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Old 02-17-2020, 09:33 AM   #26
rtv900
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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%.
^this
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Old 02-17-2020, 02:22 PM   #27
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Anyone have a Viper they want to donate for testing purposes? Pre?
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Old 02-17-2020, 03:37 PM   #28
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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 02-17-2020, 05:29 PM   #29
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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 02-17-2020, 07:18 PM   #30
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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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Old 02-18-2020, 07:30 AM   #31
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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.
no way, BSFC per unit HP and fuel economy are not related like that.
As much vacuum as possible yields the best fuel economy, and the slowest speed in top gear.

Reset your trip meter on the highway sometime going as slow as possible (like 50 or 55) on a flat road and wait for the fuel mileage to show up. It's going to be over 30 even in the STI and that will hold out long term if the road stays flat.

Get on the gas to zero psi vacuum/zero boost and you'll end up at 100mph before it finally steadies out. Your mileage will be low teens at best, probably single digit.

That's totally realistic to try in real life, do it.

Saying the viper engine is 'more efficient' at 120 depending on context and what efficiency you are talking about can be true. Hell, I'm sure it's more volumetrically efficient since it's NA and will be turning faster.

Saying it is more FUEL efficient at 120 is totally different and horrendously false. I really hope this professor you are talking about was not telling a class that a ICE powered car can be more fuel efficient at a ridiculous speed like 120.
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Old 02-18-2020, 07:47 PM   #32
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Is there a particular reason you'd do 2700 over 1800
A few reasons. Easier on the engine to maintain speed since roads are not completely flat and has some grade, better throttle response in case I had to accelerate or decelerate for that matter using engine braking, and while not a big deal, mpg does not seem to get affected much.. in some cases I get better mpg running higher rpm as the load is less on the engine.

The benefits outweigh the negatives for me. And I've owned many Subaru's in the past, never had ringland failure even though I drive my cars pretty hard.
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Old 02-19-2020, 08:24 AM   #33
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I really hope this professor you are talking about was not telling a class that a ICE powered car can be more fuel efficient at a ridiculous speed like 120.
At best, he was cherry-picking a unique combination of things that nobody would ever choose when driving. I can picture where the BSFC of an engine with a big cam would pretty much suck at only a few rpms above idle, while having 8+ liters worth of suckage might actually make enough power for 60-ish mph. Lugging-R-Us


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Last edited by Norm Peterson; 02-19-2020 at 08:48 AM.
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Old 02-19-2020, 08:40 AM   #34
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Is there a particular reason you'd do 2700 over 1800
Throttle response, which with a turbocharged engine necessarily involves turbo lag when the revs are too low. At least in OE trim, Subaru's turbocharged engines are much more responsive from a little above 2000 rpm than they are at rpms below that point.

Normally aspirated engines do not exhibit anywhere near as much change in response over such a small change in rpms. Though personally, I probably wouldn't run a 2.0 to 2.5 liter engine in a car weighing 3300 lbs much below 2000 rpm either.

You pick up on this pretty quickly once you learn to shift by paying attention to what the engine is telling you that it wants rather than focusing on specific road speeds or by watching the tach. Feel what it wants, as opposed to simply reading numbers.


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Old 02-19-2020, 11:56 AM   #35
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At best, he was cherry-picking a unique combination of things that nobody would ever choose when driving. I can picture where the BSFC of an engine with a big cam would pretty much suck at only a few rpms above idle, while having 8+ liters worth of suckage might actually make enough power for 60-ish mph. Lugging-R-Us


Norm
I'll give him the benefit of the doubt and assume there was a misunderstanding and he was probably stating that the engine is operating more efficiently from a BSFC standpoint at 120 vs 60 in the same gear, which means it may have been 10% more efficient from a hp per unit fuel position despite the hp requirement being 5-600% greater at 120. . . .easily misunderstood to think he was saying it was 10% more FUEL efficient at that speed, obviously outrageous.
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Old 02-19-2020, 01:02 PM   #36
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That makes sense. Drag is taken out of the equation and we're simply testing the engine vs rpm. Put it on a dyno. Easy enough to do.
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Old 02-20-2020, 12:52 AM   #37
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I'll give him the benefit of the doubt and assume there was a misunderstanding and he was probably stating that the engine is operating more efficiently from a BSFC standpoint at 120 vs 60 in the same gear, which means it may have been 10% more efficient from a hp per unit fuel position despite the hp requirement being 5-600% greater at 120. . . .easily misunderstood to think he was saying it was 10% more FUEL efficient at that speed, obviously outrageous.
I think the professor was talking about driving a viper in space, ya know? Like in a vacuum. Thatís how they get more fuel efficient at 120.
But when you press the button for ludicrous speed, volumetric efficiency of the flux capacitor takes over and,,,,BOOM, youíre back in 1955.
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Old 02-20-2020, 07:21 AM   #38
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I think the professor was talking about driving a viper in space, ya know? Like in a vacuum. Thatís how they get more fuel efficient at 120.
But when you press the button for ludicrous speed, volumetric efficiency of the flux capacitor takes over and,,,,BOOM, youíre back in 1955.
ha ha, well I will admit in space it would in fact be more fuel efficient at 120

