Horsepower is in raw units form 550 ft-lbf/s

If you go back F=m*a or lbf = lbf/32.2ft/s^2 * ft/s^2 you will see that the units in the system cancel out. Therefore FORCE which is units of lbf relative to your mass is what generates your acceleration. So you get your applied force by compensating for rim radius (your lever arm) and using your vehicle mass... which then gives you your acceleration rate theoretically.

Now that's all fine and dandy but then you need to be able to have the horsepower available to balance out forces in a system... So in other words if the vehicle is seeing 50 lbf of resistance over 1 ft for 1 second you'll need ~.1 hp to achieve that. But to accelerate at a given rate you need to have the torque necessary to accelerate the mass/overcome a resistance force. Horsepower put simply is what is necessary to overcome a certain amount of force over a certain distance in a certain amount of time. lbf-ft/s Hence top speed is limited by horsepower as you have to be able to provide force at a great enough rate to overcome the drag in the system.

So torque is what gets you moving and allows you to accelerate, but horsepower is what keeps you moving/allows you to STAY fast.

Hope that helps! I apologize in advance if I made any mathematical/reasoning errors, but I believe I should have it spelled out ok.. but feel free to correct!.

 

wow. should i feel lucky that i understood that? nice work on breaking that down. that explains why the 120hp lancer can accelerate faster then a 127 hp honda with less weight. we have 130 tq, while they have 114. of course those numbers are at different rpms ranges and have variations in the powerband. but what makes the civic Si accelerate faster with 160hp/130tq? same torque rating, but i think the extra horsepower helps to cancel out some of the resistence in the system, allowing the same amount of torque to accelerate the car that much quicker... and having less weight is also a factor.

 

but yeah.. that's more or less the point

I'm glad it was clear enough that you got the general idea out of it.

And yep you are right.. the powerband matters a good bit too.. the total "area" under the powerband matters as to how much "overall" torque the vehicle will have and so forth.

Weight is also an important factor as well which you pointed out nicely... Keep in mind that if a Honda weighs 2500 lbs to our 2700 that means they weigh 7.5% less than us... yet us having 130 ft-lbs to their 114 ft-lbs means we run 12% more torque... soo

Later!

Steve

 

Horsepower does become a factor when you start getting to a point where the resistance exhibited for the speed is large enough that you need the power available as well. Additionally, if they are the same torque rating, but it is at a lower rpm (and doesn't drop off too badly as the rpms go up) etc that will also have a factor in things... lots of variables... I just isolated/showed the raw part of the issue that most people completely miss(understand)

 

exactly. a car with a steady say 100 tq for a full 6000 rpm range is going to accelerate faster then say a car with 90 tq for 5000 rpms, then 120 tq for the remaining 1000 rpm range. if you plot the tq on a graph, then take the antiderivitive of the curve, you will find the total 'area' under the curve, which shows you the potential for the acceleration. honda's vtec is a good example of the power spike at the end of the rpm range. it may be rated at 127hp max, but that is from around 5000rpm to redline, while the lower rpms ranges may only be at 115hp. same applies for the tq. 114 is the max of the spike, but the motor only holds that for a short amount of time.

 

nice explanation man!

 

how can we add more torque to our cars then?

 

same way you add horsepower. Horsepower is a factor of torque, it's just torque times rpm's more or less. The point here was to make it clear that it's torque that matters the most for you.

 

I know this is OT but sdhotwn you are one of the most informative persons on the site along with some other people.. I just want to say thanks!

 

No problem! Glad my efforts are appreciated. Smart tuners make good tuners!

And as always just ask if you have any questions you'd think I'd be able to answer too. No question is "stupid" ! And I'll make sure I don't make you feel like you are stupid for asking either. Enough asses in this world, don't need another .

Later.

Steve

 

man reading this post really explains why my old jetta tdi with a whole 90 hp and 149 lb torque could run with the bigger dogs until around 65 mph. thanks sd

 

Okay, still kind of lost... But thats probably because cars and me aren't met to be together...

If torque is good, shouldn't we all shift at then end of the torque band, instead of the end of the HP band (in our case, what is it 6250 RPM?) ?

 

hmm, good point dem. im not the expert on torque and hp but that question seems pretty nteresting.

 

Here's a site with some more info on the torque vs power debate....

Torque vs. Power

 

yeah that link pretty much covers it . If you are interested in accelerating you want to always shift to maintain maximum torque, not maximum power.