Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - BSFC and Subar Impreza 2.5i

JRMichler · 07-12-2012, 10:13 PM

Are you sure about the 30 hp at 65 MPH? My truck is larger than your car and it needs about 32 hp at that speed on a cold day, based on a coastdown test.

Time for some math......

30 hp X 0.746 = 22.4 Kw. From the second graph, 22.4 Kw at 2500 RPM puts the engine BSFC right at 210 grams / PS / hr. You should be at slightly less than half throttle, say about 16 to 18 inches Hg vacuum.

If you could add one more gear to the transmission to drop the RPM to about 1800 at 65 MPH, the BSFC would improve to 195. Steady state gas mileage would improve by 210 / 195 = 8%.

You can pass in either gear. If you pass at or near WOT, the BSFC decreases to 190 grams/PS/hr in either gear.

If you stay in top gear, the power increases from 22 Kw to 50 Kw. That should be plenty for most passing. The most efficient passing is with the least increase in speed because you then spend less time at partial throttle (300 BSFC or worse) to slow back down.

07-12-2012, 10:13 PM	#4 (permalink)
JRMichler Master EcoModder Join Date: Feb 2011 Location: Phillips, WI Posts: 1,018 Nameless - '06 GMC Canyon 90 day: 37.45 mpg (US) 22 Maverick - '22 Ford Maverick XL 90 day: 43.95 mpg (US) Thanks: 192 Thanked 467 Times in 287 Posts	Are you sure about the 30 hp at 65 MPH? My truck is larger than your car and it needs about 32 hp at that speed on a cold day, based on a coastdown test. Time for some math...... 30 hp X 0.746 = 22.4 Kw. From the second graph, 22.4 Kw at 2500 RPM puts the engine BSFC right at 210 grams / PS / hr. You should be at slightly less than half throttle, say about 16 to 18 inches Hg vacuum. If you could add one more gear to the transmission to drop the RPM to about 1800 at 65 MPH, the BSFC would improve to 195. Steady state gas mileage would improve by 210 / 195 = 8%. You can pass in either gear. If you pass at or near WOT, the BSFC decreases to 190 grams/PS/hr in either gear. If you stay in top gear, the power increases from 22 Kw to 50 Kw. That should be plenty for most passing. The most efficient passing is with the least increase in speed because you then spend less time at partial throttle (300 BSFC or worse) to slow back down. __________________ 06 Canyon: The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 33,021 miles 2016-2018 at 35.00 MPG. 22 Maverick: Summer 2022 burned 62.74 gallons in 3145.1 miles for 50.1 MPG. Winter 2023-2024 - 2416.7 miles, 58.66 gallons for 41 MPG. Last edited by JRMichler; 07-12-2012 at 10:19 PM..