Why Alternative Engines Rarely Save Money

If you want to start a lively (never-ending) debate on any experimental aviation internet forum—or in the airport pilot lounge—just bring up the idea that you’re thinking about an alternative engine for your next project. “I was thinking about putting a radial on my next RV,” or “I heard that Chevy V8s are cheap and provide plenty of power—what do you think about putting one on a Kitfox?” But before you bring these topics up, you might consider checking your options for good cover—or begin digging a foxhole. Opinions on such matters run high…and hot.

The truth is, most airframes are designed for a particular powerplant. Designers might tell you that you can put anything you want on the nose (or the tail—there are still pushers out there), but any good aeronautical engineer knows that you start with a powerplant in mind and begin designing from there. Most airplanes grow horsepower as the design matures (partly because weight always grows), but this generally happens within a family of engines. An aircraft designed originally for a Lycoming O-235 eventually flies with an O-320—and before you know it, the O-360 is the norm…and someone is plotting to hang an O-540 on the nose.

So when you start thinking about an alternative to what your designer was thinking, you’re generally going to run into some interesting design problems. Weight and CG issues rear their ugly heads, coming up with a cooling system is always challenging—and who knows where you’re going to find a reduction drive for that 100-pound engine that produces 200 horsepower…at 9000 rpm.

Yes, we’re all involved in “experimental” aviation—but some folks are more interested in experimenting than others. The truth today is that the vast majority of builders want an airplane cheaper and more capable than what they can find on the certified market—but that stays well within the “norm” of general aviation types. Very few are actually interested in experimenting—which is probably good, because making great strides in aircraft performance is difficult at the current level of maturity of both aerodynamics and power. Most improvements happen in the single-digit percentages…and even then, an advance in speed generally means a decrease in payload or some other desirable parameter.

When it comes to engines, there are aircraft engines, automotive engines—and then some completely new forms of powerplant, like electric…or small turbines. Let’s pass on the electrics and turbines for today’s discussion—and by the way, I have built and own examples of each—so I am far from agnostic on the topic of “alternatives.” But let’s look at the choice between traditional aircraft engines and automotive engines converted to aircraft use. I have flown many examples of the latter and am still here to talk about it—and while quirky, they can be done safely. How efficient and how well they match airframes…well, that is another matter.

Automotive conversions are frequently marketed as far cheaper than their aircraft counterparts, and generally speaking, the bare engines are just that—far, far cheaper than their aviation equivalents. But the devil is in the details—and the details are where the idea that automotive options are a cheap way to get in the air comes off the rails.

I’ve been flying and working on airplanes for more than half a century, and in my experience, there are a number of good reasons to use “alternative” engines—those that the airframe was not designed for, or those not designed originally as aircraft engines. But saving money is not one of those good reasons. Do it for the tinkering, do it for the “wow” factor at fly-ins and airshows, do it because you want to experiment with new ideas. But if you do it to save money, you’ll probably be disappointed.

I have known many people who have gone the alternative route, and well more than half have eventually ripped the powerplant off at the firewall and replaced it with a traditional aircraft engine to make their airplanes reliable and usable.

Automotive conversions are rarely failures because of the core powerplant. They fail because of accessories—reduction drives, cooling systems, ignition or induction systems. There are lots of things that are designed into car engines because they are designed for cars. You can overcome these deficiencies with lots of time, effort, and money—all while your friends are flying off to pancake breakfasts and lunch runs while you’re sweating away trying to figure out why your exhaust keeps cracking.

The truth is that automotive conversions start out cheap but often cost a great deal in upgrades and modifications—and they rarely seem to come up with the same horsepower for a given weight as aircraft engines. And you know what—that makes sense! They were designed for cars, not for airplanes!

Now, as I said in the beginning, I am not against experimenting with different powerplants. As I noted, I have two very different powerplants in two different airplanes. I think people should experiment—it’s how we advance. What I caution against is new people looking at alternative powerplants in order to get into aviation less expensively. Because it rarely works out that way.

People used to get into aviation inexpensively by building their own aircraft using an old engine they found in the back of a hangar that still had some life left in it. I get that new aircraft engines are priced in the stratosphere—personally, I am done with them. There are still used engines out there—to run, or to use as rebuild cores—that can get you into the air without driving you to bankruptcy. I honestly believe that just as those types of engines were the genesis of the homebuilding movement, they are also our future.