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Bigger turbo.
Posted by JohnB (more from JohnB) on Tue, 6 May 2003 12:41:13
In Reply to: ? can I do to get high altitude performance gains?, speeed, Sun, 4 May 2003 19:32:50
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You can only get gauge pressure boost, i.e., you'll get 14psi boost at sea level or 10K feet, but at 10K feet if atmospheric is say 10psi absolute you're running 24psi absolute vs 28psi absolute at sea level. So you'll have to boost to say 18psi gauge at 10K feet to get the same sea level power, and you'll probably get less than sea level because the pressure ratio of the turbo at 10K feet to get the same absolute manifold pressure is higher than at sea level and the higher pressure ratio may/probably is outside the turbocharger pressure map...i.e., the turbo is less efficient at higher pressure ratios...inefficiency results in excessive charge heating.
A bigger turbo compressor wheel spins slower at a higher pressure ratio, heats the gas less, (although you'll still get more adiabatic compression heating alone at the higher pressure ratio than at sea level) but will probably spin up slower UNLESS you go to a modern technology ceramic ball bearing ceramic turbine turbocharger that runs hotter and spools up faster than your OG900T stock turbocharger. So yes, you can get better high altitude performance, but around 6000 feet I wouldn't do any more than I would at sea level with a saab turbocharged engine. The stock turbo should be enough to maintain performance...if you want more put a bigger turbo, more fuel (bigger injectors, increased fuel pressure), and perhaps an SPG9 vacuum advance pressure retard device on.
posted by 206.38.11...
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