Critical Understanding #4, ANY loss of RPM is Detonation.


Every Corvair manual I have ever published has contained very stern warnings never to fly any engine that you even suspect might detonate. That is pretty plain, but still a number of builders damage their engine each year by ignoring that warning.


Here is the golden rule of detonation: Anytime your engine experiences a loss of rpm, even as small as 25 rpm, it is detonating, and you must instantly stop or you will damage and possibly break the engine. There is no tolerance for detonation. Where a modern car might reduce the timing or go into limp mode, a pure aircraft engine only has one system to protect it: You. If you do not act, the engine will break.


This is particularly true any time the engine is at wide open throttle. If you are on the take off roll, and the engine rpm sags off even 25 rpm, you must have previously committed your mind to ABORT TAKE OFF. If you get to even 100′ or more, and you have the option of landing straight ahead, even on the over run, ABORT THE FLIGHT, land. Detonation does not get better until one of the factors is removed. If you do nothing, even for a few seconds, the engine will be damaged, in another few seconds it will blow a head gasket, and shortly after that it will potentially stop altogether. Yet every year, we have a number of pilots, mostly who never personally set the timing on their engine, sit there like a bump on a log doing nothing while the power of their engine is decreasing from raw detonation. They are hoping it will go away, the mental mindset of a sheep.


Most common causes of detonation (all preventable)


(one) Not setting the timing.

A subject I have addressed before:


When to check your timing, Lessons learned Pt#2

Ignition Timing on Corvairs

Ignition timing on Corvairs, Part 2


(two) Not Using the Correct Fuel:

I have always recommended using 100LL through all of the test hours. 93 octane fuel can be used once the rest of the systems are validated. UNDER INDIVIDUAL CONDITIONS, some Corvairs with lower compression, perfect timing at a reduced setting, and proven cooling, can use fuels lower than 93 Octane, but this has no across the board approval, and it never has. In 2014, we had a guy destroy his plane on the first flight because he first refused to set the timing for months, then reluctantly did it incorrectly, and went on to fuel his plane with 91 octane car gas. He was at an airport with 100LL, but instead, perhaps to save a few dollars, he instead used 91 car gas. Mind you this is a 150 hour pilot in his 30’s who clearly didn’t have much respect for anything I have to share.

Read: Food for thought on Fuels.


(three) Having a Carb that is running too lean:

If an engine has perfectly set timing and 100LL fuel, it can still be made to detonate by just letting it run too lean.  Traditional aircraft fuels had dual octane ratings like 115/145 “Purple Fuel” the 115 is the octane rating of it running on “Auto Lean” setting and 145 is the octane rating when the same radial and fuel are running on the “Auto Rich” setting. All fuels work like this, so you don’t want some monkey playing around in your carb and you don’t want to use some carb like this:How I became a genius in 6 minutes.

Read this story about aircraft carbs to understand that they automatically run richer at wide open throttle. There is a lot of information in the story: Air / Fuel ratios on Corvair carbs.


(four) Using the wrong spark plugs:

If there is a single issue I will never understand, it is why some builders compulsively must use spark plugs which we don’t recommend. What they gain out of it I don’t understand, but I do know what they stand to lose. Read: A Tale of Two Spark Plugs……


(five) Having an engine with incorrect or  deficient cooling:

I have had many people tell me “I am using your cooling system” but when I look at it, their cowling has untrimmed inlets, no inlet rings, and no lip on the underside of the cowl. All of these make the engine run very hot, and hot engines detonate. Our cooling systems work very well, but this means the builders has to actually build our system, not something that looks like our system. Read more here Corvair Cooling, something of a human issue…..




Your Corvair, if built and installed correctly, need never detonate, not even once. It is a machine, and it will work correctly if set correctly. It doesn’t have a mind of it’s own, it will never turn on you nor will it decide to harm you. There are plenty of builders who have flown hundreds of hours over many years, who have never experienced it once. Conversely, we have had a person, who I directly warned in writing, proceed to destroy his plane on the very first flight, and put himself as well as his illegal passenger in the hospital. Either outcome success or failure, can be replicated, it is completely up to the builder to decide which he will have.


Note Book Section:

Make line 4.1 in your Hand book a hand written entry, to seal in your mind that you recognize that any loss of RPM is Detonation, and it is never acceptable.


Make line 4.2 in your Hand book a notation that you have set the timing on the “A” side of the ignition with a timing light, at full static rpm and it is 30 degrees total. (28 degrees for 93 auto fuel)  Note the actual RPM also.


Make line 4.3 in your Hand book a notation that you have set the timing on the “B” side of the ignition with a timing light, at full static rpm and it is 30 degrees total. (28 degrees for 93 auto fuel)  Note the actual RPM also.


Make line 4.4 in your Hand Book a notation that the first 40 hours will be flown on 100LL fuel.  As the hours are flown, amend the entry with the actual measured fuel burn on an average hour of flight, the minimum hour and the maximum in an hour.


If you choose to use 93 fuel after 40 hours, make line 4.5 in your Hand Book a note stating the timing was reduced to 28 degrees.  Note the new full static rpm. Enter a new test period of 5 hours of solo flight to evaluate the compatibility.  Amend the entry with the actual measured fuel burn on an average hour of flight, the minimum hour and the maximum in an hour.


Make  line 4.6 in your Hand Book read that the only acceptable spark plugs are AC-R44F, or Denso IWF16-5359, IWF20-5359, or IWF22-5359. No other plugs are considered airworthy.


