power to weight or just more HP????
06-07-2008, 03:31 PM
#1
Race Director
Thread Starter
power to weight or just more HP????
I've been thinking about the effects of power to weight ratio..... Of course a lower number is always better....but then I started thinking about a scenario like this:
two cars with identical power to weight....1 weighs 1000lbs and has 100 hp or 10 to 1......the other car is 5000lbs and has 500hp also 10 to 1...lets say the aerodynamics are exactly the same for both cars.....in theory they should accelerate at the same rate...BUT I would think at higher speeds the 500hp car will pull away, since it has more power to overcome drag and should have a far higher top speed? Of course the lighter car will handle far better, but thats outside the scope of this question....
My guess is somewhere north of 60mph the 500hp car would start to pull away....
two cars with identical power to weight....1 weighs 1000lbs and has 100 hp or 10 to 1......the other car is 5000lbs and has 500hp also 10 to 1...lets say the aerodynamics are exactly the same for both cars.....in theory they should accelerate at the same rate...BUT I would think at higher speeds the 500hp car will pull away, since it has more power to overcome drag and should have a far higher top speed? Of course the lighter car will handle far better, but thats outside the scope of this question....
My guess is somewhere north of 60mph the 500hp car would start to pull away....
06-07-2008, 03:47 PM
#2
Race Car
Lateral acceleration (turns) will not be equal.
Off the line is harder to manage with a heavier car.
Yes the higher HP car will fight air resistance better, when air resistance becomes the more signficant part of the equation. I wouldn't be surprised if the break point was below 60mph but don't want to dust off a 20 year old physics book to figure it all out.
The problem with the heavier cars is that we seldom have the opportunity to enjoy them where they perform best - at illegal speeds. At the track, the problem is we have to figure out how to get heavier cars to actually turn.
500 hp heavyweights don't sound like much fun, but would definitley cruise really well at very high speeds.
In practice, lightness beats heft except **** maybe ***** in areas of safety, which is a generalization.
Off the line is harder to manage with a heavier car.
Yes the higher HP car will fight air resistance better, when air resistance becomes the more signficant part of the equation. I wouldn't be surprised if the break point was below 60mph but don't want to dust off a 20 year old physics book to figure it all out.
The problem with the heavier cars is that we seldom have the opportunity to enjoy them where they perform best - at illegal speeds. At the track, the problem is we have to figure out how to get heavier cars to actually turn.
500 hp heavyweights don't sound like much fun, but would definitley cruise really well at very high speeds.
In practice, lightness beats heft except **** maybe ***** in areas of safety, which is a generalization.
06-07-2008, 04:00 PM
#3
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Go watch a go kart race. A 360 lb shifter Kart w/driver and 28 hp 80cc motor will put up a faster time at most medium size tracks than everything other than a formula car.
06-07-2008, 04:03 PM
#4
Lifetime Rennlist Member
The higher HP Car will kill the lower HP car at higher speeds - certainly anything over 100mph. But is the aero is significant (say 500-1000 lbs of downforce at 100mph) the ligher car will be MUCH faster in the corners and will do faster lap times at most tracks. It will also significantly outbrake the heavier car due to that downforce from high speeds.
A real world example is my car vs. a race prepped Vette. I can drag race them up to about 120 mph then they pull away. If both are on slicks, we can both take low speed corners at ~1.5 g's. But in a 110mph corner, he Vette is still 1.5 g's and I am almost 3 g's. At many tracks my lap time would be 10 seconds faster on a 1:30 laptime.
A real world example is my car vs. a race prepped Vette. I can drag race them up to about 120 mph then they pull away. If both are on slicks, we can both take low speed corners at ~1.5 g's. But in a 110mph corner, he Vette is still 1.5 g's and I am almost 3 g's. At many tracks my lap time would be 10 seconds faster on a 1:30 laptime.
