| A supercharger, as you know, is driven mechanically by the crankshaft - thereby taking some away directly from the part of the engine making power. On the other hand, a turbocharger is driven by the energy contained within the exhaust gases. There's a LOT of wasted energy in piston engines which the turbocharger is designed to harness. So instead of using power right from the crank to force air into the engine, you are using what is normally just wasted energy. The additional backpressure in the engine's exhaust due to the turbine does take away a little bit of energy from the engine during its exhaust stroke, but that loss is rather small. Where the turbine gets a majority of its energy is from the thermal element of the exhaust gases, which is "leftovers" from the engine's cycle. "Backpressure" in the exhaust is one way of looking at what people consider being good for torque production, but it gives a big false impression. I assure you that if you stuff your exhaust full of your choice of vegetables to create "backpressure" on the engine, it certainly wont produce more torque. The easiest way to look at what causes better torque production is by way of the gas velocity. An exhaust system that has what one may call a lot of backpressure is a system that has a comparatively smaller diameter exhaust pipe. Common sense tells you that the smaller pipe makes is harder to move a quantity of air through as compared to a larger diameter pipe. The reason is due to the velocity at which the gases must move through the pipe. As the velocity doubles, the resistance squares. Smaller pipe = higher velocity = more resistance = more "backpressure" on the engine. But gas velocity can be a very big friend to you if you are shooting for making good low-RPM torque. The reason is because at lower RPMs, the smaller pipe requires the gases in the pipe to move faster. The gases in the pipe have momentum and want to keep moving even once the exhaust valves have closed. That column of fast moving gases then "pulls" on the gases upstream of the pipe, causing them to drop in pressure with the peak lowest pressure right at the backside of the exhaust valves. At the next exhaust stroke, this low pressure in the exhaust acts to pull the gases out of the cylinder, helping it clear the chamber of the inert gases to allow even more clean air to enter the chamber on its next stroke (which would be the intake stroke) than it would have without the scavenging effect being present. On turbocharged cars though, there is a turbine wheel right in the exhaust tract very close to the engine. The turbine wheel acts as a pulse damper - this lessens the effect of "exhaust tuning". On top of that, you want the turbine to have the greatest pressure differential between its inlet and outlet so it can extract more energy from the exhaust gases. Since the power produced is so directly related to how much boost your turbos are providing, getting them spooled up as quickly as possible will produce the most gains over the small gain of trying to advantage the scavenging effect in an already compromised pulse-damped exhaust and choking up the turbine.
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