Engine performance vs air temp / pressure / humidity

[Niels_at_home]

Hello!

Circuit altitude and ambient temperature affect the engine performance, that much is clear.

For aerodynamics I think its fairly clear, altitude changes pressure significantly, which changes air density. With temperature, pressure and humidity known, you can calculate the air density just by going to Wikipedia

For engine performance, assuming perfectly working cooling systems regardless of the ambient conditions, what happens? Air is kinda important for engines but is it linear with air density? When air density goes from 1.2kg/m^3 at sea level to 1.0kg/m^3 at altitude, does my 120hp engine drop to 100hp?

Or are there more factors, again assuming no cooling or other system abnormalities?

[Zico]

A general rule of thumb for power loss with increases in altitude is 3 percent for every 1,000 feet above sea level. If you raced on a circuit at 5,000 feet instead of at sea level, you can expect to lose about 15 percent of the engine’s potential power output, if the engine is tuned properly.

[Niels_at_home]

Cool, that isn't far off from the change in air density then, so it may be pretty linear with density..

(5000ft is 1500m)
http://docs.engineeringtoolbox.com/d...de-density.png

from 1.2 to 1 kg/m^3, which I guess is seen as a 1 / 1.2 = 83% or 17% loss..

Thanks

[Zico]

You mentioned temperature but you should also consider the importance of humidity. This link should be of interest.- http://www.carcraft.com/techarticles...g/viewall.html

[phoenix]

Simply put, engine power is affected by air density because the density determines how much oxygen is available to combine with the fuel to make heat.

So, if air density is down 10%, power will be down 10% (assuming the amount of fuel is corrected such that a constant air:fuel ratio is maintained.

Calculating the precise effect of the combination of altitude, temperature and water vapour/humidity is well explained here:

http://en.wikipedia.org/wiki/Density_of_air

[GORDON STREETER]

I remember driving up Pikes Peak in a small engined hire car, that proved it as it was gasping at the top (14000 ft) along with out of condition obese people

[zefarelly]

Likewise we rode over the Alps en route to Milan on late 50's Lambrettas . . . 2 strokes don't pull well at high altitude.

its also very easy to fudge dyno/rolling road figures by telling the computer the weathers changed. which does explain all those 200 BHP crossflow's and twin cams on ebay for 20% of the cost of the parts!