06-07-2008, 06:22 PM
#7
Rennlist Member
Good points all the way around. yes, in a vacuum same hp to weight ratio, at all speeds, (remember, acceleration= (power/(mass x velocity), will yield the same acceleration at any speed. you said 1000lb car vs 5000lb car with the same aerodynamics. Ill take this to mean same car, but maybe one is made of steel and the other of carbon fiber. The other way we could think of "same aerodynamics " would be to think both had the same Coefficient of Drag. (Power for acceleration with consideration of the drag factors has to be net power). With two scale versions of the same car, one being smaller than the other, both cars would accelerate at the same rate at any speed. cornering and braking would be a challenging comparison, so back to the original question.
If we are talking a same aerodynamics (same shape and size car), then yes, as the speed went up, the HP to drag ratio, independant of the HP to weight ratio would get worse. a good example of this is just look at a nice rear wing.
at 300lbs of downforce at near 150mph, the drag associated with it would be about 1/10th, or near 30lbs. based on a 1ft radius wheel and tire, that works out to 30ft/lbs of torque acting on the axles. Through the gear box of 3.2:1 for a 6000rpm engine, the effective torque at the engine would be approx 10ft-lbs. in otherwords, at 150mph, one car would have about a 10hp loss against the higher HP car. keeping things simple, say you have 500hp vs 100hp for the 5000lb vs1000lb cars. suddenly, the 1000lb car just lost 10% of its hp, while the 5000lb car only lost 2%. since this factor changes at the square of speed, you can see that the lighter car has some hude disadvantages at speed.
However, thats just acceleration. since race car basic performance is based on 3 factors. acceleration braking, and cornering. (ignoring a bunch of others)
the braking and cornering factors would FAR exceed the losses at the higher speed comparsions. Great example of this is the superbikes. At laguna, the bikes own acceleration (slow speed) but are weak under braking and cornering compared to a ALMS GT2. In the end, their times are near the same 1:26ish.
Now, give the motorcycles 4 wheels, (shifter karts) and they end up being as fast as anything on the track because they now can stop and corner as well as accelerate as fast as anything that hits the track.
Back to the question is where the 100hp car starts to loose to the 500hp car.
I think with this large difference in weight, we now need to look at rolling friction as well, but ignorning that, all we need to do is look at the drag coefficient x the frontal area. we will get some drag value. that number will go up with the square of the speed. Just looking at the wing drag alone, you can see that the effect is 10% HP loss vs 2% hp loss at 125-150mph'ish (for the light vs heavy car) This would be true for the entire drag total as well. If its 10x, then you see that the light car could have a top speed of 150mph, loosing all of its effective HP, while the heavy car might only have only lost 20% of his effective hp. Just look what a dramtic difference that makes on the "effective" or "Net" HP/weight ratios at speed! at speed, the 500hp 5000lb car has only lost 100hp, keeping in mind, the HP required to accelerate goes up with the cube of speed.
Take those numbers and work them backward and you can see where it might not matter that much. from the moment they both start working their way through the air, the higher hp car with a better "HP to drag" ratio will always have an advantage. If you look at the example above, what would be the drag at 60mph if at 150mph it was 100lbs? Well, it would be somewhere near 25lbs. through a 7:1 total gear ratio, the net effect on "Net HP" at 60mph might only be 4ft-lbs/4hp at 5250rpm.
So, on the 100hp, thats a 4% loss, on the 500hp car, thats near less than 1%. There is the ball park answer. (if total drag was near 100lbs at 150mph which is probalby a little light
)
mk
If we are talking a same aerodynamics (same shape and size car), then yes, as the speed went up, the HP to drag ratio, independant of the HP to weight ratio would get worse. a good example of this is just look at a nice rear wing.
at 300lbs of downforce at near 150mph, the drag associated with it would be about 1/10th, or near 30lbs. based on a 1ft radius wheel and tire, that works out to 30ft/lbs of torque acting on the axles. Through the gear box of 3.2:1 for a 6000rpm engine, the effective torque at the engine would be approx 10ft-lbs. in otherwords, at 150mph, one car would have about a 10hp loss against the higher HP car. keeping things simple, say you have 500hp vs 100hp for the 5000lb vs1000lb cars. suddenly, the 1000lb car just lost 10% of its hp, while the 5000lb car only lost 2%. since this factor changes at the square of speed, you can see that the lighter car has some hude disadvantages at speed.
