power to weight or just more HP????
06-10-2008 | 04:45 PM
#31
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From: saratoga, ca
So, as i posted:
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 )
Edit: Doing a quick calculaton of a WR speed setting car like mine, at 300hp/300ftlbs of torque at a max speed of 180mph, looks like 100lbs of drag is about 2x off. so call the losses going up to 2% vs 8% at 60mph for the heavy and light cars, and at 150mph, the light car might peak out at 100mph top speed! while the heavy car might have only lost 40% of its "net" power. I think a better comparison in this case would be comparing a 5000lb car with a 2000lb car, at least this way, they both could operate at 150mph.
Head hurt now??
mk
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 )
Edit: Doing a quick calculaton of a WR speed setting car like mine, at 300hp/300ftlbs of torque at a max speed of 180mph, looks like 100lbs of drag is about 2x off. so call the losses going up to 2% vs 8% at 60mph for the heavy and light cars, and at 150mph, the light car might peak out at 100mph top speed! while the heavy car might have only lost 40% of its "net" power. I think a better comparison in this case would be comparing a 5000lb car with a 2000lb car, at least this way, they both could operate at 150mph.
Head hurt now??mk
Last edited by mark kibort; 06-10-2008 at 06:35 PM.
06-10-2008 | 05:48 PM
#32
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I dont know why the previous poster's head was hurting.
basically, just keep in mind the "Three Factors". depending on the track. (said already by a few already) the factors need to be looked at more closely.
What i like to do, is find situations where emperical data can be found.
Well, look no farther than the SpeedGT world challenge again. there has been two, and now 3 porche turbo 996s that have run. I know that many of them can get near 500rwhp with only a few mods, some of which are legal. their 3200lbs weight vs the GT3 cup cars with the upgraded engine having weight under 2900lbs and RWHP in the 420 range. 300llbs can be worth near 30hp, and close ratio gear boxes might be worth a few more. The weight however, helps in braking and cornering. These becomes the major factors.
With your comparison of two very different cars, and near 450lbs of weight difference, with only 35hp as a power difference, the turbo would get smoked in every possible way. Even if the power difference was greater, the lighter car would have the advantage in 2/3s of those dependant factors.
Ive spent a good part of the last few years competiting against cars fitting in both ends of the performance catagories. The C5 i race with is 250lbs heavier, but has 90more hp. His bigger tires and more effective downforce, gives him some near same handling, but underbraking, i have a sligth edge. Sure there might be driver differences, but over 100s of laps together , you start to see some of the differences. on the other side, i had an S2000 and a M3 that were much lighter than i was on smaller tires, but were able to spin the same lap times with only 15 less HP. (the S2000 being 300lbs lighter and had 50 lessHP. The point here is that there are a bunch of trade offs.
Back to your turbo, if you can gut it, pull out the FWD and bump up the HP , you could have a real monster on your hands. But, the GT3 with most of its weight already out of the car, might be a better place to start and build from.
mk
basically, just keep in mind the "Three Factors". depending on the track. (said already by a few already) the factors need to be looked at more closely.
What i like to do, is find situations where emperical data can be found.
Well, look no farther than the SpeedGT world challenge again. there has been two, and now 3 porche turbo 996s that have run. I know that many of them can get near 500rwhp with only a few mods, some of which are legal. their 3200lbs weight vs the GT3 cup cars with the upgraded engine having weight under 2900lbs and RWHP in the 420 range. 300llbs can be worth near 30hp, and close ratio gear boxes might be worth a few more. The weight however, helps in braking and cornering. These becomes the major factors.
With your comparison of two very different cars, and near 450lbs of weight difference, with only 35hp as a power difference, the turbo would get smoked in every possible way. Even if the power difference was greater, the lighter car would have the advantage in 2/3s of those dependant factors.
Ive spent a good part of the last few years competiting against cars fitting in both ends of the performance catagories. The C5 i race with is 250lbs heavier, but has 90more hp. His bigger tires and more effective downforce, gives him some near same handling, but underbraking, i have a sligth edge. Sure there might be driver differences, but over 100s of laps together , you start to see some of the differences. on the other side, i had an S2000 and a M3 that were much lighter than i was on smaller tires, but were able to spin the same lap times with only 15 less HP. (the S2000 being 300lbs lighter and had 50 lessHP. The point here is that there are a bunch of trade offs.
Back to your turbo, if you can gut it, pull out the FWD and bump up the HP , you could have a real monster on your hands. But, the GT3 with most of its weight already out of the car, might be a better place to start and build from.
mk
06-10-2008 | 07:38 PM
#36
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From: KC
Huh? Let me see if i clearly understand that first comment.
Are you saying that a heavier car can brake and corner as well as a lighter car the same exact shape and size. (that is the discussion by the way, as we are talking about performance differnces due to drag at the higher speeds for two cars the same aero, or same shape and size, but different weights and HP is different to provide same HP/weight ratios)
Basic physics will say, for the same grip, the heavier car will take as much hp difference to stop the same as it would to accelerate the same. In other words, for the heavier car to stop as well, it will need to find some BIG braking hp advantage. in order for it to corner as well, it also will need to find some cornering power advantage. (pop up splitters and wings for example)
I provided a ball park differnce of the HP losses for any speed based on a known drag at speed. Im sure the values are close enough for discussion,
but bascially, we are talking a near 2% loss in power at top speeds for the high powered heavier racer, vs near 10% losses for the lighter racer due to aerodynamic drag. These losses go down with speed, inverse to the square of the speed.
