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Old 06-08-2013, 01:43 PM   #10
Scooby Specialist
Member#: 329526
Join Date: Aug 2012
Chapter/Region: SWIC
Location: Ground Control
2013 Impreza Spt cvt


Originally Posted by Commander Keen View Post
No... BSFC tells us how much fuel is needed to generate a unit of power over time (Kwh / horsepower-hour).
I stand corrected (actually I sit corrected).

Pretty much all true, except that time is not involved. In your "Kwh / horsepower-hour" time cancels out. It breaks down to joules/joule in terms of energy in / energy out. Power is energy / time, which is not part of it.

Originally Posted by Commander Keen View Post
It's likely that the FB20 can make 30 HP more efficiently (Kwh/gal) than 20 HP. Say that 20 HP produces a 55 MPH cruise on level ground, and 30 HP at 70 MPH. While the fuel->power conversion might be more efficient at 30 HP, the power->distance conversion falls off at a faster rate due to drag at the higher speed, so MPG as a whole decreases. BSFC is just one link in the chain.
Absolutely, and why you're correct in that BSFC is not linearly related to mpg.

Originally Posted by Commander Keen View Post

The CVT can choose a point to follow on a curve within that BSFC map based on power demand. For example, to run at 2500 RPM and 40% throttle or 3000 RPM at 30% throttle, but how much power you demand, and the speed you choose to travel is not within its control. Say the BSFC peak is 50 HP @ 2400 RPM and 60% load. The CVT could only hold that for as long as you had a use for 50 HP, and ratios allowed.
Or, if it was more power than needed the car would continue to accelerate until it needed 50hp to maintain.

Originally Posted by Commander Keen View Post

I would guess that between aerodynamics and drivetrain, it takes more energy to push an Impreza 68 MPH than a Corvette. Also, the V8 probably requires premium fuel, which the engine can extract more energy from. The Impreza could use a taller top gear, at least in the 5MT.
Taller gears are not what's needed in the CVT, the top ratio is way above what the engine is capable of.
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