However, thats just acceleration. since race car basic performance is based on 3 factors. acceleration braking, and cornering. (ignoring a bunch of others)
the braking and cornering factors would FAR exceed the losses at the higher speed comparsions. Great example of this is the superbikes. At laguna, the bikes own acceleration (slow speed) but are weak under braking and cornering compared to a ALMS GT2. In the end, their times are near the same 1:26ish.
Now, give the motorcycles 4 wheels, (shifter karts) and they end up being as fast as anything on the track because they now can stop and corner as well as accelerate as fast as anything that hits the track.
Back to the question is where the 100hp car starts to loose to the 500hp car.
I think with this large difference in weight, we now need to look at rolling friction as well, but ignorning that, all we need to do is look at the drag coefficient x the frontal area. we will get some drag value. that number will go up with the square of the speed. Just looking at the wing drag alone, you can see that the effect is 10% HP loss vs 2% hp loss at 125-150mph'ish (for the light vs heavy car) This would be true for the entire drag total as well. If its 10x, then you see that the light car could have a top speed of 150mph, loosing all of its effective HP, while the heavy car might only have only lost 20% of his effective hp. Just look what a dramtic difference that makes on the "effective" or "Net" HP/weight ratios at speed! at speed, the 500hp 5000lb car has only lost 100hp, keeping in mind, the HP required to accelerate goes up with the cube of speed.
Take those numbers and work them backward and you can see where it might not matter that much. from the moment they both start working their way through the air, the higher hp car with a better "HP to drag" ratio will always have an advantage. If you look at the example above, what would be the drag at 60mph if at 150mph it was 100lbs? Well, it would be somewhere near 25lbs. through a 7:1 total gear ratio, the net effect on "Net HP" at 60mph might only be 4ft-lbs/4hp at 5250rpm.
So, on the 100hp, thats a 4% loss, on the 500hp car, thats near less than 1%. There is the ball park answer. (if total drag was near 100lbs at 150mph which is probalby a little light
)mk
I've been thinking about the effects of power to weight ratio..... Of course a lower number is always better....but then I started thinking about a scenario like this:
two cars with identical power to weight....1 weighs 1000lbs and has 100 hp or 10 to 1......the other car is 5000lbs and has 500hp also 10 to 1...lets say the aerodynamics are exactly the same for both cars.....in theory they should accelerate at the same rate...BUT I would think at higher speeds the 500hp car will pull away, since it has more power to overcome drag and should have a far higher top speed? Of course the lighter car will handle far better, but thats outside the scope of this question....
My guess is somewhere north of 60mph the 500hp car would start to pull away....
two cars with identical power to weight....1 weighs 1000lbs and has 100 hp or 10 to 1......the other car is 5000lbs and has 500hp also 10 to 1...lets say the aerodynamics are exactly the same for both cars.....in theory they should accelerate at the same rate...BUT I would think at higher speeds the 500hp car will pull away, since it has more power to overcome drag and should have a far higher top speed? Of course the lighter car will handle far better, but thats outside the scope of this question....
My guess is somewhere north of 60mph the 500hp car would start to pull away....
Last edited by mark kibort; 06-07-2008 at 06:54 PM.
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06-08-2008, 04:23 PM
#8
Race Director
Thread Starter
Interesting....about what I thought it would be.....of course the 1000lb car would destroy the 5000lb car at every racetrack since it is far easier to turn and stop.....but I was only considering straight line performance....comparing the effects of drag against power to weight ratio & total HP or drag-HP....