or 120 sitting on a dyno just turning rollers
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Old 02-21-2020, 04:14 PM   #39
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what i gather from this thread so far...

1. I cant google LSPI and get thousands of very well studies done by REAL engineers...

2. I don't know how to drive manual because I choose to stay in a lower gear and keep my rpm higher than 2000 when cruising in the car the I own...

3. Vipers are more economical when going 120 MPH...

4. Going 40 in 5th gear and then WOTing to pass the school bus is perfectly normal and my engine will last over 101k doing this all the time.

Very informative!

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Old 02-23-2020, 09:41 AM   #40
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Originally Posted by willthetech View Post
what i gather from this thread so far...

1. I cant google LSPI and get thousands of very well studies done by REAL engineers...

2. I don't know how to drive manual because I choose to stay in a lower gear and keep my rpm higher than 2000 when cruising in the car the I own...

3. Vipers are more economical when going 120 MPH...

4. Going 40 in 5th gear and then WOTing to pass the school bus is perfectly normal and my engine will last over 101k doing this all the time.

Very informative!

Correct on everything except number 3, the collective understanding is that a Viper is in fact NOT more fuel efficient at 120, although it is possible it's engine is operating at a slightly higher overall efficiency when it is delivering 4-6 times more power and of course using 4-6 times more fuel to do so.

but otherwise your other points are mostly accurate
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Old 02-25-2020, 02:53 AM   #41
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Originally Posted by willthetech View Post
what i gather from this thread so far...

1. I cant google LSPI and get thousands of very well studies done by REAL engineers...

2. I don't know how to drive manual because I choose to stay in a lower gear and keep my rpm higher than 2000 when cruising in the car the I own...

3. Vipers are more economical when going 120 MPH...

4. Going 40 in 5th gear and then WOTing to pass the school bus is perfectly normal and my engine will last over 101k doing this all the time.

Very informative!

I started this thread and I don't even know if I got an answer.
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Old 02-25-2020, 07:30 AM   #42
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I started this thread and I don't even know if I got an answer.
You got a clear answer. Ignore the internet myths that you need to cruise at 3000 rpm's because "it's hard on rod bearings" since that is false.
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Old Yesterday, 12:48 AM   #43
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Originally Posted by rtv900 View Post
You got a clear answer. Ignore the internet myths that you need to cruise at 3000 rpm's because "it's hard on rod bearings" since that is false.
While on the topic, does lower cruising speeds overall increase oil dilution from fuel since the oil won't be getting as hot and therefore not evaporating the fuel into the PCV valve?
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Old Yesterday, 08:04 AM   #44
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I wouldn't worry about that as long as the cooling system gets up to temperature - basically meaning as long as the thermostat isn't stuck wide open and that your driving isn't all just very short trips.

This does assume that the engine (rings and cylinder bores particularly) is in good mechanical condition.


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Old Yesterday, 05:13 PM   #45
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While on the topic, does lower cruising speeds overall increase oil dilution from fuel since the oil won't be getting as hot and therefore not evaporating the fuel into the PCV valve?
the whole point of the 'cruising' rpm thing is that you are at very light load, so just because you are winging the engine at 3000 at light load doesn't mean you are building any more heat than at 2000 rpms.
Technically you'd be at even less throttle at 3000 rpm's and pumping much faster so if anything your oil would probably run cooler if I had to guess.

Regardless, for overall average driving it's irrelevant as you will have higher load times and the oil will still run around 200ish

It just isn't worth analyzing details like that when talking about what gear to drive in. Even 1500 rpm's runs fine in these cars when you are cruising along at 9% throttle on a flat road going 40mph.
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