Quote:
Originally Posted by CanM3
Dont get me wrong, and I dont mean to sound like a di*k....but your explanation is wrong.....
A cars compression ratio is fixed, its determined by swept volume, piston configuration, cylinder deck height......
What you are talking about is volumetric efficiency, that is to say how well that fixed volume is used.
At higher altitudes, the air is 'thinner' there is less o2 available, so the VE is decreased. The use of forced induction packs the cylinder, increasing the VE....this will increase the cylinder pressures, but trust me, the compression ratio is a fixed ratio, it does not change...but cylinder pressures change constantly....
The only advantage a turbo has over a Supercharger is the turbo is utilizing a waste stream (the exhaust) to charge the cylinders...it uses a little mini gas turbine, same principle as a steam turbine, just on a much smaller scale. The turbo 'works' as hard at sea level as it does at high altitude, the thing that changes is the waste gate position, less boost required, more air is bled off thru the waste gate.
I much prefer supercharging to turbocharging....there is a bit of a tradeoff as there is a bit of a parasitic element to supercharging, but they are so much more reliable. Turbo's run at extremely high temps, require zoomy metallurgy, require an extensive oiling system, very sensitive to downstream pressure changes, then the infamous turbo lag, not present in supercharging.
At full throttle, a supercharger and a turbo charger would both put out max rated boost......as mentioned before this would be dependant upon waste gate or bleed valve settings.
So, dont mix up compression ratio and cylinder pressures, they are two totally different things.......
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No worries--this is a better explanation and it uses the right terms (volumetric efficiency). And yep, that's what I meant by "compression effectively drops" (probably conceptually sort of okay) but the language was sloppy. Not so much confusion as imprecision and sloppiness. Fair enough and perfectly okay to jump in and make corrections and this moves us all in the right direction (precision, less ambiguity).
Back to the OP’s issue, do you concur that at altitude a tune isn’t going to offer much improvement (or as much as at sea level) with a N/A engine, and if so, is FI probably the best (although not cheap) option to compensate for altitude related performance issues? What about the question of 100 octane seeming to cause worse performance than 91 octane in this application?
Now, as to which is better, S/C or Turbo....I'm not going there! That's DCT vs. 6MT territory or worse! 
I do have a couple more questions on the S/C and turbo stuff though (I’m not trying to sound like a d*ck here either, just trying to fill in some areas where I'm not certain and learn something):Does a S/C adjust, via ECU or other mechanism, to keep boost at a target PSI based to some degree on intake air temp, engine temp, fuel quality, throttle position, detonation detection, and altitude—and does that target change if some parameters would make a “normal” boost target unsafe under some conditions?
Doesn't a modern (say 335i) turbo setup use the ECU to monitor and adjust the wastegates to achieve PSI boost targets (which can vary) to hit target based on intake air temp, engine temp, fuel quality, throttle position, and altitude, and doesn't it dial back boost if it’s unsafe for the engine?
I'm guessing both aren't perfect remedies for high-altitude as the air is thinner (guessing more turbo lag--slower spin up--no sure about the S/C) and it would seem you'd have to spin the turbo faster to achieve the same boost (same with S/C?) since the air is less dense (and there's a point where you'd spin the little guys into pieces).
Given the preference for S/C I take it you're not thrilled with the idea of the next generation of M3 probably being a turbo-based FI approach....