Measuring Cylinder Head Temps on Corvairs.
October 18, 2012 Leave a comment
In the last week I have been on the phone with Jeff Cochran, who is flying off the test time on his 2850cc powered Zenith 750. Jeff was concerned about a fairly high Cylinder head temp he was seeing on test flights. I knew that Jeff’s engine was in good shape, he had an overhauled MA3 carb, and he was using our standard cowling. This assured that the temp wasn’t caused by a basic issue. I verified that he had the timing set correctly and was wisely using 100LL for his test hours. In short, there wasn’t a good reason for seeing the high CHT numbers. Jeff’s probes are the kind that go under the spark plugs. Although these work on countless planes, they can have variations in how accurately they reflect the actual temp of the head. In some cases they are a better reflection of the temp of the spark plug. Although the corvairs plugs are on the top/cool side of the head, CHT measurements taken under the plug will often read higher than those taken from the boss on the bottom of the cylinder head, on the hot side, where GM put the CHT probe in the car. I asked Jeff to re-arrange his CHT probes to leave one under the #1 plug, and move the other to be on the GM factory location on the bottom of #1. Jeff’s letter back below confirms what I suspected, that his engine is actually flying in the middle of the heat range of the engine and that his CHT probes under the plugs were providing a false high number.
“William, Just a quick update. Yesterday I flew two hours in the morning without any changes. Came back stripped off the nose bowl and cowling, changed oil, relocated sensors and added the cowl lip extensions. On the sensors, I left one under spark plug #1 and moved the one from spark plug #2 to the location under cylinder #1. That way we get a direct comparison. After all of the changes I flew another hour. On climb out the spark plug CHT got to 458 deg. F and stayed over 450 for the rest of the climb to 3800 ft. The sensor under the cylinder reached a max of 349 deg. F for about one minute.at the end of the climb. During cruise the temps were around 420-425 vs. 325-330. The average difference between the two was normally over 90 deg. F. The largest difference was on climb out when the lower sensor heated up slower and I saw 119 deg. F difference. I feel much better about the engine now.-Jeff “
Above, Jeff stands on the left with his 750 on the day of airworthyness.
Above, Jeff sent this photo of the CHT probe moved to the bottom of the head. The hole he put it in is the stock GM location. On a 95 or 110 head this hole is threaded 3/8-16. Get a bolt 5/8″ long with this thread and an area washer, and you can clamp an under plug style sending unit right to the flat boss. If you are starting out from this position, you can use a 10mm spark plug sending unit because they are .400″ in diameter and a 3/8″ bolt is .375″, and it will be a little neater than using a 14mm sending unit. This location is where GM measured the 575F factory CHT limit from. We don’t recommend using that limit on flight engines, but they can take it as long as the engine isn’t allowed to detonate. Very high temps that are caused by detonation will obviously result in engine damage, but a Corvair that isn’t detonating has a very high cooling margin. Jeff showing numbers like 350F may sound very hot to a Jabaru operator, but on a Corvair he is 225F below the proven red line. Jeffs engine is running right in the middle of the thermally efficient zone and also is running a a temp the reduces the amount of lead build up in the engine compared to an engine running cooler. Keep in mind that a 750 is a large slow climbing aircraft and this effectively shows that the Corvair cools very well with a very wide margin in this heat challenged installation.
Above is Rich Whittington’s fabulous looking 601 HDS at Corvair College #21. Rich actually was one of the first builders to verify the under plug vs GM bottom location temp differential in independent testing last year. Rich’s results were much like Jeff’s. 601’s and 750s have very similar installations, sharing the same cowling and baffling. The 750 is bigger and climbs slower, but still cools itself well using the same parts we now have flying on more than 60 601’s. Rich’s aircraft has a 3,000cc engine with a Roy bearing
Flycorvair.com: An underutilized resource
Below are some photos and information I pulled from our main website Flycorvair.com. I went to that page, and typed in “spark plug washer” in the search box at the bottom of the front page and it spit out a number of links like the 2008 entry below. The page you are reading right now is Flycorvair.net, which I think of as our daily news paper of the Corvair movement. we have now had it for 10 months, and it had more than 100,000 words on it in 120 articles. But this is puny compared to Flycorvair.com, our main page that we have had for 12 years, which actually has more than 25 times as much information. Granted, it is not perfectly accessible, but many people fail to even try the search engine on it for basic answers. People today are often impulse driven to want an instant answer, rather than reading a little, learning and developing a real understanding of the answer to the question they asked. Flycorvair.com is a main library and the store for our business. It is a resource more builders should use. (note: below is a source info for copper plug washers in 14mm.)