The weight question gets even more "gray" in cars like the Nissan GTR....I still can't understand how a car thats over 4000lbs with driver, has relatively small tires (255 front 285 rear) can handle as well as it does....however it does seem it has some wickedly heavy springs (from all the harsh ride comments from road testers).....I stil can't understand how a Z06 with more power, larger tires and far less weight gets "beat" by the GTR....
The weight question gets even more "gray" in cars like the Nissan GTR....I still can't understand how a car thats over 4000lbs with driver, has relatively small tires (255 front 285 rear) can handle as well as it does....however it does seem it has some wickedly heavy springs (from all the harsh ride comments from road testers).....I stil can't understand how a Z06 with more power, larger tires and far less weight gets "beat" by the GTR....
06-08-2008, 08:03 PM
#9
Rennlist Member
The Nissan, in the tests ive seen has been a little bit of question mark for me.
first of all, no one really know what the HP was. It was supected to have a lot more than they admit. that would certainly be a good reason why the Nissan was pulling on the porsche turbo in that test in japan. tires are a big factor as well. I will tell you this, if the HP to weight ratios are the same, and they are with the porsche turbo , Z06 and the nissan GTR, all 3 will be pretty close. I would take professional racers (not drivers) to figure out what one has the advantage. the 4x4 of the nissan with its balance is a pretty potent package.
Look no farther than the Audi RS6 with near 300lbs of rewards weight in speedGT, and its 2:14 times at Road america! (and those guys were saying " but the boost is below stock levels". Bottom line, the car was able to brake, corner and accelerate much faster than the pumped up GT3s, vipers, vets and certainly mark andersons 500rwhp 928!
Getting back to the Nissan. I seriously doubt it had any less hp than the Vet, in fact, it was rumored to have much more than they are spec'ing that car out to have. When it all is boiled down, when you compare the cars and they are apples and apples, the differences are usually pretty small in performance.
The another good example of this suprising performance comparison was with an another test of a new M3 that had a lot less hp but was able to hang, with the Z06 in a test a few years back. 333hp vs 405hp. the handling of the M3 was better and so was its braking. (reasons?? i dont know, the M3s always seem to be more "turney than other cars its size". the difference in HP was probably almost matched by the difference in weights, even though those weights were pretty close.
mk
first of all, no one really know what the HP was. It was supected to have a lot more than they admit. that would certainly be a good reason why the Nissan was pulling on the porsche turbo in that test in japan. tires are a big factor as well. I will tell you this, if the HP to weight ratios are the same, and they are with the porsche turbo , Z06 and the nissan GTR, all 3 will be pretty close. I would take professional racers (not drivers) to figure out what one has the advantage. the 4x4 of the nissan with its balance is a pretty potent package.
Look no farther than the Audi RS6 with near 300lbs of rewards weight in speedGT, and its 2:14 times at Road america! (and those guys were saying " but the boost is below stock levels".
Bottom line, the car was able to brake, corner and accelerate much faster than the pumped up GT3s, vipers, vets and certainly mark andersons 500rwhp 928!Getting back to the Nissan. I seriously doubt it had any less hp than the Vet, in fact, it was rumored to have much more than they are spec'ing that car out to have. When it all is boiled down, when you compare the cars and they are apples and apples, the differences are usually pretty small in performance.
The another good example of this suprising performance comparison was with an another test of a new M3 that had a lot less hp but was able to hang, with the Z06 in a test a few years back. 333hp vs 405hp. the handling of the M3 was better and so was its braking. (reasons?? i dont know, the M3s always seem to be more "turney than other cars its size". the difference in HP was probably almost matched by the difference in weights, even though those weights were pretty close.
mk
Interesting....about what I thought it would be.....of course the 1000lb car would destroy the 5000lb car at every racetrack since it is far easier to turn and stop.....but I was only considering straight line performance....comparing the effects of drag against power to weight ratio & total HP or drag-HP....
