MAX Update No. 70: MAX's Theoretical Max

Reader Contribution by Staff
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What a miserable month, as is typical of the end of a Western Oregon winter. Rain rain rain snow rain, definitely not road test weather. But thanks to the HBFCI I’ve been able to learn a thing or two in the relative comfort (it’s not raining indoors and that’s good enough for me) of my well ventilated shop.

Two updates ago (MAX Update No. 68: Idle Speculation) I reintroduced the Honey Bear Fuel Consumption Indicator, and did some engine efficiency comparisons between MAX and my store-bought minivan. I was curious what we’d save if we shut our engines down at idle, plus I was curious what MAX’s minimum fuel consumption might be. Well, I learned that it takes a fair bit of fuel just to make an engine run, and to nobody’s surprise, as engine size and horsepower increase, so does the fuel required, even if the engine is just sitting there idling. 

But we’re interested in fuel economy here — the fuel it takes to get someplace — so fuel burn at idle isn’t the main issue. A more important question is, how much fuel does it take to run the engine at cruise? 

So with the van idling in park, I gradually increased throttle until my ScanGauge showed I was burning 1 gallon an hour, and noted the rpm. I did it a few times and 2150 looked about right; I double checked by holding 2150 rpm and sure enough, the gallons per hour nibbled around between 0.98 and 1.02 so I’m calling it 1 gph.

The sobering thing about this is, this van goes 54 miles an hour at 2150 RPM, so even if the van had no aerodynamic drag and no rolling resistance and no transmission losses at all, the best mileage it could ever hope to get would be 54 miles per gallon. There is no car available — not even in theory — that is so light and streamlined and friction free that it could get 55 mpg with that engine and gearing.

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