From 2008, “Above is a photo to put a few things in perspective. The eight sparkplugs in the photo are replacements we just got for the Taylorcraft. On Gus’ advice I picked up iridium plugs for the bottom of the C-85 and standard plugs for the top. Make sure you’re sitting down: These eight plugs cost $375. That’s pricey. However, consider this perspective: Small Continentals on classic American airframes are some of the most reliable and trustworthy aircraft ever built. Even with 70 years of age and stingey or inept maintenance, these planes soldier on with an enviable record of reliability. Since this reliability is the foremost attribute of the plane, I am always willing to spend money in order to maintain that attribute.
A C-85 has a 6.3:1 compression ratio. This low ratio and its magneto ignition system make it prone to fouling plugs when run on 100 low lead. By comparison, a 9:1 compression ratio Corvair with one of our 35,000 volt ignition systems is immune to plug fouling. Also in the photo is Champion sparkplug lube, Part No. 2612. This is an absolute must for plugs going into Corvair heads. Plenty of people try silver anti-seize, but that’s a mistake because it gets on the plug’s insulator. You can also get 14mm copper washers for your Corvair plugs from Chief Aircraft, Part No. CH-M673, at about 40 cents a piece. If you’d like to experiment with iridium plugs in your Corvair, Denso makes a Part No. IWF16-5359. Please note that we tried platinum plugs with 100 ll fuel and it’s a poor combination.”
Above is a shot from 2006. Evidently I didn’t always have long gray hair. Merrill films Whobiscat and I in front of Rick Lindstrom’s Quick Build Zenair 601XL in our hangar. This aircraft had a glass cockpit. It had purpose made CHT probes that were expensive and threaded directly into the 3/8-16 holes in the bottom of the heads. These were made to feed information to an engine monitor made by the highly respected IK Technologies. These reflected the same temps as 10mm ring style senders clamped down with washers. The photo is a good indication of how long we have been gathering data and techniques on these specific subjects.
Above, a photo of the Wagabond built by the hangar gang at the old Edgewater hangar. This photo is from Corvair College #10. This is a very large plane that climbs slow. To this day, I think that no Corvair powered plane has left the ground at a higher gross weight (1625 pounds) We finished this aircraft eight years ago. It gave us a lot of test data that proved useful on aircraft like Jeff’s 750. Good data and useful answers are not developed in a day, and companies that are here today, gone tomorrow place very little value on gathering information, and even less on teaching builders things for the long run.
Above, is the Wagabond’s front end in a Corvair College #9 photo. Note that it basically uses the same nose bowl and cowl as we use on Zenith 750s (the carb air inlet is different on a zenith). This is how I knew that Jeff’s plane would cool itself well, and why I suspected an instrumentation error. We have long known that this type of cowl works well, even on big, slow climbing planes. We don’t use customers as guinea pigs, we promote things after they are tested.
Above, another Flycorvair.com photo from Corvair College #9. The aircraft is the turbo skycoupe, a major test bed of ours between 2001-06. If you look on the cowl ahead of the pilot, the white 3″ outlet pipe from the turbo is visible. Note that this aircraft also uses the same nose bowl we sell. The skycoupe isn’t big, but it wasn’t real fast, and the turbo could generate a lot of heat to go along with the power. (During a static run up this plane could drag the tires on dry concrete with the brakes locked. We tested the combination to 60″ of manifold pressure.) This aircraft sustained 500F CHT’s measured in the GM location. It also ran EGT’s over 1600F. Naturally aspirated Corvairs don’t generate this kind of heat, but it is good to know our favorite engine can take it. Today this airframe is owned by Craig Anderson who is restoring it and installing a 2850 he built up after Corvair College #22. Look for the Skycoupe to be at Oshkosh 2013.
Above, Dan Wesemans “wicked cleanex” in 2007. Although powered by a 3100cc engine and flown very hard, this aircraft never had issues with running hot. In a previous post on oil systems, I have a good shot to the engine installation. Fast small aircraft like the Cleanex do not challenge the cooling. Even if they climb only 25-30 mph faster, you have to keep in mind that dynamic pressure increases with the square of airspeed, and the energy of the incoming air is a lot higher, even if the airframe is only a bit faster. In cruise, these planes run very cool. Chris Smith, who built and Flew the “son of cleanex”, often had cruise CHT’s in the low 200’sF. Likewise, fast aircraft like these do well with stock 12 plate oil coolers. The larger and slower the plane, the bigger the oil cooler it needs.