[RockyMtnRay]

Originally Posted by sandy eggo

I understand your point about torque being the main factor in chugging up the mountain but doesn't the engines ability to breathe at the higher altitudes have any impact on how efficiently that power & torque is produced? My (non-engineering) brain says it certainly should have an impact. The effect of VANOS like technologies is that it allows the engine to breathe as it normally would at sea level even at mountain altitudes. Sort of like "altitude training" for cars?

Methinks you misunderstand what the variable valve timing is doing. First, it is not going to allow your engine to compensate much, if any, for the lowered atmospheric pressure of higher altitude. Only some form of waste-gated forced induction (i.e. supercharging or turbocharging) can provide substantial compensation for altitude. Absent forced induction (with a boost pressure compensating wastegate), a given engine design will "breathe" equally well (or equally poorly) regardless of altitude. An engine that breathes well at 12,000 feet will also "breathe" very well at sea level and vice versa.

What variable valve timing can do...particularly if combined with variable intake runner lengths...is change the shape of the torque curve with respect to RPM.

As mentioned previously, to optimize the torque of an engine for low to mid RPM, it should have long intake runners, very little valve overlap (and the exhaust doesn't open until very near the end of the power stroke), and long but fairly small diameter exhaust header primaries. Unfortunately, this combination has the side effect of causing a sharp drop off in engine torque at higher RPM...the torque curve is fairly flat up to about 3500 RPM but drops precipitously thereafter. This is the classic torque curve of a truck engine...lots of low grunt but no top end.

By contrast, to optimize an engine for high RPM, it should have short intake runners, quite a bit of valve overlap (and the exhaust valve should open well before the end of the power stroke), and the exhaust should be tuned for scavenging at high volumes and high rates of exhaust flow (e.g. headers have short fat primaries and there will be dual mufflers (and probably dual pipes)). The downside to this kind of tuning is the torque curve rises from very low values to peak (and more or less stays flat thereafter) only after the engine gets over about 3500 RPM. This is the torque curve of a race car or a classic sports car...lots of go only if you keep it really revved up.

What variable valve timing does is, depending on RPM, partially retune the engine between these two extremes...as the RPMs increase, the valves are retimed from little valve overlap/late opening to more overlap and earlier valve opening. On some engines with really complex cams, the entire cam profile is switched toward longer opening duration at high RPM. Some engines (like the new '05 Tundra engine) even go one step further and have dual intake runners for each cylinder...at low RPMs the engine control unit uses manifold valving so that a longer intake runner in the manifold is feeding air to each intake valve, and then as RPMs increase, the ECU switches to using the short intake runners inside the manifold. I don't know if the BMW Elves used this particular self-tuning trick but wouldn't be surprised if they did.

The net of these self-tuning operations is a variable valve timing engine (particularly one with varying length intake runners) maintains a fairly flat torque curve from very low to very high RPM...which means pretty decent low end grunt (torque)and also very impressive high end horsepower. If you want to phrase all this in terms of an engine's ability to "breathe" (a term I'm not very fond of), you could say that variable valve timing allows the engine to breathe equally well at all RPMs...not just breathe well in a portion of its RPM range.

It's not inconceiveable (though also not likely) that the BMW Elves are using a pressure sensor to slightly tweak the response of the VANOS for differences in atmospheric pressure. However, since intake air/exhaust gas velocity doesn't change much with altitude, or more precisely, doesn't change nearly as dramatically with altitude as it does with change in engine RPM, the overwhelming majority of the VVT effect will be proportional to engine RPM and not to altitude. So I'm not going to flat out say that your engine's VANOS doesn't compensate for altitude but I would be highly surprised if it's more than a 1 or 2 percent compensation. Certainly not a really noticeable compensation.

Anyway, maybe I should just get a TM and come out to CO for a good old fashioned tractor pull.

Heehee...with my mods I'm getting around 360 ft-lbs of torque (at a relatively low 3200 RPM)...and a whole lot of torque multiplication from the 4.30 axle gearing. To quote El Presidente Bush: "bring it on"