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Quote:
Originally Posted by Firestarter
Cams,timing, comp ratio,Afr all play into it.and all need to be adjusted accordingly to a high performance turbo motor.
BMW also switched over to a turbo 2.0l over the na n54 3.0 i6. And said the new M's will be turbocharged cause it raises efficiency which means lower emissions and higher mpg
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I think you don't understand what I'm saying. A turbo does not usually increase the efficiency of the engine, if you're lucky under certain heavy load situations it will help a tiny bit. If you reduce the engine size and then turbo it, you are reducing engine efficiency most likely but the reduced engine size allows one to run the engine at higher load and thus better overall efficiency.
The biggest 2 distinctions that need to be made when talking about fuel efficiency and turbos are 1. throttle plate or fancy intake valve control system 2. electronic wastegate/bypass or not.
With a throttle plate, increasing part load efficiency is a lost cause. As long as the throttle is partially closed the turbo can't do anything because it can only blow off the pressure it builds. With a variable duration and lift system the turbo can build boost even under part load and increase efficiency because the turbo is able to "recycle" a little bit of energy from the exhaust stream (more precisely, it is the sound/shock waves/pulses that give you "free" energy).
Then there's the question of bypassing the turbine under part load, which should be done whenever the turbine efficiency drops past some point. The average turbo setup has a spring actuated wastegate that cannot do anything about this.
Air/fuel ratio and timing is going to depend on temperature and knock resistance. A relatively big cam along with turbo can potentially see higher efficiency at near maximum load, if everything is set up correctly.