Horsepower Change with Weather Calculator

Have you ever wondered exactly how much of an effect changes in air density and humidity have on engine power?

Here is a link to a calculator that you might find interesting. It assumes you have 100% power at a reference condition of: 77 degrees F, 29.235" Hg barometric pressure, 0 feet altitude and 0% relative humidity. Then you plug in your actual conditions to see what percent increase or decrease in power you would have. I think it assumes gasoline as fuel. With a highly oxygenated fuel like nitromethane the effect is probably less than this calculator shows.

My son Greg will tell you I like to blame changes in the weather for my boat running slower than at a previous outing. According to this calculator, those weather changes would only account for a 2-3% change (maybe 2 mph at 75 mph) so now I need to find something else to blame.

Here is the calculator: Relative Horsepower Calculator

Bob, FWIW, years ago I was told more important than air density was oxygen in the air. We would assume sea level more fuel, 1000 feet above less fuel. Wondered why running at Greenbrier @ 900 feet so rich, reason surrounded by trees in a national park instead of lake surrounded by major highways and pollution. Bottom line more oxygen more fuel then ratio of density. Tried all the gauges out there, came down too finding the best setting and WRITE IN DOWN as a reference for next time. My 2 cents.

Ray Sametz said:

Bob, FWIW, years ago I was told more important than air density was oxygen in the air. We would assume sea level more fuel, 1000 feet above less fuel. Wondered why running at Greenbrier @ 900 feet so rich, reason surrounded by trees in a national park instead of lake surrounded by major highways and pollution. Bottom line more oxygen more fuel then ratio of density. Tried all the gauges out there, came down too finding the best setting and WRITE IN DOWN as a reference for next time. My 2 cents.

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If your mixture was richer at the park, that would mean the air had LESS oxygen there

I've dealt with the issue for years. Racing at the Colorado Nationals as well at Al Hobbs races in Montana compared to lower level races showed the differences. Gasoline engines are a lot more sensitive than nitro engines. Our gasoline riggers were always one or two mph faster at Legg Lake than in Oregon. Nitro engines could pull the same props at Al's race's 2500 feet, but gasoline engines needed to prop down. Stock nitro outboards could run the same props in Colorado at 5,000 feet with only a needle adjustment. Higher performance nitro engines may have needed a different head shim and pipe length as well as smaller props. Gasoline engines had to seriously prop down and run leaner high and low speed needles. Everything ran slower. Electrics have no problems.

We always compensated our dyno results for weather. I'm not sure that the standard adjustments work over much of a range for piped two strokes. Exhaust gas temperatures will change with altitude, changing the effective pipe length because of the lower air density. That really hurts gasoline engines running fixed length pipes that operate at higher temperatures than nitro engine pipes.

Lohring Miller

I've been watching the water grains more closely for the past couple years and found you can have high water content without affecting the relative density (or density altitude) a whole lot.

Saw a record 168 gpp a couple years ago at the Nats in Mt. Vernon, granted it was 100* but I've run at the same relative density with less water and it was a lot better.

Glo ignition nitro motors don't burn water very well.

Eric Canto said:

If your mixture was richer at the park, that would mean the air had LESS oxygen there

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Ray Sametz said:

Bob, FWIW, years ago I was told more important than air density was oxygen in the air. We would assume sea level more fuel, 1000 feet above less fuel. Wondered why running at Greenbrier @ 900 feet so rich, reason surrounded by trees in a national park instead of lake surrounded by major highways and pollution. Bottom line more oxygen more fuel then ratio of density. Tried all the gauges out there, came down too finding the best setting and WRITE IN DOWN as a reference for next time. My 2 cents.

Click to expand...

Reread it twice before I posted and still got it backwards. Getting old.