Boost levels

[Scott]

At 09:31 03-05-01 -0600, Smeins, Larry wrote:
>Is there anybody is this great group that understands the altitude effect on
>our boost?  Reading Bentley gives the impression that absolute boost
>pressure should be constant but that is not what I observe.  It appears I'm
>not the only one seeing 1.2 bar max at 5000 feet.  If I am wrong I have no
>problem with being told that I'm full of excrement just tell me why as well.

DISCLAIMER: It has been MANY MANY years since my last physics class, so if 
I blew the math I am sorry.

To make it obvious I will use an somewhat unreal example.

Atmospheric pressure at sea level: 1 bar
Driving Altitude: 18,000ft
Atmospheric pressure at that altitude: .5 bar
Hypothetical turbo running at sea level: 2 bar

To do that the turbo would have to double the amount of air in the cylinder.
If it doubles the amount of air in the cylinder at  driving altitude it 
would result in only 1 bar of pressure in the cylinder.

In order to generate that same 2 bar the turbo would have to work 3 times 
as hard as it does at sea level (do the work once and your are at 1 bar, 
twice you are at 1.5 bar 3 times you are at 2 bar)

Needless to say you probably just melted your turbo.

Now, down to more realistic numbers.

Atmospheric pressure at sea level: 1 bar
Driving Altitude: 5,000ft (Denver)
Atmospheric pressure at that altitude:  ~.85bar
Turbo running at sea level: 1.4 bar
Turbo working just as hard at altitude: ~1.2 bar

Essentially the turbo is adding an additional %40 air to the cylinder. To 
get 1.4 bar at altitude you would need to add %65 more air to the cylinder.
This would require the turbo working %63 harder.

That could very well be overspeed.

Scott