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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
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Two turbochargers delivering same amount of boost with same discharge temperature will move same mass of air.

But a smaller turbocharger out of it's efficiency island won't be able to compress air to same pressure as efficiently as the big one, thus discharge temperature will be higher. PV=nRT thus mass of air delivered by small turbocharger will be less.

OK you say, just attach the intercooler and bring discharge temperature to same? Nope...if you run 120 deg. C air to intercooler and then ask it to lower it to 50 deg. C you will introduce big pressure drop trough intercooler. You will have to run much more boost prior interooler than post intercooler, pushing the turbo even further out of efficiency range.

And last and most important: a small turbo trying to push 1.3 bar, heating it up a lot and then letting the intercooler try to lower the temp (and lowering pressure to 1.0 bar in the end, due to ideal gas law) will introduce immense exhaust backpressure. That backpressure will be lower on big turbo. Engine is just an air pump and letting it work against exhaust pressure will lower it's output.


So to round it off:

Boost != power.

Power is produced by moving highest mass flow trough engine. To move high mass of air you need low backpressure. You also need low air temperature.

Small turbo means high discharge temperature and high exhaust backpressure = low mass flow. (Plus additional strain on engine, hot heads and lot's of fuel wasted just so you can heat up the air and then cool it with intercooler).

Big turbo means lower discharge temperature and low exhaust backpressure = high mass flow.

So boost is by no means a measure of power. It's just a funny figure that doesn't mean much alone. I have a four cylinder track car that produces more power @ 0.5 bar that same car (with same turbo) produces at 0.9 bar. How? Well the usual hard way: hotter cams, less restrictive exhaust, more ignition advance, ported head etc.

Remember, BMW M3 E36 produces 326hp from 3.3L with zero boost


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Old 07-19-2008, 04:01 AM
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