The weight question gets even more "gray" in cars like the Nissan GTR....I still can't understand how a car thats over 4000lbs with driver, has relatively small tires (255 front 285 rear) can handle as well as it does....however it does seem it has some wickedly heavy springs (from all the harsh ride comments from road testers).....I stil can't understand how a Z06 with more power, larger tires and far less weight gets "beat" by the GTR....
The weight question gets even more "gray" in cars like the Nissan GTR....I still can't understand how a car thats over 4000lbs with driver, has relatively small tires (255 front 285 rear) can handle as well as it does....however it does seem it has some wickedly heavy springs (from all the harsh ride comments from road testers).....I stil can't understand how a Z06 with more power, larger tires and far less weight gets "beat" by the GTR....
06-08-2008, 08:10 PM
#10
Rennlist Member
The Nissan, in the tests ive seen has been a little bit of question mark for me.
first of all, no one really know what the HP was. It was supected to have a lot more than they admit. that would certainly be a good reason why the Nissan was pulling on the porsche turbo in that test in japan. tires are a big factor as well. I will tell you this, if the HP to weight ratios are the same, and they are with the porsche turbo , Z06 and the nissan GTR, all 3 will be pretty close. I would take professional racers (not drivers) to figure out what one has the advantage. the 4x4 of the nissan with its balance is a pretty potent package.
Look no farther than the Audi RS6 with near 300lbs of rewards weight in speedGT (with total weight in the 3500lbs range) , and its 2:14 times at Road america, on DOTs too!! (and those guys were saying " But the boost is below stock levels". Bottom line, the car was able to brake, corner and accelerate much faster than the pumped up GT3 cup car porches, vipers, vets and certainly mark andersons 500rwhp 928!
Getting back to the Nissan. I seriously doubt it had any less hp than the Vet, in fact, it was rumored to have much more than they are spec'ing that car out to have. When it all is boiled down, when you compare the cars and they are apples and apples, the differences are usually pretty small in performance.
The another good example of this suprising performance comparison was with an another test of a new M3 that had a lot less hp but was able to hang, with the Z06 in a test a few years back. 333hp vs 405hp. the handling of the M3 was better and so was its braking. (reasons?? i dont know, the M3s always seem to be more "turney than other cars its size". the difference in HP was probably almost matched by the difference in weights, even though those weights were pretty close.
mk
first of all, no one really know what the HP was. It was supected to have a lot more than they admit. that would certainly be a good reason why the Nissan was pulling on the porsche turbo in that test in japan. tires are a big factor as well. I will tell you this, if the HP to weight ratios are the same, and they are with the porsche turbo , Z06 and the nissan GTR, all 3 will be pretty close. I would take professional racers (not drivers) to figure out what one has the advantage. the 4x4 of the nissan with its balance is a pretty potent package.
Look no farther than the Audi RS6 with near 300lbs of rewards weight in speedGT (with total weight in the 3500lbs range) , and its 2:14 times at Road america, on DOTs too!! (and those guys were saying " But the boost is below stock levels".
Bottom line, the car was able to brake, corner and accelerate much faster than the pumped up GT3 cup car porches, vipers, vets and certainly mark andersons 500rwhp 928!Getting back to the Nissan. I seriously doubt it had any less hp than the Vet, in fact, it was rumored to have much more than they are spec'ing that car out to have. When it all is boiled down, when you compare the cars and they are apples and apples, the differences are usually pretty small in performance.
The another good example of this suprising performance comparison was with an another test of a new M3 that had a lot less hp but was able to hang, with the Z06 in a test a few years back. 333hp vs 405hp. the handling of the M3 was better and so was its braking. (reasons?? i dont know, the M3s always seem to be more "turney than other cars its size". the difference in HP was probably almost matched by the difference in weights, even though those weights were pretty close.
mk
Interesting....about what I thought it would be.....of course the 1000lb car would destroy the 5000lb car at every racetrack since it is far easier to turn and stop.....but I was only considering straight line performance....comparing the effects of drag against power to weight ratio & total HP or drag-HP....
