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Old Yesterday, 09:33 AM   #26
rtv900
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Quote:
Originally Posted by car_freak85 View Post
I did NOT say this (better economy at 120 mph) applies to every vehicle on the road. I specifically mentioned a VERY slippery car (there's your drag) with a MASSIVE, inefficient engine and the ability to reach silly speeds. I didn't say this about a Forester XT.

Here's a fun brain buster: Our '99 Forester. Drove myself, the wife and another couple, their dog and all our gear down to central Oregon a mostly flat drive with one small mountain range to climb up and over. Drive in was at ~65 mph. We averaged ~27 mpg, mostly highway. On the return trip I purposely averaged ~80 mph and our mileage was above 30 mpg. Was it a scientific test? Absolutely not. But it proved my professor correct.
1) The fact that he went from working for Lamborghini to being a professor kind of proves he wasn't that great, not trying to bash teachers or anything but a career move in that direction doesn't really add up.

2) Proved him correct? No way any car gets better mileage at a higher speed, PERIOD. I don't care if you are driving a semi or a prius, drag is the predominant factor over 40 mph by far. Some other variable affected your results.

3) saying your statement only applies to the viper is erroneous at best, a viper, a pickup or a motorcycle have the exact same fundamentals working when travelling at a speed like that. A viper could probably get 25 mpg going 55 mph in top gear, and I'm sorry but at 120 it would most likely be around 6 mpg. Drag alone is quadruple and that is the dominant force. I just have no idea how you are making that argument, and if your professor seriously believes that I'm sorry but he should not be teaching.


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Originally Posted by Norm Peterson View Post
The power required at 120 mph is something like 6 times what you need at 65, provided that it's the same car and the aerodynamics haven't shifted due to aero lift or downforce. No way has the BSFC at peak torque rpm dropped off to less than 16% of what it is at half of peak torque rpm.

Your own chart shows 0.42 for 100% at something like 3400 rpm. The 25% curve at 1700 rpm reads about 1.05. That's 40%, not 16%.
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Old Yesterday, 02:22 PM   #27
car_freak85
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Anyone have a Viper they want to donate for testing purposes? Pre?
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Old Yesterday, 03:37 PM   #28
rtv900
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Anyone have a Viper they want to donate for testing purposes? Pre?
just curious, what are you saying is so special about the Viper?
serious question btw

I mean it's just a sports car with a big engine. Frankly, although they look cool as hell, I'd bet just about anything it's coefficient of drag is probably double or triple that of a Prius or a smart car or some cheese car like that.
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Old Yesterday, 05:29 PM   #29
car_freak85
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Originally Posted by rtv900 View Post
just curious, what are you saying is so special about the Viper?
serious question btw

I mean it's just a sports car with a big engine. Frankly, although they look cool as hell, I'd bet just about anything it's coefficient of drag is probably double or triple that of a Prius or a smart car or some cheese car like that.
I'm not saying ANYTHING about the Viper is special, it's just the one anecdote from class that stuck out. Might need to dust off my old notebooks and see if I wrote it down. What makes this anecdote "special" is that at 120 mph the Viper has a wider throttle opening and it's engine is at an rpm that is more efficient than when compared to the fuel consumption when the vehicle is traveling 60 mph, allowing it to use less fuel overall.

As for the Viper's cD (not great, but it'll still do 200 mph):

1996-1999:
Frontal Area 19.3 sq. ft. (1.79 sq. m.) -RT/10
20.5 sq. ft. (1.90 sq. m.) - GTS

Drag Coefficient 0.495 (top off)
0.46 (soft top)
0.45 (hard top) - RT/10
0.35 -GTS

From Viper Central

Quote:
The 2001 marketing brochure for the W450 build series cites 0.37 drag coefficient.
From the Smart ForTwo wiki page.

The Prius is actually quite slippery, I've seen numbers as low as 0.25 cD.

I hypothesize that the most fuel efficient driving speed for a WRX would be to peg the throttle at whatever position nets you "0 psi" manifold pressure (as little restriction as possible without introducing the extra fuel that is added under boost) and just let the car accelerate to whatever speed the vehicle will hit at that throttle opening in top gear. Never had enough "private road" to do that for very long.
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Old Yesterday, 07:18 PM   #30
Norm Peterson
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Quote:
Originally Posted by car_freak85 View Post
I'm not saying ANYTHING about the Viper is special, it's just the one anecdote from class that stuck out. Might need to dust off my old notebooks and see if I wrote it down. What makes this anecdote "special" is that at 120 mph the Viper has a wider throttle opening and it's engine is at an rpm that is more efficient than when compared to the fuel consumption when the vehicle is traveling 60 mph, allowing it to use less fuel overall.
Only way I can picture that even happening theoretically would be if there's enough power available down around idle speed with little or no throttle opening to actually be able to run at 65 or whatever mph. Needless to say, you'd need pretty tall overall gearing, maybe something like 50 mph or more per 1000 rpm . . . so it's not like anybody would actually operate the car at that speed that way.

Even if I stipulate to the possibility that this could actually be the case, it's not a useful capability for anything beyond classroom discussion purposes.


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