You also have a slight contradiction in your last paragraph regarding the lighter car having a cornering advantage. (you reference downforce) I agree , but keep in mind for a given downforce, there will be drag associated with it as well as rolling friction increases. I guess that would fall in to the comparisons of the aero drag forces with the two racers
mk
Are you saying that a heavier car can brake and corner as well as a lighter car the same exact shape and size. (that is the discussion by the way, as we are talking about performance differnces due to drag at the higher speeds for two cars the same aero, or same shape and size, but different weights and HP is different to provide same HP/weight ratios)
Basic physics will say, for the same grip, the heavier car will take as much hp difference to stop the same as it would to accelerate the same. In other words, for the heavier car to stop as well, it will need to find some BIG braking hp advantage. in order for it to corner as well, it also will need to find some cornering power advantage. (pop up splitters and wings for example
)I provided a ball park differnce of the HP losses for any speed based on a known drag at speed. Im sure the values are close enough for discussion,
but bascially, we are talking a near 2% loss in power at top speeds for the high powered heavier racer, vs near 10% losses for the lighter racer due to aerodynamic drag. These losses go down with speed, inverse to the square of the speed.
You also have a slight contradiction in your last paragraph regarding the lighter car having a cornering advantage. (you reference downforce) I agree , but keep in mind for a given downforce, there will be drag associated with it as well as rolling friction increases. I guess that would fall in to the comparisons of the aero drag forces with the two racers
mk
06-10-2008 | 08:20 PM
#38
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From: MA
As I've discovered so far this season. Where I never had problems heating my tires in the past, the wider rubber and reducution in overall weight has made me think that, at least in the spring and fall, I certainly should be running narrower rubber.
06-10-2008 | 09:04 PM
#39
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From: KC
Or increase toe in cold weather to generate more heat.
06-11-2008 | 12:42 AM
#41
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From: saratoga, ca
You have to start driving at their edge now, not the speeds you used to drive at !
06-11-2008 | 12:49 AM
#42
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From: saratoga, ca
I understand the point you were trying to make, but by ignoring real world forces, would be analogous to ignoring aero in the question that started this thread. (if HP to weight is the same and the test is in a vacuum both cars will accelerate the same at any speed) (i.e acceleration = power/(mass x velocity) Now, if you are talking scale models of the heavier car, making the smaller one 1/5th scale in all respects, then you would be correct.
the driver of the lighter car has to drive at the edge to generate the same heat in the tire. As long as he is not following the heavier car and cant make the pass, he will be able to stop and corner better once he is in front
Mk
the driver of the lighter car has to drive at the edge to generate the same heat in the tire. As long as he is not following the heavier car and cant make the pass, he will be able to stop and corner better once he is in front
Mk
The point I am making is that in a theoretical car (ignoring the real world of grip vs load curves, etc) there is NO advantage in cornering of braking due to lighter weight. A tire with a Coefficient of friction of 1.0 will produce 1.0 g's for the 1,000 lb car and will do the same for the 5,000 lb car. This is basic physics and kinematics. When you add the real world, it can go either way. The lighter car may do better but OTOH, it may struggle to get enough heat in the tires and the heavier car will excel.
06-11-2008 | 05:31 AM
#43
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Huh? All else being equal, there is no reason a lighter car will corner better nor stop in a shorter distance - basic physics tells us that grip is proportional to weight. So unless you are reading more into the question and making assumptions that were not part of the question, there is no difference in cornering nor braking. As I said earlier, if each has the same pounds of downforce then the lighter car wins out on cornering but the higher HP will have the acceleration advantage.
Comparing different cars on top speed vs. lap time has nothing to do with the original premise of this thread.
Comparing different cars on top speed vs. lap time has nothing to do with the original premise of this thread.
06-11-2008 | 07:06 AM
#44
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Hahahahaha... To quote Bugs, 'he dont know me very well... do he?' You are absolutely right, though, I do have to learn to spend more time on the edge instead of over it. But forget about that. Previously, with the car at 2250 empty, running 225/245s I'd routinely raise pressures, even on a cool day, from 32 starting to > 44#s. Now with some 200#s less and 255/315s on an overcast day with ambient in the mid fifties on a hard compound tire, pressures go up on the order of 2-3 pounds total. Same car, well, same track, same driver. Perhaps as was suggested by Sunday, more thinking around setup might be beneficial. In the meantime, I'll try to heed your advice.
06-11-2008 | 09:32 PM
#45
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you need more tire ACTION! 44psi!!?????? you need to be in the 38psi range hot. with the bigger meats, you might need to start out at 28psi to get them working and then drive the snot out of them too! the skinny tires and fat tires are quite a bit different. also, look at set up. you migh have too much camber or something for the fatter tires. so many factors, so little time!
Mk
Mk
Hahahahaha... To quote Bugs, 'he dont know me very well... do he?' You are absolutely right, though, I do have to learn to spend more time on the edge instead of over it. But forget about that. Previously, with the car at 2250 empty, running 225/245s I'd routinely raise pressures, even on a cool day, from 32 starting to > 44#s. Now with some 200#s less and 255/315s on an overcast day with ambient in the mid fifties on a hard compound tire, pressures go up on the order of 2-3 pounds total. Same car, well, same track, same driver. Perhaps as was suggested by Sunday, more thinking around setup might be beneficial. In the meantime, I'll try to heed your advice.