The Corvair Engine

[RJW note:  I received this article in March, 2004, and have received Mr. Collins' permission to reproduce it on my web page.  I've made some minor formatting changes.  While Mr. Collins is not building a Fly Baby, his article might be of use to those who are considering using the Corvair engine.]

It is said that engine development drives aircraft development. It works on another level, too.

My first glimmer of interest in building a plane came when Neil Hulin brought some Corvair engine parts to an EAA chapter meeting and described how it would power his Zodiac 601 XL. Then at Sun-n-Fun 2003 I went to a forum by William Wynne on the Corvair engine. He preached a powerful sermon that morning and I was hooked. I was volunteering in the Forums area and the headquarters was near the tent for auto engine conversions. I hung around and listened to the patter between Wynne and Corvair builders and potential builders.

Before I left Lakeland, I bought a conversion manual from Mr. Wynne and when I got home I began to study it and to look for a 1965-69, 110 hp Corvair engine, which is the optimal engine for conversion for aircraft use. Such engines can be found for $100-200 and can be made into a direct drive 100 hp aircraft engine that weighs the same as an 0-200 Continental for about $3000. That is less than half the cost of a newly overhauled 0-200. There are some compromises with our usual concept of an aircraft engine but there are some nice benefits as well – they run almost electric-motor smooth. The search for a suitable plane for the Corvair led to the Sparrow II XTC by Carlson Aircraft. Garry Howell decided he would like to go in with me and that made the whole project feasible. We found an engine at John Coyne’s Boot Hill salvage yard.

History

Pietenpol did not do a lot to the engine. It retained the blower fan for cooling and the stock camshaft. He worked out a retention method for the drive flange and a way to use the oil supply to heat the carburetor for carburetor ice protection. The starter was removed and most used some kind of alternator since the standard distributor was retained.

The engine set up this way produced about 75 hp. Conversions done this way have been quite dependable. So the Corvair has been flying for over 40 years.

Basic Specs

We have all heard about torsional vibration problems in aircraft power systems, and a dampener is a great help on the Corvair even though it is direct drive. The harmonic balancer is the external crankshaft pulley that was used to drive the original cooling fan. It is made in two pieces that are bonded together with rubber between. The 110 hp engines built from 1965-69 have the better head gasket and the harmonic balancer. In the car, these engines had two carburetors, one for each bank of cylinders.

The engine was also made in a 140 hp version with two carburetors on each bank. Strangely, these engines will develop less horsepower than the 110 hp engines at the rpm needed for a direct drive propeller.   GM made a few turbo-charged Corvair engines rated at either 150 or 180 hp. They had different cam, heads, distributor and induction systems but they had the same crankshaft, connecting rods, pistons and cylinders as the 110 hp engines.

Even though the last Corvair engines were made 35 years ago, about 1,700,000 of them were built and they are still quite easy to find. It is interesting to note that Lycoming has taken 50 years to produce 230,000 aircraft engines.

Myths

Some say it was General Motors’ answer to the VW, but they had very different design goals. The VW was designed to be the cheapest car possible. The Corvair was an attempt to produce a small sporty car with the quality of the larger GM offerings. All Corvair engines have hydraulic lifters, six cylinders and oil filters. Most VWs did not have these features.

And lastly, Ralph Nader did not kill the Corvair. By 1969 the Corvair sold for about the same price as the Nova but cost more to make so GM killed it for simple economic reasons.

The Aircraft Conversion

It also presents the opportunity to learn a lot about engines. When modified as Wynne directs, you end up with an engine about the size of a Jabiru 3300, (28 inches long and 28.5 inches wide). It will put out 100 hp at 3200 rpm and 90 hp continuous at 2800 rpm. The most powerful Corvair auto engines produced 180 hp at 5500 rpm, so asking it to produce 90 hp at 2800 rpm is conservative.

The prop will turn backwards by American standards (unless you buy a special cam and make a few other changes). It will have a standard SAE #1 prop flange and use a wood or Warp Drive prop of about 68 inch length. It will use any of a variety of standard aircraft carburetors or the Aero-Carb and can be set up for gravity feed or use a fuel pump. It will have an electric starter and a small alternator and when so equipped it will weigh 225 pounds. The distributor will be extensively modified and have two sets of points, either of which will run the engine. It will not have fully redundant dual ignition systems. It will have dual coils and points, which have given good dependable service. It will have 9.25:1 compression and use 100 LL fuel.

The crankshaft is a 26-pound, very rugged, carefully-balanced item as it comes out of the car engine. The output end has a six-bolt flange that appears to be part of the crank but was actually pressed on by GM. While it takes about 20 tons of force to get it off, Wynne feels it needs a retention system since the prop hub bolts to it. The maximum thrust is only about 400 pounds but the flight environment is quite different than the auto situation so this makes sense. There are at least two methods of retention. There have been no cases of crankshaft failure in flight engines during an estimated 40,000 flight hours.

The major factor in getting 100 hp at direct drive prop speeds (3200 rpm max) is a cam change. This costs about $200. No big effort other than forking over the money.

New hydraulic lifters are $2.00 each. The same item for an 0-200 is close to $85.00.

The stock cylinders are useable and the cheapest way is to have them bored 0.030” oversize. An option that came along just this past Fall is to use modified 88 mm VW cylinders that will fit the Corvair case. They boost the displacement to 2850 cc and reduce weight by four pounds.  Forged pistons are a requirement for flight engines and are about $450 for a set of six.

To get rid of the cooling fan, a simple flat plate of aluminum is cut to cover the top of the engine. We got a friend to make one for us but it is available from Wynne. A flight engine needs a larger oil supply than an auto engine and a deeper pan is available from Wynne for $269.

The conversion uses a Ford Taurus starter ring and Subaru starter motor. We got the starter from Auto Zone for about $90. Two expensive items are the prop hub, which costs $319 and the modified distributor at $219. We will leave the stock oil filter and oil cooler in place and the Aero-Carb from Sonex seems to be a good choice for the carb. It will work on gravity feed and is somewhat less prone to carb ice than conventional carbs.