Automotive Powerplant Conversions: Caveat Emptor
I offer my opinion and experience on the subject of using high performance water-cooled automobile engines in your homebuilt airplane. Although I am not a mechanic, I have flown behind several professionally built and maintained auto conversions. In fact one of those car engines put me into a field and destroyed a beautiful replica mustang. I have also had some close calls testing various auto-derived powerplants, which I believe makes me qualified to speak on the matter.
Disclaimer: There are actually some really viable automotive powerplants for airplanes. Corvair, VW, and sub-2 liter variants, like the Geo Metro and Honda motors. Also, this discussion is limited to 4 stroke only powerplants.
Notice from my disclaimer that most of the engines that seem to work well are not fire-breathing V12 Merlin type engines. Where we get into trouble is when we want to put a 400+hp Chevy Corvette motor in our airplanes and think if we gear it down and derate to 300 hp and there won’t be any issues. That’s not how it works in real life. Not to mention that most folks are bolting these to airplanes with stall speeds that make an engine-out off airport landing dicey at best.
These big engines are complex compared to most certified aircraft engines and although they are generally the latest in technology, none of that technology is specifically designed to be used in an airplane.
Let’s look at each area of concern:
All big bore auto conversion need to be geared down. They run at RPMs that would drive the prop tips way past supersonic and cause emmense drag. So you have to come up with a gear reduction of somekind. Generally you are limited by what is out there in the experimental market to fit your application or you have to be an engineer with a machine shop to create your own. Some drive units use cog belts, some use a gear system with oil bath. The gear reduction is a major achilles heel for high power output auto conversions. They are often engineered on shoe-string budgets and not supported by R/D like the big manufacturers would do. In the experimental market you might find yourself on your own with a gearbox because the small manufacturer stopped making or supporting the unit. Not a pretty scenario.
The electrical system on a car is very complex and although robost, it is not designed specifically for aerospace operations where components and connections are subjected to high frequency vibrations, G loading, and rapidly changing temperatures and pressures. Often the connectors are completly ill-suited for the mission and must be replaced. Another achilles heel is the lack of a mechancial backup, i.e. a magneto, for the ignition. If you loose your primary power you better have a totally independant backup to power the ignition or it’s going to get very quiet. Unfortunately electrical issues killed a local man with his auto-engined homebuilt.
This is another item that causes homebuilders grief. It is difficult to build an adequate cooling system that doesn’t weigh a ton. Plus where do you put the radiator(s)? Better make sure you don’t introduce steam pockets in your system – cars don’t pitch up or down. Also, like the oil and fuel lines, if one of these coolant lines blow the engine will quickly be rendered unusable.
Again because these engines are not designed for airplanes you need to pay at least some attention to the oil system. If you are running an oil supplied gear box and/or a oil-driven constant speed prop you have just one more complication and one more point of failure that didn’t exist in that Continental or Lycoming.
Because aerodynamics are important, packaging an auto-conversion can be tricky. Actually in some designs, like replica fighters, the auto-conversion helps, but most kitplanes are not designed with an auto-engine in mind. Infact you will probably have to fabricate almost everything to make it work including your engine mount. Read: more time and expense
Some snake oil salesman will report how light their auto-engines are compared to the tractor-technology we have in our production airplanes, but you better check the fine print. Once you add in all the required systems and redencies to make it work you’ll likely be back where you started.
Like the weight, mentioned above, the price per horsepower seems attractive at first and in the low HP engines it is, but what you’ll often find is most auto-converters end up spending as much or more money for their troubles.
Most folks that don’t have the technical knowhow to do a conversion will shy away from airplanes on the market in this category – and rightfully so. Unless you are really, really handy you will probably be over your head. Worse yet, is someone that buys an airplane like this and thinks it’s a lycoming – they will almost certainly learn the lessons written here the hard way. And good luck finding a mechanic to work on it and sign it off.
I know I have been harse and there are several folks who have made these work and work well, but I have lost friends and seen others die and I want everyone that is considering high performance automobile derived power plants to go into it with eyes wide open! Nuff said…
Have a question, post it below in the comments. I’ll give you an honest answer and it won’t cost you a dime. Also, if you have comments or feedback feel free to post them below as well. We know this editorial will spark a nerve with some. If you disagree, we want to see your comments too.
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