[swamp2]

Originally Posted by joema

That is easily possible. Altitude-induced power loss is about 1% per 100m, so at a typical Johannesburg altitude, hp will be down 17.6% from altitude alone.

However -- the engine doesn't feel altitude above sea level, but density altitude -- which includes temperature and humidity. This can easily result in a much higher effective altitude.

E.g, on a muggy 85F summer day in Johannesburg, the density altitude could be 9,000 ft (2743 m). That equates to about 27.4% power loss.

Like horsepower, torque is also reduced by altitude. Even at sea level the 335i has more torque than the e90 M3, across a much wider range. This differential would be magnified at higher altitudes.

Because of all these effects, it's easily possible a 335i could out-accelerate an e90 M3 at higher density altitudes.

However the same is true for many normally-aspirated vs turbocharged comparisons. In Johannesburg, or Flagstaff, AZ on a hot day, a Dodge Neon SRT-4 could probably out-accelerate a Porsche 911 Carrera.

It doesn't mean I'd rather have a Neon than the 911, but it illustrates how great the effect of density altitude on vehicle performance.

• You are right that an engine only "feels" air density, not altitude but "density altitude" is not a real term.
• At Sea Level the 335i has about 1.5% more torque than the M3 but is delivered across a much NARROWER rpm band than the M3, NOT A BROADER band.
• Although torque and hp are affected by density the width of the curves is NOT affected.