Preheat Your Oil, Not Your Cylinders
"There is only one reason to preheat a piston engine; and it's not to make it easier to start."—Lew Gage
By Mark Phelps
11/13/2008
Lew Gage is a retired airline pilot and A&P mechanic who knows more about operating early E-engine Bonanzas than Warren Buffett knows about the side streets of Omaha. And he's written much of it down -- as a regular contributor to the American Bonanza Society magazine. Gage has published a book of his articles (E-Series Bonanzas—available from ABS); and it includes guidance on a variety of issues. Take, for example, what he has to say about preheating. Just hosing down the cylinders with hot air isn't nearly enough. It's the oil that counts when it comes to thawing out a frosty engine. Gage wrote: "During oil preheat, the rest of the engine will also become heated, however, heating the cylinders by blowing hot air over the top of the engine may take the cylinders down to Miami and leave the oil up in Minneapolis."
Supporting Gage's theory, I have heard stories of bush pilots who would drain their engine oil soon after shutdown, keep it warm indoors by the fire overnight, then pour it back into the engine at dawn as a failsafe preheat in sub-sub-zero temperatures. Even under more balmy conditions, a high percentage of engine wear takes place in the time between engine start and when the oil warms up and begins to circulate. A mechanic I once knew described an engine trying to circulate cold oil as like trying to "suck pudding through a cocktail straw." Oil that is warm to begin with will help reduce wear and extend engine life. Most recommendations call for preheating a piston engine if the ambient air (which could be inside an unheated hangar) has dropped below 20° F for several hours.
Gage also recommends "long-term" heating (a plug-in sump heater or even a light bulb under the cowl for six hours or more) as a better strategy than that 15-minute stint with the fire-breathing preheater. But if that's your only option, he also recommends using a good insulated blanket to cover the engine and plugging the cowling inlet holes to keep the hot air where you want it most.
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