weight vs. hp

[Myjunkistight]

Is hp to weight a linear relationship or is it something else, the reason I ask is because on this "junkyard wars" this guys says that hp is the square of weight. So he says that if your car weighs twice as much as my car, it will need four times the horepower to keep up with it. Is this true?

[crxlvr]

dont know if that relation is correct.

but HP and weight are related in some ways. if your car has 100hp and weighs 1000lbs, and my car has 50hp and weighs 500lbs, we should have a dead even race, but if i had 100hp and weighed 500lbs, id beat you by a mile.

[Myjunkistight]

So then your saying that horepower to weight is a linear relationship?

If so then does anyone approx. know the ratio of horepower to weight? (how many lbs. = 1hp)

[got v-tec?]

Originally Posted by Myjunkistight

So then your saying that horepower to weight is a linear relationship?

If so then does anyone approx. know the ratio of horepower to weight? (how many lbs. = 1hp)

Damn good question! pretty interested myself

[Mrock]

I am curious too. I remember hearing this discussed over at Honda Tech before...but I completely forget what the forumla was. I'd really like to know as well.

I know that the (horsepower/weight) gives you the power to weight ratio..but I'm not to sure what the squaring does for the forumla.

As for lbs = 1hp...I KNOW I've seen that in a post somewhere at Honda Tech...was a long time ago. I didn't answer the question, but I am searching now..

Edit: Just for anyone interested...a little page that discusses power-to-weight ratio's. It's off of "HowStuffWorks"..

http://auto.howstuffworks.com/horsepower3.htm

[rubix777]

Just make a scatter plot using ms excel with the Weight as a function of Horsepower, F(x) => W = f(hp).

Put Horsepower in 1 column, weight in another column, the more cars you include in the listing, the better closer your equation will be.

make a scatter plot. see if it appears to be linearly or exponentially related. You can then plot a regression line, and determine the slope for the line. You'd get some type of equation, y=mx + b, or something else if it is exponentially or power related. I'm sure it's gotta be power related.

if you're too lazy to plot, look,

http://forrest.psych.unc.edu/researc...egression.html

This

You see they plotted the weight of many cars with their respective horsepower. Their equation was Wt = .37 + .02*Hpwr
Their correlation was R = +.92, which is fairly good, i fyou're taken statistics.

But I think it would be biased because taking that this is all based on statistics, all cars are aerodynamically shaped differently, but this would be a rough estimate of what you are looking for.

I'll make a graph later today and see what equation I get.

[Myjunkistight]

To do it this way you would have to have all cars that were evenly matched right? Because you would have to somehow include how fast the car is. If you just plot horsepower vs. weight for different cars, they won't make a good line or exponential.

What would work would be to take a car with a certain horepower. Say 100.

Then on the graph, change the weight of the car from increasing to decreasing. And on the y axis, quarter mile time.
If the line is straight then it is a linear relationship, if it curves it is exponential.

I found a quarter mile time calculator so I am going to make a graph and figure it out.

[rubix777]

Originally Posted by Myjunkistight

To do it this way you would have to have all cars that were evenly matched right? Because you would have to somehow include how fast the car is. If you just plot horsepower vs. weight for different cars, they won't make a good line or exponential.

It would be impossible to have something be equally matched because even in the same exact make, model, trim, one 95 Civic EX will still be different than another 95 Civic EX due to minor differences such as weight, even off by a few ounces, or maybe the room temperature was hotter in the room when one car was built compared to another, maybe they had to scrape off more metal on one than the other to make things fit. The only thing we can do is make a rough estimate of the relationship we see between all these cars.

Testing every single car and logging all their information would be the most accurate, but for simplicity sake, we get the information from a few random sample of cars and take that as how the majority of the cars would behave. The sample that is taken has to be very random in order to account for all cars, like not neglecting, VWs, Kias, Hyundais, pickups, and suv's.

If you take the quarter-miles of all the cars, not all cars have the same gear ratios, so with gear ratios changing, that wouldn't be the best estimate either unless you disregard that difference. BTW, the equation they use for that quarter mile time calculator was derived by statistics too.

Drag would be another issue. for example racing a scion xA with an xB, they're both the same horsepower, same engine, probably a little different weight by about +100lbs, but one is box shaped and another is hatchback.

I think if you do the quarter-mile vs weight graph, that would be the best estimate. You can then calculate acceleration from the quarter-mile time and the 1/4 mile distance taking the (final velocity - initial velocity)/time.

Get actual quarter-mile times instead of the calculated ones because things perform less than how they should theoretically.

[Ricochet]

So how much hp per lb do I have in a 2300lb Civic with 200hp?

[crxlvr]

Originally Posted by Ricochet

So how much hp per lb do I have in a 2300lb Civic with 200hp?

well thats just simple math 2300/200 = 11.5LB:1HP

[Ricochet]

With that math a 6000lb car with my same 200hp would have 30hp per pound and be faster...

[whtteg]

you would have .086 hp per pound I think
so the 6000lb car would have .033 hp per pound if it had the same 200 hp
correct me if I am wrong but it seems to work

[rubix777]

Originally Posted by whtteg

you would have .086 hp per pound I think
so the 6000lb car would have .033 hp per pound if it had the same 200 hp
correct me if I am wrong but it seems to work

yeah that works. being inversely proportional. now we gotta get something going with weight vs quarter-mile, or acceleration. I think it can be safe to assume the car with the highest hp/weight ratio would win the race assuming both drivers are equally good.

I plotted the 50 values taken from modernracer.com of weight vs the quarter mile times and got almost no correlation. It makes sense because some people like to put small engines into big cars.

[Milliardo]

I wouldn't mind the math; I never liked it. But then common sense is really all there is in determining power to weight ratio. An object which is heavier would need more power to move it compared to something much lighter. So, if 2 cars with the same hp, but one is heavier than the other, were to go together, the lighter one would go faster because less weight is being moved as compared to the heavier one. That is why Honda itself confirms that as far as power to weight ratio is concerned, the 1999 Honda City (a car which is only available here in Asia) can match the 1999 Honda Civic SiR, whose engine is a B16 as opposed to th D15 of the City. At least in the initial stage both cars would be evenly matched; the SiR would eventually push ahead in the upper gears (3rd to 5th) eventually though.

[PreludeMOFO]

hahah just a few days ago in math, i was debating with the teacher when the hell id ever need to know in real life functions and y=mx+b....

Fate has a way of biting you in the ass i guess...