On normally aspirated engines, a change in density altitude will affect the performance of the engine, especially in its ability to produce horsepower. It's the reason why a piston propelled airplane will need a longer runway to take off on hot humid days as compared to cold and dry weather. The heat and humidity will decrease the amount of O2 per cubic foot of air. Therefore on hot humid days even if you are at sea-level, your engine will feel like its at a higher elevation (where the air density is lower). The opposite is true: generally on colder drier days, the engine will behave or suck air more efficiently and therefore feel with more horsepower or at its optimum hp even though you may be at sea-level, behaving like you are below sea-level where the air has more available O2 than on what aviation people call standard day (or something to that effect). I used to be a private pilot (up until I married 7 years ago and now with children) and density altitude is a very critical factor that is considered in aviation every time a prudent pilot is about plan their flight. You don't want to overload your airplane on a very hot and humid day with a runway that is short. You would run out of runway before you have enough speed to take off, all because your engine is not producing all the horsepower it is rated for, and it behaves like if you are taking off from Denver, Colorado even though you may be in San diego CA. Turbocharged or supercharged engines will not feel this change in density altitude up to point, meaning that the envelope of performance is wider. Mostly because the turbos will compress the air into the engine and increase its performance. Also, don't forget what the engine's computer does to retard the ignition once it detects pinging.

 

Example of the density altitude factor frm my 172P POH:

0 deg C at SL, 795 ft ground roll and 1460 ft to clear 50 ft obstacle

40 deg C at SL, 1065 ft ground roll and 1945 ft to clear 50 ft obstacle

Don't forget the engine management system will lean the mixture with high density altitude. Pilots do this with a mixture control **** to get maximum rpm when the density altitude is high and the runway is short.

 

the 99 has motronic 5.2 which "learns"... I'm not so sure about model years 00 and beyond which use motronic 7.x - less parameters in this system for tuning...

 

i believe the only thing that learns is the tiptronic tranny, not the engine.