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Old 02-08-2018, 12:08 AM   #20
JohnMc
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Join Date: May 2004
Location: St. Louis
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That old Saab commercial claiming that the turbo takes wasted energy from the exhaust and recycles it as boost isn't quite completely correct. Exhaust backpressure reduced engine output. The turbo sitting in the exhaust stream restricts flow. You pay for the boost with a reduction in power produced, it's just that you lose a lot less than you gain. But if you got the 20 psi of boost from some OTHER source (like a giant room sized tank of pressurized air) and had a wide open exhaust, you'd make more power.

Take a normally aspirated motor, and put a restrictor in the exhaust, say 1/2" in diameter. Then drive the car around. Floor it, and the intake manifold pressure goes up to 1 bar, 14.7-ish PSI worth of air. Each time the intake cycle happens the cylinder gets a gulp of that 14.7 psi of air. And then has to force it out into a highly pressurized exhaust system because the air can't get past that restrictor. 14.7 psi of air in the intake, full gulps of air in the intake cycle, and you're not making much HP at all.

Take that restrictor out and floor the throttle. Same exact pressure in the intake manifold - 14.7 psi. Same amount of air in the cylinder on the intake cycle. Only now on the exhaust stroke there's practically no pressure there, and the motor makes a lot of HP.

Nowe just mentally replace that restrictor with the turbo. A tiny turbo has to work a lot harder to pump air up to a certain PSI than a bigger turbo does. When you say 'work harder' it means that it requires more pressure in the exhaust system to spin the turbine faster to pump more air out of the compressor. The smaller the turbo, the more of a restriction it is in the exhaust, and the more HP it subtracts from the engine to create the boost. A larger turbo makes the same amount of boost with less backpressure, so it subtracts less HP.
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