The weight question gets even more "gray" in cars like the Nissan GTR....I still can't understand how a car thats over 4000lbs with driver, has relatively small tires (255 front 285 rear) can handle as well as it does....however it does seem it has some wickedly heavy springs (from all the harsh ride comments from road testers).....I stil can't understand how a Z06 with more power, larger tires and far less weight gets "beat" by the GTR....
The weight question gets even more "gray" in cars like the Nissan GTR....I still can't understand how a car thats over 4000lbs with driver, has relatively small tires (255 front 285 rear) can handle as well as it does....however it does seem it has some wickedly heavy springs (from all the harsh ride comments from road testers).....I stil can't understand how a Z06 with more power, larger tires and far less weight gets "beat" by the GTR....
06-09-2008, 03:43 PM
#11
I've been thinking about the effects of power to weight ratio..... Of course a lower number is always better....but then I started thinking about a scenario like this:
two cars with identical power to weight....1 weighs 1000lbs and has 100 hp or 10 to 1......the other car is 5000lbs and has 500hp also 10 to 1...lets say the aerodynamics are exactly the same for both cars.....in theory they should accelerate at the same rate...BUT I would think at higher speeds the 500hp car will pull away, since it has more power to overcome drag and should have a far higher top speed? Of course the lighter car will handle far better, but thats outside the scope of this question....
My guess is somewhere north of 60mph the 500hp car would start to pull away....
two cars with identical power to weight....1 weighs 1000lbs and has 100 hp or 10 to 1......the other car is 5000lbs and has 500hp also 10 to 1...lets say the aerodynamics are exactly the same for both cars.....in theory they should accelerate at the same rate...BUT I would think at higher speeds the 500hp car will pull away, since it has more power to overcome drag and should have a far higher top speed? Of course the lighter car will handle far better, but thats outside the scope of this question....
My guess is somewhere north of 60mph the 500hp car would start to pull away....
It depends on what you are trying to do: go fastest around a auto-X, road course, straight line drag or top speed run. For an Auto-X, you area always better with lighter more nimble cars than power. This is why Miatas run circles around 350Zs and why Lotus Elises/Exiges are beating up Corvettes.
On a round course it is overall balance and grip. Given the same power to weight, the lighter car should easily have the advantage here. Without as much weight, there should be better grip and far better response giving significantly better track times.
In a straight line drag it is anybodies game. The heavier car may have 4WD, which would give it an advantage off the line.
In a top speed run, you alwasy want more power. Top speed is power versus aerodynamics. Weight has no factor on top speed. The only thing weight effects on a top speed run is how long it takes to get there. Hence why a Bugatti Veyron has a higher top speed than a McClaren F1, yet is much slower accelerating all the way up to 230 mph. The McClaren has a better power to weight and better aerodynamics (lower coeficient of drag and smaller frontal area). Despite this advantage, it hits the aerodynamic wall 9 mph lower than the Veyron.
06-09-2008, 10:51 PM
#12
Rennlist Member
The question for me is do I shed 100 lbs from the car to optimize my pwr to weight (14.5 lbs per hp) or do I do i add the ~7 hp.
Mike
Mike
06-09-2008, 10:53 PM
#13
Rennlist Member
Rubbish. All depends on the track. At a track with very short straights and/or lots of relatively low speed corners, you might be correct. At a track with longer straights and/or high speed sweeping corners, you'd probably be wrong.
