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

Quote:
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.

Quote:
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.

Quote:
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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