cylinder that was removed from John Neff’s engine with a blown head gasket. The commentary on the original 2011 photo says this:

“If you look closely you can see that it has molten aluminum stuck to the side of the iron cylinder. Again, this did not stop the engine from running or producing power.”

You can read the full comments here, where they have been for the last 6 years:  The fact the ignition timing on this engine was not set with a light before it was flown was the root cause of this. This is what severe detonation does to a Corvair flight engine. A large part of why the engine keeps running is the forged pistons do not blow out first, thus the engine can keep turning and making power. 100% of original Corvair cars had cast pistons which tend to break or get holed before the head gasket fully blows. Thus Corvair car experience and opinion has no value when looking at flight engines and evaluating them for detonation. If car mechanics were qualified to work on aircraft installations, an ASE car mechanic could do an annual on a Cessna 172 engine. They can’t because the two different experiences don’t overlap very much, and in some ways work against each other.



Above is Gary Coppen’s Corvair Skycoupe. I have been friends with Gary for nearly 20 years. He will gladly tell anyone the story of how he flew a 90 mile cross country in the plane pictured above, with ZERO differential compression in cylinder #4.


In 2004, Gary had the plane based at Spencer airfield in north central Florida. It is a rough 2,000′ grass strip surrounded by 50 foot trees.  At the time our shop was at Edgewater airport, 10 miles south of Daytona Beach. Gary called to say the airplane seemed slightly down on power, but he had been operating it that way for 10 hours or so. I told him I would come up and get a look in a week. His solution was to fly it down to our shop. I tested it when he got there, and it has absolutely zero differential compression in #4. At the time the plane had bolt on head pipes, and instead of using the specified Clark’s C-12A gasket, he had made one, and it had caused a vacuum leak that leaned out and detonated #4 until the head gasket was blown. The plane had flown the cross country without issue, Gary just said the ROC was down from 800’/min. to 600’/min.


These are but two of many stories I know that show the engine will run and produce flight power with a blown head gasket and zero differential compression. The secondary moral of these examples is they were both preventable.


Beyond this, I have has cases where builders forgot to put a base gasket under a cylinder, or even missed a head gasket. I have had cases where car head shops failed to deck the bottom of the head after cutting the head gasket areas, so the cylinders ‘shouldered’ on the underside of the head. When these motors run, they let out a distinctive ‘chirp’ sound, but people miss this, even though it is audible over the open exhaust and prop. Again, if you use our parts, and follow the directions, these are not issues you will encounter. On the other side of the coin, people who don’t use timing lights, nor respect direct warnings have trouble in their future.






About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 30 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at and in more than 50 magazine articles.

One Response to Compression and Detonation Testing, #2

  1. Dan Branstrom says:

    Gary Coppen at least has 1 real world data point for Critical Understanding #5, Knowing “+ROC/5” Rate of Climb on Five cylinders. It’s probably a good thing he wasn’t flying at gross out of his field.

    It’s a testimony to the advantage of having 6 cylinders rather than 4.

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