06-09-2008, 11:01 PM
#14
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Good points all the way around. yes, in a vacuum same hp to weight ratio, at all speeds, (remember, acceleration= (power/(mass x velocity), will yield the same acceleration at any speed. you said 1000lb car vs 5000lb car with the same aerodynamics. Ill take this to mean same car, but maybe one is made of steel and the other of carbon fiber. The other way we could think of "same aerodynamics " would be to think both had the same Coefficient of Drag. (Power for acceleration with consideration of the drag factors has to be net power). With two scale versions of the same car, one being smaller than the other, both cars would accelerate at the same rate at any speed. cornering and braking would be a challenging comparison, so back to the original question.
If we are talking a same aerodynamics (same shape and size car), then yes, as the speed went up, the HP to drag ratio, independant of the HP to weight ratio would get worse. a good example of this is just look at a nice rear wing.
at 300lbs of downforce at near 150mph, the drag associated with it would be about 1/10th, or near 30lbs. based on a 1ft radius wheel and tire, that works out to 30ft/lbs of torque acting on the axles. Through the gear box of 3.2:1 for a 6000rpm engine, the effective torque at the engine would be approx 10ft-lbs. in otherwords, at 150mph, one car would have about a 10hp loss against the higher HP car. keeping things simple, say you have 500hp vs 100hp for the 5000lb vs1000lb cars. suddenly, the 1000lb car just lost 10% of its hp, while the 5000lb car only lost 2%. since this factor changes at the square of speed, you can see that the lighter car has some hude disadvantages at speed.
However, thats just acceleration. since race car basic performance is based on 3 factors. acceleration braking, and cornering. (ignoring a bunch of others)
the braking and cornering factors would FAR exceed the losses at the higher speed comparsions. Great example of this is the superbikes. At laguna, the bikes own acceleration (slow speed) but are weak under braking and cornering compared to a ALMS GT2. In the end, their times are near the same 1:26ish.
Now, give the motorcycles 4 wheels, (shifter karts) and they end up being as fast as anything on the track because they now can stop and corner as well as accelerate as fast as anything that hits the track.
Back to the question is where the 100hp car starts to loose to the 500hp car.
I think with this large difference in weight, we now need to look at rolling friction as well, but ignorning that, all we need to do is look at the drag coefficient x the frontal area. we will get some drag value. that number will go up with the square of the speed. Just looking at the wing drag alone, you can see that the effect is 10% HP loss vs 2% hp loss at 125-150mph'ish (for the light vs heavy car) This would be true for the entire drag total as well. If its 10x, then you see that the light car could have a top speed of 150mph, loosing all of its effective HP, while the heavy car might only have only lost 20% of his effective hp. Just look what a dramtic difference that makes on the "effective" or "Net" HP/weight ratios at speed! at speed, the 500hp 5000lb car has only lost 100hp, keeping in mind, the HP required to accelerate goes up with the cube of speed.
Take those numbers and work them backward and you can see where it might not matter that much. from the moment they both start working their way through the air, the higher hp car with a better "HP to drag" ratio will always have an advantage. If you look at the example above, what would be the drag at 60mph if at 150mph it was 100lbs? Well, it would be somewhere near 25lbs. through a 7:1 total gear ratio, the net effect on "Net HP" at 60mph might only be 4ft-lbs/4hp at 5250rpm.
So, on the 100hp, thats a 4% loss, on the 500hp car, thats near less than 1%. There is the ball park answer. (if total drag was near 100lbs at 150mph which is probalby a little light)
mk
If we are talking a same aerodynamics (same shape and size car), then yes, as the speed went up, the HP to drag ratio, independant of the HP to weight ratio would get worse. a good example of this is just look at a nice rear wing.
at 300lbs of downforce at near 150mph, the drag associated with it would be about 1/10th, or near 30lbs. based on a 1ft radius wheel and tire, that works out to 30ft/lbs of torque acting on the axles. Through the gear box of 3.2:1 for a 6000rpm engine, the effective torque at the engine would be approx 10ft-lbs. in otherwords, at 150mph, one car would have about a 10hp loss against the higher HP car. keeping things simple, say you have 500hp vs 100hp for the 5000lb vs1000lb cars. suddenly, the 1000lb car just lost 10% of its hp, while the 5000lb car only lost 2%. since this factor changes at the square of speed, you can see that the lighter car has some hude disadvantages at speed.
