October 9, 2013 Leave a comment
A number of builders just getting into Covairs miss the distinction between these two engines. Below I have a number of links to illustrate the difference, but in a nutshell, the 3,100 was the “big bore” option on corvairs between 1998-2008. It has since been largely superseded by the 3,000 cc Corvair with good reason. There are still a number of 3,100s flying, and there will be for a long time, but very few, if any new ones are being built these days. A number of builders who previously flew 3,100s have elected to build a 3,000 as their next engine. There are reasons for this, and I will detail them below.
Above, the 3,000 cc Corvair that is flying in the Panther prototype. Notably, Dan Weseman successfully put several hundred hard hours on his 3,100cc engine in his “Wicked Cleanex, ” but opted for an all-out 3,000 cc Corvair in his aerobatic Panther. The engine both cary the same 120hp rating, but the internal differences make the 3,000 more durable, and it is far easier to build.
On the surface, the difference between a 3,000 and a 3,100 is simple: the 3,000 has a 92mm bore and the 3,100 has a 94mm bore. But the details go far deeper. The 3100 was originally developed for dune buggy’s in California, and piston/rod cylinder kits were sold by a number of companies in California. I bought one from Bob Sutcliffe in 1998, and his quality was good. At early Corvair colleges we assembled at least 15 and perhaps as many as 20 of these engines. It was very apparent that the quality of the kits varied a lot. The other issue was that builders who knew nothing about engines spent lots of money on poor kits from California, and them brought them to Colleges and said “I just spent $5,400 with a dune buggy place that has never seen an airplane, and now I would like you to build this for me for free this weekend.”
The main problem with this was several fold, first, ‘free work’ isn’t what colleges are about; second, these engine required a lot of hand work and fitting to even be assembled. Often they had no standardization, and individual pistons and cylinders could only occupy specific locations on engines. The big one was that each of these engines required a custom set of pushrods to be made so the valve geometry was correct. We and others like Mark Langford repeatedly told people that this was not a first engine to build. Most people listened, but there was a certain type of person that just wanted a bigger engine although they were unwilling to learn the required detailed information to understand and assemble it.
Internally, the 3100 had many VW 94mm parts in it. Here are the things about that that I didn’t like: The pistons, even good ones made by Mahle, were cast. The VW wrist pin is 22mm, and the dune buggy mentality was to just bore out the Corvair’s .800″ (20mm) rod to take the larger pin and run it as a steel on steel no bushing floating pin. This worked, but was a poor way to do it. The California companies would occasionally send out a rod with a .060″ wall thickness around the pin and think nothing of it. These engines also had the head gasket cut so large that it broke into the upper head bolt areas. Using the VW pistons made the compression height wrong for the Corvair, requiring custom pushrods to correct it. The Compression on these engines was really too high to safely have Mr. average pilot run car gas in it. Many of the people attracted to them didn’t recognize that Mr. average pilot was a polite name for them. If you would like to read about 3100s that worked well, look at:
Above is the 3100 of Dan Weseman in the Cleanex. This engine and the one in Chris Smith’s ‘son of cleanex went on to log about 500 hours each. Note the reverse gold oil filter housing on the engine. Have a look at four 3100s taking off in a row from Corvair College#16 in South Carolina, Langford, Weseman, Smith and Horton:
The first Correction actually came from Brady McCormick, Owner of the now defunct Magnificent Machine. What Brady did was have a new forged piston made in 94mm. This was a good idea, and he incorporated the Corvairs deck height and pin diameter in his design, automatically correcting the geometry. But it still had the head gasket issue, and the piston design Brady chose was a light weight one for high rpm engines. It’s compression was still high, and It could not tolerate having a dish machined in it. Still it was an improvement, and perhaps 15 engines were built this way. Examples would be Jim and Rhonda Wesemans Celebrity; Mike Robitie’s Cleanex engine;( Guest writer: Phil Maxson, flying a 3100cc Corvair in his 601XL ) and several production engines we built like this one: http://www.youtube.com/watch?v=y_1ov0DAbe8&feature=plcp
Brady’s pistons are no longer available, and the forging blanks they were made from have been acquired by Clark’s Corvairs as the basis for their USA made forged pistions. (They are only made to .060″ over bore now, and 94mm is a .264″ overbore)
Above Jim and Rhonda at CC#23, with Dan in the cockpit. the engine in this plane is a one of a kind reverse rotation Corvair, a 3100 with Brady pistons. Jim is a life long professional aircraft mechanic by trade, and a very clever guy. He had the experience to create a very unique Corvair for his plane. Most builders are far better off building an engine that is a regular “bolt together” experience. Jim and Dan developed their 5th bearing as a father/son team. Jim and Rhonda make Corvair cowling and baffling kits, but they made their best contribution to experimental aviation back in 1975…by having Dan.