CC Outtake: Spoiler Alert! – Double Your Mileage and Keep the Engine Cooler! Maybe

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– Posted on August 26, 2018


I’d seen this ’63-’66 Beetle around town from time to time for quite awhile by the time I finally caught up with it in an Office Despot car park some years ago. The spoiler seemed a period piece, with its sheet metal construction and paint matching not only the colour but also the wear and patina of that on the rest of the car.

I got my money’s worth of social-media mileage out of this picture, captioning it in accord with the title of this post. In the process I got a bit of education about these cars, then more or less forgot about it until last week when I saw the car again. Its drivers, looking like a longtime husband-wife team, pulled up to a bakery where I was grabbing a quick breakfast before a several-hour road trip. I ducked outside; Misses had gone into the bakery while Mister stayed in the car, and he was quite happy to chat about the spoiler. Picked it up about 30 years ago (maybe 40?), he said, after seeing one like it on the Beetle of an acquaintaince who’d made it by hand and subsequently began making more and selling them.

“It’s primarily about form drag,” said Mister. “Any shape you’re pushing through the wind, there are going to be areas of high pressure and low pressure.” So far, so good. He continued: “The spoiler changes the form drag of the car, creating a low-pressure area below it instead of the high-pressure area that would normally be there.”

Now, I know rather not very much about ACVWs—except for their lights, and a real thing I had for the “Herbie” movies when I was a gradeschooler—and that mostly suits me just fine…mostly. When I took this picture I didn’t know the engine cooling airflow on these, and just assumed the grill above the engine cover is the air outlet. Hot air rises, right? So you’d put the outlet above the engine right? Wrong. That’s the inlet, as someone kindly illustrated for Wikipedia:

Cool air enters via the grill above the engine cover, is forced across the engine’s cooling fins by the fan, and is dumped out below. Very simple, until someone will make a driveway aerodynamicist of themself and come up with something like this what’s on the blue car: Er…Wolfsburg? Ich glaube, wir haben hier vielleicht ein kleines Problem. Now we are creating a low-pressure area under the spoiler—okeh, that’s what a spoiler is meant to do—but in that process we’re deflecting air away from the grill. Which strikes me as maybe not what we might best want, as that deflected air was meant to cool the engine. Maybe it’s not such a big issue, though. After all, the car’s ancient and still going.

Mister went on: “…I don’t take this car on the highway much any more, but I got forty-some miles to the gallon! Compare that to twenty-five without the spoiler.” I must have seemed a little shocked that a length of bent sheetmetal could alter the aerodynamics of the car enough to almost double the fuel economy. He added “…But the form drag is just one part of it; there’s also the carburetion. When you tune a car, it’s standing still and the air pressure’s equal all over the car. When you’re driving, that’s not the case—you’ve got those low and high pressure areas—so your carburetor tuning is way off. The spoiler brings the pressure in the engine compartment back closer to what it is when the car’s standing still.”

I’m not so sure about any or all of that. I’m inclined to doubt, but then again I don’t own a Beetle—with or without a spoiler. Anyhow, even if the fuel economy benefits are other than claimed, I’m sure the spoiler generates appreciable downforce to keep the car firmly glued to the road at speeds above…er…how fast do we reckon a stock early-’60s Beetle will go? Asking for a friend.