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Not true...you still want high flow velocity in a turbo setup.
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You want the lowest pressure possible on the exit side of the turbine so you can have the largest pressure drop across the turbine. This means you need a large pipe, or no pipe which is pretty common on non-street legal turbo cars. Whenever you have a large pipe flowing the same amount of air, it flows slower, simple fuild mechanics. So maybe I should have said: you want less pressure in the exhaust and to achieve that you use larger diameter pipe and that means you have slower flow as well (this is mass flow rate)
300hp is 300hp, I don't care how you get it. Also, they do not use the exhaust to tune for torque at all, they use boost. Diesel engines may use a different air/fuel ratio and may use more or less air (mass) per HP, I'm not sure about that and that would affect the size of the pipe as well. In my book I think you would be just fine in a 300hp I-4 engine in a 85,000lb semi as long as the torque curve was wide enough to support the gearbox that is chosen. It would just get considerable worse gas milage due to the brake specific fuel comsumption being higher then a diesel engine.