Re: thought experiments vs reality... - Saab Performance Bulletin Board - Saabnet.com
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Re: thought experiments vs reality...
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Posted by Dean (more from Dean) on Wed, 7 Jul 2004 06:38:43 Share Post by Email
In Reply to: thought experiments vs reality..., zeke, Wed, 7 Jul 2004 04:18:14
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There are many issues at play: The pressure drop across the IC makes the turbo compressor work harder to get the target boost at the intake manifold, or the boost justs drops off. The wastegate makes the turbine work harder to spin the compressor so there is a higher exhaust manfold pressure. For 15 PSI of boost, the exhaust pressure would be around 30 PSI or more. Higher exhaust gas pressures increase the amount of gas that cannot get out of the cylinder, which when expanded, decreases the amount of fuel/air that get get in. So you have reduced air/fuel getting into the cylinders and the back pressure also robs shaft HP from the engine. So thats a double penalty. And while the compressor is working harder to overcome the IC pressure drop, its is generating more heat to be rejected by the IC and is pushed into operation within its compressor map where its efficiency is declining, so more heat yet again and more turbine shaft power required. When you reduce the IC restriction, all of these stacked losses are reduced and you get, in comparison, stacked synergystic improvements.

An IC is designed to reject heat, not efficiently heat the external cross flow air. So external cross flow air is by defintion excessive. At low speeds the air flow is rather pathetic. If the IC is not aready heat soaked, its heat sink capacity can get you up to speeds where the cross flow can start to remove heat.

An IC that is too tightly finned that looks like an engine rad, can limit cross flow air in a self defeating manner.

posted by 68.93.19...

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