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It really depends on the HW set-up as a whole what is lost and what is not, when speaking of altitude affects to power output.
Genarally speaking, the turbo will need to work harder (wastegate more closed) to achieve the same MAP or airmass flow. When this is the case VE will be lower which leads to drop in power IF all other parameters remain the same.
Also as compressor effiency drops more heat will be introduced into the system, so it will be up to the IC to take care of it. During a short dyno run this should not be an issue.
The compressor is not an independent component, so one should look into the turbine side as well to see if it can cope with the additional task.
This is a bit harder than a simple compressor analysis, because manufacturers dont hand out detailed maps for them.
On a MAP car if compressor effiency is lower and IC is not the best one will see a drop in airmass at the manifold, ie. absolute pressure remains the same but temp goes up. This will of course lead to drop in power as well. On a T7 car you will have the same airmass, but boost will be higher due to the higher manifold temp. Even thogh the airmass is the same power wont, since the VE is lower.
To make matters more difficult altitude is not always for the bad ;)
IHMO altitude has the same affect on a turbo car as "stroking" the flow to the compressor. In both cases the turbo will need to do more work to achieve the target airmass/MAP. Sometimes it is for the better to limit flow to comprssor (widely used trick) to get manifold pressure up = aim at better compressor effiency.
This of course depends on what the HW set-up is... I will take my car as a small exemple, it runs with high airmass but low on boost. At high rpm I have 0.9bar boost and 33lb/min worth of airmass. Pressure ratio will be about 2 (boost+ loss in IC ~0.1bar). From the map below we see that the compressor runs with 68% efficiency, but the compressor could run 72% with 2.25 pressure ratio, ie. boost would be 1.15bar.
On a non monified car like the viggen there is CAPASITY left over so it will not loose that much power with altitude. Of course there is a limit...
If there would not be capasity, there would not be any gains chip tuning etc, would there? I know from my car that a stock viggen runs appr. 200g/s at high rpm, a modified one will do 220g/s. So there is slack on the comprssor/turbine which will work agaist the alitude fairly well.
Viggen is no diffent from the other stock turbo cars, they all have capasity left over just like all the other componets on a stock car. Some more some less so it is absurd to suggest that these cars would automatically loose big HP as ambient pressure gets lower.
On the other hand we have the basic tuning kits, which pretty much run the comprssor to the limit, so go over and the target is not reached if the conditions are not "ideal". A car like this is much more vulnerable to abient changes, like temp, altitude aso, because everything is already taken out at the "ideal" conditions.
my 0.02
posted by 81.197.226...
, Thu, 1 Dec 2005 10:47:40, Tue, 6 Dec 2005 20:13:50, Wed, 7 Dec 2005 06:04:12
, Wed, 7 Dec 2005 21:37:05, Fri, 9 Dec 2005 14:00:12, Wed, 7 Dec 2005 21:12:55, Fri, 9 Dec 2005 13:44:33, Sat, 10 Dec 2005 10:06:48, Sat, 10 Dec 2005 16:32:03, Sun, 11 Dec 2005 04:05:03, Sun, 11 Dec 2005 09:31:34, Sun, 11 Dec 2005 14:41:26, Mon, 12 Dec 2005 13:21:10, Fri, 2 Dec 2005 08:11:38, Thu, 1 Dec 2005 15:50:21, Thu, 1 Dec 2005 16:58:28
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