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Old 06-08-2013, 01:43 PM   #4376
stevehnm
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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.

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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.

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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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Old 06-08-2013, 01:57 PM   #4377
myrt1987
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Originally Posted by G2Spfld View Post
...in my corvette ... almost 500 HP v8.
Showoff...



lol jk, lucky b****
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Old 06-08-2013, 02:14 PM   #4378
Angelus911
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I still think the impreza would get almost as good mpg with the Forester's 2.5 and offer much better performance. I would gladly trade 1-2mpg for the 2.5's power
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Old 06-08-2013, 03:08 PM   #4379
Commander Keen
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You can use a number of unit types for BSFC:

-Energy over time vs power (GPH per HP)
-Energy vs power over time (Gal per HP hour)

HP and Kw describe power.
HP/hour and Kwh describe work.

Work is power over time.

The joule is a unit of work, so it doesn't need the /time conversion like the others. All forms are valid.
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Old 06-08-2013, 04:33 PM   #4380
stevehnm
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Quote:
Originally Posted by Commander Keen View Post
You can use a number of unit types for BSFC:

-Energy over time vs power (GPH per HP)
-Energy vs power over time (Gal per HP hour)

HP and Kw describe power.
HP/hour and Kwh describe work.

Work is power over time.

The joule is a unit of work, so it doesn't need the /time conversion like the others. All forms are valid.
True, and it basically means the ratio of joules out over joules in (or whatever unit of energy, or work, you want to use. Whenever time is in the equation it's in there somewhere else to cancel out.
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Old 06-08-2013, 07:43 PM   #4381
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Quote:
Originally Posted by myrt1987 View Post

Showoff...

lol jk, lucky b****
I only mentioned it for an example, I hesitated to mention it at all, I'm not a precocious person. I just worked very very hard long hours building businesses. It's not the coolest car I have though nor is it near the fastest

I still love driving my SP CVT for everyday runaround car. Best buy I've made I think!
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Old 06-08-2013, 11:14 PM   #4382
myrt1987
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Haha its all good.
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Old 06-09-2013, 12:14 PM   #4383
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Well temps are heating up and I was running the A/C for the entire last tank. 80% city 20% highway I'd estimate, and the mileage came out to 20.5 mpg. '13 sport 5sp. Ugh.
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Old 06-09-2013, 12:22 PM   #4384
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Originally Posted by GMU-BRZ View Post
Well temps are heating up and I was running the A/C for the entire last tank. 80% city 20% highway I'd estimate, and the mileage came out to 20.5 mpg. '13 sport 5sp. Ugh.
You have a severe issue then. It's been hot here, I've hauled wife and two kids last two tanks with air on, driving thru hills and driving the piss out of it, using M mode lots, and redlining it lots to make sure rings are seated and it breaks in like I want it. I have 48xx miles on 2013 CVT SP, and tanks were 26.5 and 27.4 hand calcul. I'd never drive it this way normally everyday. I can't believe the 5-sp can be that much worse. Maybe it is, but then that says a lot for the CVT. that would irritate some on here if that's true. I do run ethanol free 91 octane though.

Last edited by G2Spfld; 06-09-2013 at 01:31 PM.
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Old 06-09-2013, 02:19 PM   #4385
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Short drives and a heavy foot could give you very low mpg's I guess.

I have never seen mpgs anywhere close to 20mpg with my 5 speed (even in the winter with snows and winter gas), but I never drive it only a mile or two at a time and I am not a stoplight drag racer.

The car is broke or the driver knows something that isn't being shared, yet, about how the car is being driven.
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Old 06-10-2013, 02:17 PM   #4386
hemophilic
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Lowest I've gotten is 24.
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Old 06-10-2013, 03:27 PM   #4387
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Quote:
Originally Posted by hemophilic View Post
Lowest I've gotten is 24.
I had one tank at 23 over the winter, warmest temp was -10f, wind was 20-30mph, speed 75-83
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Old 06-11-2013, 07:00 AM   #4388
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32.8 this last tank. Used AC almost the entire time. Mainly back and forth to work.

John
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Old 06-11-2013, 05:02 PM   #4389
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Last tank was the worst in a long time, 30.2 mpg. Different work place so instead of mostly hwy, the cummute is now 80% city.
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Old 06-11-2013, 06:50 PM   #4390
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Originally Posted by Guzzi 1 View Post
32.8 this last tank. Used AC almost the entire time. Mainly back and forth to work.

