I'll give you an layman's explanation (archive)

We all know that air density decreases as altitude increases. In a normally aspirated engine at altitude, the engine is forced to make due with less air (therefore oxygen) per "gulp" than it would at sea level, since air is less dense at altitude and there is a finite volume (size of the cyulinder). Since maximum combustibility and therefore performance are obtained at a specific air-fuel ratio (around 14-1 I believe), with less air there is either 1) less fuel delivered (most common with todays engine management computers) or 2) less than ideal air-fuel raio, either way power output gets reduced, pretty significantly.

A turbocharger works on the principle of compressing air before it enters the cylinder. More air + more fuel = more power. Note the turbocharger can compress air to a constant pressure. It matters less to a TB engine if that air started at 29.92 inches mercury or 28.9, it can still compress it to say 35 inches.

Now all this assumes an absolute reference type turbo with excess capacity, which I believe the S4 has (99% sure). At higher altitudes you will notice more "turbo lag". This is because before the turbos kick in you are running in N/A mode and its less HP and Torque due to less air, therefore the turbo effect seems like more than at sea level.

Follow Ups:

Re: I'll give you an layman's explanation ceb 00:15 2/12/2001 (1)
- Only the engineers know exactly...(m) Steve_D 00:25 2/12/2001 (0)

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