However, thats just acceleration. since race car basic performance is based on 3 factors. acceleration braking, and cornering. (ignoring a bunch of others)
the braking and cornering factors would FAR exceed the losses at the higher speed comparsions. Great example of this is the superbikes. At laguna, the bikes own acceleration (slow speed) but are weak under braking and cornering compared to a ALMS GT2. In the end, their times are near the same 1:26ish.
Now, give the motorcycles 4 wheels, (shifter karts) and they end up being as fast as anything on the track because they now can stop and corner as well as accelerate as fast as anything that hits the track.
Back to the question is where the 100hp car starts to loose to the 500hp car.
I think with this large difference in weight, we now need to look at rolling friction as well, but ignorning that, all we need to do is look at the drag coefficient x the frontal area. we will get some drag value. that number will go up with the square of the speed. Just looking at the wing drag alone, you can see that the effect is 10% HP loss vs 2% hp loss at 125-150mph'ish (for the light vs heavy car) This would be true for the entire drag total as well. If its 10x, then you see that the light car could have a top speed of 150mph, loosing all of its effective HP, while the heavy car might only have only lost 20% of his effective hp. Just look what a dramtic difference that makes on the "effective" or "Net" HP/weight ratios at speed! at speed, the 500hp 5000lb car has only lost 100hp, keeping in mind, the HP required to accelerate goes up with the cube of speed.
Take those numbers and work them backward and you can see where it might not matter that much. from the moment they both start working their way through the air, the higher hp car with a better "HP to drag" ratio will always have an advantage. If you look at the example above, what would be the drag at 60mph if at 150mph it was 100lbs? Well, it would be somewhere near 25lbs. through a 7:1 total gear ratio, the net effect on "Net HP" at 60mph might only be 4ft-lbs/4hp at 5250rpm.
So, on the 100hp, thats a 4% loss, on the 500hp car, thats near less than 1%. There is the ball park answer. (if total drag was near 100lbs at 150mph which is probalby a little light
)mk
OUCH! My head hurts. Make it stop!
Honestly though, you need to consider 1 hp gain vs 2-4 lbs of weight reduction. Which is cheaper, which does not reduce reliabilty, which has better drivabiltiy characteristics.
If I could shave 100 lbs off my car for nothing, that's what I'd do first. Conversely, if I could gain 10 hp and take 10 lbs off my car at the same time (removing the cat), then that is the best of both worlds.
The answer to you question is easy. Take the free road first. After that is exhausted, do a cost/benefit analysis and go with the answer that offers the greatest benefit for the least amount of money.
There! Thread Closed!
06-09-2008, 11:44 PM
#15
Rennlist Member
ok, so after all the physics talk...lets use a simple car comparison...
996 gt3 with 395 hp and around 2950 lbs VS a 996 turbo at 3400 lbs with 430 hp (so there isnt a HUGE amount in between the two, but noticeable). Assuming that the turbo had enough suspension work, how do you think the scenario would play out at different tracks?
(reason why is I have both currently in the stable...and feel like the gt3 will be faster nearly everywhere...short of MAYBE 160 plus (which is very rare to hit anywhere other than pocono or VIR)...
996 gt3 with 395 hp and around 2950 lbs VS a 996 turbo at 3400 lbs with 430 hp (so there isnt a HUGE amount in between the two, but noticeable). Assuming that the turbo had enough suspension work, how do you think the scenario would play out at different tracks?
(reason why is I have both currently in the stable...and feel like the gt3 will be faster nearly everywhere...short of MAYBE 160 plus (which is very rare to hit anywhere other than pocono or VIR)...