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

I don't know of a turbo-charged car sold in the states that runs its turbo/turbos at max capacity. In fact, turbos are typically not matched to an engine configuration to provide a max capacity, they're matched to provide max efficiency at a given level of capacity.

So an S4s turbos are designed to run a given level of boost. Their capable of running higher boost, but they're more efficient at their stock boost level (moving a given amount of air at a specified pressure). You'll notice upgrade stages normally start with just a chip. After exhaust upgrades, people will swap out to larger turbos. The larger turbos are usually more efficient at moving larger amounts of air (taking advantage of the upgraded exhaust) at a specific boost level.

So if the S4 indeed has this extra capacity turbo then it always compensates for the lighter air by running itself 10%(whatever) harder/faster than a s4 at sea level, right? What if you've changed the air/fuel mixture and boost with a chip (in the S4), are you saying the turbo will always find that extra capacity to compress it to lets say 35 inches? At what point, when chipping the S4, will lighter air play a factor in it's performance, if ever?

Thanks

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 19:24 2/12/2001 (1)
- Re: I'll give you an layman's explanation Josh 10:57 2/13/2001 (0)

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