# Are you considering an alternative engine?

Engine spec sniff test.

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Are you considering an alternative engine? Will you be going to the various shows this season to see what’s available? Perhaps you are concerned that the glossy sales brochures might be loaded with hype. If so, put the numbers through this little “lie detector” system to help separate the wheat from the chaff.

The first alarm bell rings if the company in question isn’t actually offering a product. Maybe it is just fishing for investors. In descending order, give a lot more credence to someone who is offering a product that you can either take home with you from the show (or that can be delivered within one to two weeks after the show) than someone who just has a flying prototype and is conducting demo flights. Further down the ladder would be someone who has a running display at the show, and below that, a static display would rate above someone with a pile of really cool parts and the infamous glossy brochure.

## Claims Department

The next test of a company is its claims. Check the brochure. Is the horsepower claim on par with the displacement? Is the fuel burn consistent with the horsepower claim? Are torque and horsepower the same at 5252 rpm? I won’t bore you with the math from which these rules of thumb are derived, but the following can be used to decipher the numbers. Note that these rules apply only to naturally aspirated, four-stroke, gasoline engines, including rotary powerplants.

## By the Numbers

Horsepower will always be torque x rpm ÷ 5252. Expressed another way: On a graph, the lines representing torque and horsepower will cross or otherwise meet at 5252 rpm. But you must read the graph carefully, as some are laid out with two different scales representing torque and horsepower, many of which will graph rpm along the bottom. If they don’t meet at 5252, consider that the graph is a figment of someone’s imagination.

Horsepower = cubic inches x rpm ÷ 5250. This works for direct-drive engines or for an engine with a redrive, but you have to use engine rpm, not prop rpm. Remember, this is a rule of thumb, and you have to consider that a modern engine with liquid cooling, fuel injection and an electronically controlled ignition can easily return 10% more power at the same rpm than an old, air-cooled, carbureted engine running on points or a magneto.