Ignition timing on Corvairs, Part 2

Builders:

Eighteen months ago I wrote a comprehensive story on ignition advance and timing on Corvair flight engines. I consider the story one of the most important and fundamental elements of mastering your Corvair. If you have not seen it in a while, or you have joined up since then, I suggest making 15 minutes in your schedule to read this and give it your full attention. The story is here: Ignition Timing on Corvairs.

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I use the term “suggest” above because I am my brother’s keeper, but not his jailer. I care about people, but I can only appeal to their willingness to learn and do a good job, their ethical responsibility as an airman to take risk management seriously. I can not force anyone to do anything with their own property. If you would like a real world example of this, we have this story from last year: Understanding Flying Corvairs Pt. #6, 98% DNA not enough. In it you can read about a person who destroyed his plane on it’s first ‘flight’,  severely injuring himself and a passenger, simply because he didn’t care to understand timing, long refused to buy a timing light, and refused to stop what he was doing when I warned him.

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For Part 2 , a slight update: On the internet, there is a suggestion being made by a non-pilot, that fliers using Corvairs should only use 24 degrees total of ignition advance on their planes, because this will allegedly make the same power as using the full advance we recommend. After 25 years of doing this, building several hundred Corvair flight engines, countless tests and having spent 5 years of my life at Embry-Riddle, I an assure builders that they should simply follow my timing recommendations for best results. To offer some further ‘why’ to expand the understanding.

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Ignition advance isn’t magic, it is Chemistry in action, particularly a branch of study in it called ‘rate of reaction.’ While I am not a Chemical Engineer, I was fortunate enough to have a number of classes with Dr. D. Cameron at Embry-Riddle, and he was an outstanding professor who really understood and could teach the physical properties of this branch, and he also knew internal combustion engines very well. This doesn’t make me an expert, but compared to a guy who slept through 9th grade Chem class, I am Alfred Nobel on the topic.

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How long it takes to burn the air and fuel in the cylinder, and thus how much ignition advance the engine needs, is a rate of reaction problem. Combustion dynamics in a real running engine are very complex, driven by the fact that once the combustion starts, the reaction itself is changing the dynamics of remaining unburned fuel and air. This acknowledged, the principles still apply, and they can be seen in action and tested easily for their proportion and effect.

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I could list 10 factors playing a role, but let’s look at just two of the ones that make using too little ignition advance an issue:

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The lower the pressure in the cylinder, the more advance it will need to make full power: Rate of combustion is greatly affected by the pressure in the cylinder when the spark happens. Three factors on this are the compression ratio, how wide the throttle is open, and what is the atmospheric pressure outside. While a max power test on a high compression engine, at sea level with the throttle wide open, may show OK results with less advance, That isn’t how planes go flying. Lower the compression to what most builders are using, understand that much of flying is cruising at part throttle, and the critical item a car mechanic never sees, the reduction of atmospheric pressure as the plane climbs, all call for more ignition advance for the engine to make best power and run efficiently.

The lower the starting temperature (given the same density)  the more advance it will need to make full power: While cold dense air burns fairly quick, cold thin air does not, and it needs more ignition advance to run efficiently. Again, this is a common factor to planes that few car mechanics consider. As a plane climbs it will do much better with even a slight increase in timing. Many people know that Klaus Savier’s Vari-eze is one of the most efficient homebuilts ever made, particularly in any contest where he can get some time at altitude, and he primarily credits his ignition that has far greater total timing than the magneto it replaces.

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There are plenty of myths about aircraft timing. I have people tell me every year that “aircraft engines all have 25 degrees advance” Really? evidently this people missed all the manufactures data working A&P mechanics use. Look at this Mandatory Service Bulletin from Continental:   http://www.tcmlink.com/pdf2/msb94-8d.pdf  Notice how the A-65’s all use 30 degrees of advance on both mags. People tell hangar stories about ‘the big bore of aircraft engines needing two plugs’ ignoring the idea that an O-200 continental’s piston is just 5/16″ larger in diameter than the one in a 3,000cc Corvair. The internet theories are endless, but mostly based on things easily disproven on inspection.

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Limited timing does appear on some other engines, like Jabarus and some VW’s, but this is driven by the mechanical design of those ignitions. Such engine are not noted for easy starting nor high altitude efficiency. Some people tout that Continental reduced the timing on O-200’s years ago, but his was actually driven by pilots using auto fuel that didn’t meet the STC requirements, and doing damage to the cylinder mounting studs on certain models. Car mechanics don’t know this, but ask anyone who flew a 150 before and after the timing reduction, and they will tell you the 4 degrees Continental ‘dumbed down’ the engine made a power difference.

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On it’s face, saying the Corvair makes full power at 24 degrees doesn’t make sense. What would GM, the original manufacturer use far more advance than this if that was all it took to make full power? Even the most torque oriented Corvair engine, the 95hp model, which had a peak torque at just 2,400 rpm and made it’s full rated power at 3,600, arguably closest to the flight engines we build, used 32 degrees of total advance (with the vacuum advance disconnected). If the engine made full power with 24 degrees, GM would have made them that way.

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The only possible motivation for a car mechanic to recommend using less ignition advance is if he is concerned about an engine having been assembled with substandard parts, like Chinese valves, and he is trying to convince people to lower the power output to protect the cheap parts. This also applies to telling people the engine can not fly with cht’s that touch the 400’s like this: CHT info taken from test flight of 601XL  Many of the issues where builders have been told they hurt their engine by running it to hot and be re-evaluated. There have been plenty of builders who made poor cowling choices who damaged engines, but we have positive evidence and factual data that shows the Corvair can run the CHT at my recommendations, provided of course, it doesn’t have sub standard valves in it.

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Above, a closer look at an E/P distributor in my distributor machine ( circa 2008). The machine has a large electric motor inside that spins the distributor. I have made hundred’s of Corvair ignition systems over the years.

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From  Ignition Timing on Corvairs :

“If a builder reads and follows the directions, he has mastered level 1). If he reads, considers and understand this story, he has moved his understanding up to level 2). Does he need to know more than this to effectively use the engine? No, but if he would like to know far more, it is one of the things I have a good understanding of in engines. This did not come from years of being a mechanic. The further understanding came from a number of years in Engineering classes at Embry-Riddle, Particularly the Chemistry classes. While the subjects we studied were academic examples for almost all of my younger classmates, I was 26-28 years old then, and the information was enlightening when I had a sudden understanding of combustion dynamics that I had observed for years in automotive racing, but didn’t have a detailed view of how the factors worked together, far less that you could make calculated and predictable changes.”

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Above, as you see it, this is a non-running model, but it has a serious purpose. The red parts you see are plastic, and were made for us by Corvair/Panther builder Paul Salter. Read the story here: Ignition system, experimental “E/E-T”

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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 www.FlyCorvair.com and in more than 50 magazine articles.

One Response to Ignition timing on Corvairs, Part 2

  1. David McBride says:

    The link to the service bulletin didn’t work for me. MSB94-8d and others can be found on continentals website.

    http://www.continentalmotors.aero/Support_Materials/Publications/Service_Bulletin_Search/

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