John
Thats really good. Using the AC kills my mileage by at least 3-4 mpgs over the course of a tank. I think the best mileage I got out of a tank using the AC was 26 or 27 mpg.
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Old 06-12-2013, 03:51 PM   #4391
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My commute changed from 40 miles of mostly highway to 6 miles of mostly suburban traffic. So my fuel economy has gone from about averaging 30 mpg to 25 mpg. I'm going to adjust my style to try to get as much out of it, but 25 mpg is more than the 16 mpg my last car was getting on a similar commute.
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Old 06-12-2013, 04:03 PM   #4392
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Originally Posted by foghelmut View Post
My commute changed from 40 miles of mostly highway to 6 miles of mostly suburban traffic. So my fuel economy has gone from about averaging 30 mpg to 25 mpg. I'm going to adjust my style to try to get as much out of it, but 25 mpg is more than the 16 mpg my last car was getting on a similar commute.
That's where I am at 25/26. Blotchy commute, and driving this car 1-6 miles around Waldorf shopping centers.

Annoyed me at first but overall It's not so bad.

Maybe when they push the hybrid on down we could see 28/30 regular tanks. That'd be one hell of an improv lol.
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Old 06-12-2013, 05:59 PM   #4393
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So...this may make no sense but my 2013 5dr CVT gets about .4 mpg better when I have the AC on. I have tried it about a dozen times and every time my average mpg goes up when I have the AC on. I dont get it...how could this be possible?
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Old 06-12-2013, 06:10 PM   #4394
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Quote:
Originally Posted by srderby View Post
So...this may make no sense but my 2013 5dr CVT gets about .4 mpg better when I have the AC on. I have tried it about a dozen times and every time my average mpg goes up when I have the AC on. I dont get it...how could this be possible?
Well AC is a power drain, but perhaps it is due to extra drag from having your windows open when not using AC?

Or, if your car has Auto climate control, it is possible the AC is cycling more than you realize in auto mode...

Or you drive it differently when you have a cool head...

Can't think of another reason, AC isn't free, the compressor robs power from the engine to provide cooling breezes, so that makes the engine consume more gas, all other things being equal

Last edited by Zeeper; 06-13-2013 at 06:27 AM.
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Old 06-12-2013, 09:18 PM   #4395
hemophilic
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I think mpgomatic got higher with ac on. Weird.
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Old 06-12-2013, 11:16 PM   #4396
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mythbusters proved that A/C on is more fuel efficient than windows down though!
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Old 06-13-2013, 12:14 AM   #4397
cny12owner
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Quote:
Originally Posted by foghelmut View Post
My commute changed from 40 miles of mostly highway to 6 miles of mostly suburban traffic. So my fuel economy has gone from about averaging 30 mpg to 25 mpg. I'm going to adjust my style to try to get as much out of it, but 25 mpg is more than the 16 mpg my last car was getting on a similar commute.
8 mile commute here. Around 3-4 miles suburban with minimal traffic lights, 35-45 speed limit. Another 2-3 miles of interstate, 55-65 speed limit, then 2 miles of downtown traffic, stop and go. Car barely gets warmed up before I'm parking it. Pretty inefficient, averaging 27-28 mpg, and it's not the speeds that does it, it's the short distance and the warmup.

Did a 25-30 mile run tonight, mix of suburban, village, and highway (55 mph) miles, and was solidly in the 32-34 mpg range, and I wasn't light on the gas. If I had really tried, would have been in the high 30's.

So I can understand what you're experiencing. Still reasonable in my book for the circumstances.
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Old 06-13-2013, 10:47 AM   #4398
myrt1987
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Quote:
Originally Posted by Zeeper View Post

Or you drive it differently when you have a cool head...
My guess is this one. You might be changing your driving habits while using AC. Seeing as how you are at least somewhat aware AC has an effect on mpg I suspect this to be the case.

I know I drive slower with AC on.
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Old 06-13-2013, 12:00 PM   #4399
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Quote:
Originally Posted by foghelmut View Post
My commute changed from 40 miles of mostly highway to 6 miles of mostly suburban traffic. So my fuel economy has gone from about averaging 30 mpg to 25 mpg. I'm going to adjust my style to try to get as much out of it, but 25 mpg is more than the 16 mpg my last car was getting on a similar commute.
fits my experience, too.

for my 4mi commute between home and the train station, average speed in suburbia is 18mph (lots of lights on average) with 24-25mpg reported by the computer. mixed in with some weekend driving which involves longer stints and short stretches of freeway (so not optimal for mileage because of the need to accelerate to 65mph and only drive it for 4-5miles), it boosts me into the 29mpg range on the computer. in the end, my cars lifetime computer mileage is 29.4mpg* at an average speed of about 26mph.


* which is i know optimistic, even with odometer errors accounted for. it actually is closer to 28.5 when everything is accounted for.
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Old 06-17-2013, 03:00 PM   #4400
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32.4mpg (with an avg mph of 32)...

i have to say, im completely satisfied with these numbers in this fun little DD cuz I drive in m mode and push her to perform from time to time.. DGM 2012 impreza sport cvt, btw.
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