Make  line 4.7 in your Hand Book read the EGT of the engine after running for 30 seconds at full static RPM.  While running, pull the mixture out slowly, and the RPM MUST RISE, and the EGT MUST DROP. Note the numbers in the Hand Book.


Make  line 4.8 in your Hand Book a note of measuring the diameter of inlets, noting minimum as 4.75″, and that they have been equipped with inlet rings. After completing the “Two Minute Test”, note the Static RPM and the CHT in this entry also.





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 20 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.

9 Responses to Critical Understanding #4, ANY loss of RPM is Detonation.

  1. Andy Elliott says:

    WW: Could you please comment on the following scenario, please?
    As an experiment, I had a local prop builder, Gary Hertzler, who is well-respected among the EZ community, build me a “point design” cruise prop for my 3100-powered 601XL taildragger.
    In the EZ community, they generally concentrate very heavily on cruise performance, not on any kind of balanced performance. They are willing to sacrifice a *lot* of takeoff performance and they usually climb at airspeeds well above best AOC or even best ROC..
    Gary started with a Sensenich wood prop for a fast-ish 601XL and using his design code, moderately modified the airfoil, twist, planform and tip shape. The goal was for WOT to produce ~3150 RPM at 5000′ DA at 120 KIAS. The stock prop produced about 115 KIAS at those conditions and 2750 at static at 2000′ DA.. It took two iterations to get it right.
    The surprising (to me) development was that the new prop also produced 2750 static RPM. *BUT* during the takeoff roll, as I accelerated, the RPM stayed relatively constant until about 30 KIAS, and just as I would bring the tail up, the RPM would dip slightly, maybe 50 RPM or so, and the plane would accelerate much more aggressively.
    Gary said that the new twist and airfoil put a lot more of the blades into stall at very low airspeeds compared to the stock prop. And when the forward speed came up enough, the blades would quickly “un-stall” and the bite was what caused the RPM drop and the acceleration change. The performance was very consistent and repeatable, and since the engine was quite powerful for a 601, climb was decent at 90 KIAS and about 2850 RPM. Take-off performance was not carefully measured (sorry!), but I *estimated* that I increased my TO rolls by about 20%..

    • jaksno says:

      I think I would have let ol’ Gary test my plane to be sure his ‘experiment’ with you as guinea pig would fit hit own parameters of risk’reward ration. My ‘boredom threshold’ is quite high, so I guess I don’t understand why this experiment was worth risking so much to you. but, hey, just an opinion from a ‘looker on’. Merry Christmas, glad you’re still around. {;^)

    • Andy, the effect you are referencing really only applies in aircraft with fixed pitch props faster than 90% of Corvair powered planes. Today, the only planes in the Corvair fleet affected are really Panthers. I will address the affect in a later post, I am sure you know what it is, but for the benefit of others, it is having a large section of the blades stalled at static rpm, but adding forward velocity unstalls them, and they actually absorb more power unstalled and the rpm drops. but pulls smoothly from there. I will see if we can get a cockpit Panther video to show this, and differentiate it from Detonation.

  2. jaksno says:

    William: This POH guidance is the smartest thing you’ve every done, and that’s saying a lot! (especially considering my slim knowledge of all you’ve done being following your posts and reading both manuals cover to cover 4 or 5 times) You planted your flag on the moral high ground this time. Those who refuse to follow the deep groove to success and safety you’ve carved out for them have NO excuse, if they ever did have one. ( Side bar: If there is ever a motion picture of your life and work, I suggest Benedict Cumberbatch be on the short list.) When I finally get involved, I will hold myself to no excuse and you have permanent permission to be very harsh with me should I have a brain fart attack and appear about to deviate. Merry Christmas to you and Miss Grace!

  3. feregas says:

    Mr. Wynne,

    In the “Ignition Timing on Corvairs” you state “Total advance on a Corvair flying on 100LL is 32 degrees. On 93 auto fuel it is 30 degrees” and in this post: “full static rpm and it is 30 degrees total. (28 degrees for 93 auto fuel)”. May I ask you why the difference in degrees? I just want to make sure I have the proper information written down. Thank you.

  4. David Swann says:

    HI William,
    In November I attended a local EAA meeting with the intention of joining the local chapter. But now im having 2nd thoughts about joining this chapter because one of the members stood up in front of the group and told the story of his 1st through 3rd flights of his newly built KR that he spent 5 years building having to make dead stick landings and joking about not being able to leave the pattern until he got the problem fixed. From what I understand his fuel tank vent had a bend in it that would fill with fuel on takeoff and block the vent and stop fuel flow. As far as I am concerned he dodged dead 3 times. I dont understand this mentality. When questioned about the first flight he said he had no flight plan and no tail wheel training. He said he “taxi tested” the plane until he felt like he was ready to fly. So the first flight was totally unplanned and I doubt very seriously if he could tell you what the take off distance would be. This is just another story of a smart guy doing stupid things. I hope you can use it an another example of what not to do. I think I am going to find another EAA chapter to join because this behavior did not seem to phase the other members of the group so maybe this is considered normal among them? Oh and most of these people think the corvair engine in an airplane is the work of the devil. And they should know because none of them own one?????

    • Kevin Purtee says:

      Mr. Swann – I know you didn’t ask me but I’m pretty sure WW would agree: your EAA chapter experience was not unusual. I don’t know what you’re building, but you’re better off sticking with the Corvair Piet or Zenith sites if they apply. People say stupid things on them but get smacked down pretty quickly. I’ve found that EAA chapters work ok for socializing on a Saturday morning, but are frequently populated by people who are less than thoughtful and have an astonishing tolerance for risk.

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