bigdean
As for adjusting for altitude, isn’t that the point of the turbo? As the air gets thinner it naturally tries to keep the set manifold pressure.
Unfortunately, it ain’t that simple. If you reference the turbochargers wastegate signal to the manifold, then yes, it will, within it’s capability, attempt to maintain a constant Manifold Absolute Pressure, which will result in a constant manifold air density at altitude. BUT, to do that the turbocharger has to work a lot harder, because the work done by the compressor is determined by the massflow and the pressure ratio across it. At sea level, to make say 1barG of boost pressure, the pressure ratio would be 2 (2barA / 1barA) but at the top of pikes peak, to make the same 1barA manifold pressure, with an inlet pressure of 0.6barA, that pressure ratio has risen to 3.33!
Of course, the efficiency of the compressor also falls with press ratio, and as a result, post compressor air temperature is going to be a lot higher, and hence you either need more intercooling capability, or more boost pressure (which leads to more heat!) to recover that density and hence engine mass flow.
And then, because the compressor requires more power (it has to spin faster to deliver the same mass flow) the turbine must generate more power, and because that power must come from the exhaust flow, the wastegate must shut further (assuming it isn’t already fully shut) and hence pre-turbine EBP climbs. This reduces the effective engine pressure ratio, which results in both a fall in manifold volumetric efficiency (and hence a lower intake mass flow) and also results in a higher exhaust gas residual content at EVC, which leads to a higher end gas temperature at IVC and a corresponding loss of spark efficiency to avoid excessive PkpMax. And obviously, because the effective compression ratio of the engine has fallen, EGT at EVO will rise, which may result in the requirement for extra overfuelling below Stoichiometric to stay within exhaust line thermal limits.
And we haven’t even talked about load determination, which on this motorcycle engine with throttle bodies was probably via a basic throttle position sensor (which is terrible for altitude compensation) or intake air temp compensation, or Barometric compensation for both fuel mass, (transiently and statically) and ignition angle. And of course, lets not forget that the ionisation voltage for the spark plug is directly dependant upon the charge density at the Ignition point, requiring Dwell compensation as well.
But of course, i’m sure everyone knew that^^^ anyway 😆
