Hondata

I know that elevation affects max power output, but does it affect the ideal ignition timing and fuel maps, part throttle and all?

You tune an engine to run correctly based on the manifold pressure. If you're on another planet where the atmospheric pressure was twice as high, WOT would be the equivalent of running 1000 mBar (14.7 psi) of boost on this planet.

On the other hand, a planet with half of the atmospheric pressure would mean that at WOT the manifold pressure would never get above 500 mBar. Isn't that just the equivalent of running at part throttle on the high pressure planet?

Do flow rates and throttle restriction factor into ideal ignition timing? i.e. On the high pressure planet, the throttle is nearly closed and heavily restricting flow to hold 500 mBar in the manifold, while on the low pressure planet, the throttle is wide open and unrestricting, with the same 500 mBar pressure in the manifold.

I guess you have to factor in the back-pressure as well, considering the exhaust has to fight whatever the atmospheric pressure is.

So back to planet Earth, where we're not talking about huge variations in pressure, but small variations caused by elevation.

Can anyone answer the original question, or have I already answered it?

Yes elevation will change how it runs. It will actually read from different fuel and ignition columns since there is less manifold pressure.

I completely understand how the ECU controls the timing and fuel based on the manifold pressure. I understand the columns in the tables represent the manifold pressure. I understand that the maximum pressure one can get in the manifold is whatever the pressure of the atmosphere is at any location. I understand exactly why it would run differently, but my question is much more technical.

Theory...

You have a dyno inside a chamber where the (absolute) air pressure is 2 atm, or 2000 mBar, or 29.4 psi. You take a normally aspiriated car, and you properly set the ignition timing and fuel values for all of the tables, from near vacuum, all the way up to 2000 mBar. You end up using the boost columns, although it's not actually boosted. So the ECU ends up with twice the operating pressure range as a standard NA engine. You then take the car out of the chamber.

Although the ECU "knows" how to run the engine at 2000 mBar, the manifold pressure will never exceed 1000 mBar, so the higher columns never get used. I understand that.

What I'm asking is, would you have to tune it again at 1 atm pressure to re-set all of the ideal timing values, or did the tuning you performed at 2 atm already take care that (for all of the lower manifold pressures)?

The only factor I can see that could affect the ideal timing would be due to the exhaust gas fighting against a higher ambient pressure in the chamber.

The scenario I just described is a huge exaggeration of "tuning at sea level vs tuning at higher elevation", but it's correct in concept.

Within the limits of elevation change, the difference in back pressure isn't huge, and IF there is a difference in ideal timing, it would probably be a few percent, or even fractions of a percent.

That is only a guess, but it's based on the fact that the BMEP is so much higher than the ambient pressure.

Ok, Power stroke is independent of the friction losses and exhaust back pressure.

No change in ideal ignition timing.