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MPH and Gear Ratios - Audi
Comments
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Yep, and it’s a big effect too, if a car travelling at 50 mph needs 30 horse power to maintain the speed then the same car needs eight times the power so 240 horse power to cruse at 100 mph.
On the 85 being more economical than 75, as per my post above about max efficiency “sweet spot” it is possible if moving from 75 to 85 brings the engine from a less to a more efficient operating point although in practice it’s only really likely to be true on a very highly tuned engine with a narrow power band and hence a very steep slope away from the sweet spot.
On a normal road car any efficiency gain is likely going to be swamped by the increased drag (15% more speed needs [STRIKE]120%[/STRIKE] 52% more power)0 -
On a normal road car any efficiency gain is likely going to be swamped by the increased drag (15% more speed needs 120% more power)
?
Your previous example was a cube relationship - double the speed requires 8 times the power. By the same relationship, 15% more speed would need 52% more power, not 120%...0 -
?
Your previous example was a cube relationship - double the speed requires 8 times the power. By the same relationship, 15% more speed would need 52% more power, not 120%...
??
It’s been many years since I last did fluid mechanics in school but I thought…….
[STRIKE]
50 to 100 is 100% more speed times eight so 800% more power
75 to 85 is 15% more speed times eight so 120% more power
[/STRIKE]0 -
??
It’s been many years since I last did fluid mechanics in school but I thought…….
50 to 100 is 100% more speed times eight so 800% more power
75 to 85 is 15% more speed times eight so 120% more power
75 to 85 is 1.15 x the speed
1.15^3 is 1.52 x the power
So 52% more is right - you can't multiply 15 by 8 to get your answer.
EDIT: and for your first example, it is 2x the speed, where 2^3 = 8, you need 8 times the power, not 800% more power (which would be 900% of the original power). You need to look at multiples, not percentages
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FWIW the cubic relationship is correct:
Aerodynamic drag force = 0.5 * ρ * Cd * A * v^2
Where
ρ = air density
Cd = drag co-efficient
A = frontal area
v = speed
So drag force is related to speed squared, and...
Power = Force * speed
...giving the cubic relationship.
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IMO it can only really be 1.) Going down hill, more kinetic enrgy and 2.) Going up the other side, as you may be in or above peak torque, until you have to start making the engine work again.How can 85mph be more economical than 70mph?
Heard a discussion on the radio re possible increase in Motorway Speeds recently, a contributor was adamant his high spec auto wouldn't actually change up until he was well over the limit and that it was more economical to actual keep that speed when it did. :rotfl::rotfl::rotfl::rotfl::rotfl:"If you act like an illiterate man, your learning will never stop... Being uneducated, you have no fear of the future.".....
"big business is parasitic, like a mosquito, whereas I prefer the lighter touch, like that of a butterfly. "A butterfly can suck honey from the flower without damaging it," "Arunachalam Muruganantham0 -
Off thread I know but it is worse with boats/ships as the drag becomes almost impossible unless it comes out of the water of course. Don't they hit a "brick wall" around 30 ish Knots require ridiculous increase in of power and fuel to take them faster?FWIW the cubic relationship is correct:
Aerodynamic drag force = 0.5 * ρ * Cd * A * v^2
Where
ρ = air density
Cd = drag co-efficient
A = frontal area
v = speed
So drag force is related to speed squared, and...
Power = Force * speed
...giving the cubic relationship."If you act like an illiterate man, your learning will never stop... Being uneducated, you have no fear of the future.".....
"big business is parasitic, like a mosquito, whereas I prefer the lighter touch, like that of a butterfly. "A butterfly can suck honey from the flower without damaging it," "Arunachalam Muruganantham0 -
It can't 99% of the time in a normal car under normal conditions, this is from the states but the same hold true in the UK.How can 85mph be more economical than 70mph?
In the June special edition (No. 1165) of Auto Express there was an interesting 24 page section on saving dosh which included how to use less fuel. They used a Honda Civic 1.8 and set a benchmark of 49mpg and got 3 readers to have a go and then gave them tips on how to improve.
Usual advice - conserve momentum, anticipating the road ahead and traffic lights, using the brakes less, slow down, decluttering, use aircon at faster speeds only. Also interestingly removing a boot spoiler on a Vauxhall which they said would be costing fuel, particularly at higher speeds. The readers got between 39 and 45mpg!
Thay also did a test at a constant speed with a Laguna round a banked track at 60, 70 and 80mph, the results were as you would expect - 47.7, 40.2 and 33.1mpg - hell of a difference though, 44% more fuel used at 80mph compared to 60mph
If at first you don't succeed, try, try, try - oh bu99er that just cheat
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grizzly1911 wrote: »Off thread I know but it is worse with boats/ships as the drag becomes almost impossible unless it comes out of the water of course. Don't they hit a "brick wall" around 30 ish Knots require ridiculous increase in of power and fuel to take them faster?
Not off thread at all (or a least no more off thread than others)
I’d guess the same relationship applies to boats, the density of the fluid changes but the drag is still going to be related to the cube of the speed so to double the speed you need eight times the power.
Fast boats are designed to “plane” which means the hull rises up out of the water as you go faster and ideally is just skimming the surface. This will have the expected dramatic reduction on drag (and can be very dramatic if they get it wrong as boats become planes)
Something similar also happened with a car at the Le Mans 24 hour a couple of years, google will throw up lots of example of flying boats & cars
http://www.youtube.com/watch?v=WLYIWY00X8M
http://www.youtube.com/watch?v=h0t5juiDfr8
drag & aerodynamics generally is a very powerful force0 -
Well i started a bit of a debate there! Thanks for all the help and advice.
Its a 1.9 tdi A4 5 speed and at 70mph Im doing just over 2350 rpm in 5th. The sweet spot of the engine seems to be around 1900 rpm.
It would just feel altogether better with a bigger ratio
(sorry for the dealy in responding as i could not log in on my desk top- just tried it on on lap top and bingo !Hi, we’ve had to remove your signature. If you’re not sure why please read the forum rules or email the forum team if you’re still unsure - MSE ForumTeam0
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