Advanced Engineering Tech - Why do people get so caught up in horsepower?

I just do not understand why people talk about horsepower and not torque. I know that P=Tw so with engine speed and one you can find the other, but it just seems that Power can easily be manipulated by high engine speeds, so why talk about it? I don't know a single formula that uses power to calculated acceleration so why would you use power and weight to calculate et's? If I was going to go about it I would say,

Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass

I just do not see where horsepower is needed. Is there something I am missing?

,Chase

Why do people post non-Advance Engineering questions in the Advanced Engineering section?

if you really like torque get a Detroit diesel

I just do not understand why people talk about horsepower and not torque. I know that P=Tw so with engine speed and one you can find the other, but it just seems that Power can easily be manipulated by high engine speeds, so why talk about it? I don't know a single formula that uses power to calculated acceleration so why would you use power and weight to calculate et's? If I was going to go about it I would say,

Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass

I just do not see where horsepower is needed. Is there something I am missing?

,Chase

You are right that you don't understand.

TQ can be easily "manipulated" by gearing etc. where as HP can not.

You have it backwards.

TQ means little on it's own at all and neither does RPM.

Put them together and then you can see what your rear wheel TQ is at some speed and gear and now you know how hard you can accelerate.

You are right that you don't understand.

TQ can be easily "manipulated" by gearing etc. where as HP can not.

You have it backwards.

TQ means little on it's own at all and neither does RPM.

Put them together and then you can see what your rear wheel TQ is at some speed and gear and now you know how hard you can accelerate.

How would putting torque and engine speed together give you rear wheel torque? However I did account for rearwheel torque by multiplying flywheel torque by gear reductions and then by efficiences. I still do not understand where a hp # would be used to figure acceleration unless you used it and it's corresponding engine speed to find that instant torque. Again I am just trying to find out why everyone talks about hp. I am not arguing its not important.

,Chase

400HP at 12,000RPMs or 2,000RPMs is still simply 400HP. The motor running 2,000RPMs is running a lot more torque, yet realistically it will not make the same car accelerate faster. Basically, if two engines running the same power at different RPMs were geared to the same speed at their peak RPMs they would be applying the same torque to the wheels. This assumes no difference in frictional losses of course.

Average power to the wheels is what matters...

here was a quick story i made last time this topic came up... keep in mind power is a measurement of how often that torque is applied...

You are trying to build a wall (aka drive down the drag strip) and you want to get it built as quickly as possible (ET). You have a few people to help (rpm) and they can each carry a certain size rock (torque/force) at a rate of one rock each per minute

Gasp.. you have one group that cant get along with the other to determine how to build this wall the fastest. One team of 3000 people big surly men think they can get it done faster than the team of 7000 women. The team of men cary big heavy rocks (350lbs) and each one can move one rock every minute. The women are weaker so they carry slightly smaller rocks (262 lbs) and they also can each get one rock placed every minute.

Which team is building at a faster pace?

A: The team of men are building their wall at a rate of 1,050,000 lbs per minute, and the team of women are building their wall at a rate of 1,834,000 lbs per minute. This is calculated by multiplying the weight of the rocks by the number of rocks being placed

Moral of story.. the size of your rocks dont matter unless you have alot of helping hands to build the wall. A single 4000lb rock is impressive but if nobody moves it.. no wall will be built


assume the lb/minute rate they are building the wall as thrust force moving your vehicle down the track. Becuase multiplying the torque by the amount of times you can apply it (rpm) will give you the resultant thrust force to move the vehicle (that is a very simple explanation)

Why do people post non-Advance Engineering questions in the Advanced Engineering section?

I am sorry I just assumed that this was engineering related since in one of my upper level engineering classes we spent a chapter or two on simulating acceleration.

Average power to the wheels is what matters...

here was a quick story i made last time this topic came up... keep in mind power is a measurement of how often that torque is applied...



assume the lb/minute rate they are building the wall as thrust force moving your vehicle down the track. Becuase multiplying the torque by the amount of times you can apply it (rpm) will give you the resultant thrust force to move the vehicle (that is a very simple explanation)

Thanks for the positive help. I am going to keep reading this and see if I can't get something to soak in. lol So how would power be applied into an equation to calculate acceleration?

,Chase

has anyone one else heard the term "hp is how fast you can go, tq is how fast you can get there"? does that have any sense in itself?

Why do people post non-Advance Engineering questions in the Advanced Engineering section?

:jest: :poke: :jest:

has anyone one else heard the term "hp is how fast you can go, tq is how fast you can get there"? does that have any sense in itself?

Yeah I used to hear that a lot, but I am looking to get a little more indepth than that. Thanks for the input though.

,Chase

I am sorry I just assumed that this was engineering related since in one of my upper level engineering classes we spent a chapter or two on simulating acceleration.

If your asking this question and you are in a upper engineering class you need to take a step back.

has anyone one else heard the term "hp is how fast you can go, tq is how fast you can get there"? does that have any sense in itself?

that actually makes zero sense and just confuses the general population..

although i hear it alot

If your asking this question and you are in a upper engineering class you need to take a step back.

So you have a equation to find acceleration by using power?

So you have a equation to find acceleration by using power?

yeah.. thats pretty easy

F=ma

use power, engine rpm, gear ratio, vehicle weight and wheel speed..

the part that you are missing is that more power allows more torque multiplication via gear ratio

for a given vehicle speed your tire rpm is fixed, but if you can have your engine spinning 7000 rpm putting out a high toque level and then multiply it by a higher gear ratio, your tractive force will be higher than an engine with the same high torque at 3000 rpm cause they cant multiply the torque as much through the transmission.

this concept should be easy to grasp by an upper level engineering student..

yeah.. thats pretty easy

F=ma

use power, engine rpm, gear ratio, vehicle weight and wheel speed..

the part that you are missing is that more power allows more torque multiplication via gear ratio

for a given vehicle speed your tire rpm is fixed, but if you can have your engine spinning 7000 rpm putting out a high toque level and then multiply it by a higher gear ratio, your tractive force will be higher than an engine with the same high torque at 3000 rpm cause they cant multiply the torque as much through the transmission.

this concept should be easy to grasp by an upper level engineering student..


Yeah you are going about it the same way as I did earlier but you using power and dividing by engine speed to get the torque # I use. I see what you mean about if an engine spins up really high you can stuff in some low gears to bump up the tractive force which obviously makes sense.

has anyone one else heard the term "hp is how fast you can go, tq is how fast you can get there"? does that have any sense in itself?

No that statement does not make any sense. There is so much mis-information regarding this. So many people say torque is what gets your going and power is what keeps you going. Anyone who says that just simply does not understand what they are talking about. I believe that phrase comes from people associating high torque numbers with wide powerbands. Like stated above, acceleration is about average power output. So obviously a 900hp motor with a wide powerband is going to generally accelerate better than a peaky 900hp motor.

So you have a equation to find acceleration by using power?


Here's the deal. I can deliver 100 ft lbs of torque to a lug nut, or a wrench or whatever. What I cannot do is deliver a 100 ft lbs of torque at 7000 RPM. Do you see the difference in power and torque now?

The wall analogy is a good one above. There is no equation that says a car will accelerate with so much power. If you are a higher level engineer you should have already had a class for "dynamic systems" This would be a good class to solve a problem like this. You would need a little differential equations and to build a simulation to see how a car is going to accelerate.

Actually if you know the weight and the power you can use several equations to find the acceleration very easily.

If you only have a static torque value though you can not figure anything out at all as far as acceleration.

The one that I like to bring up in hp vs torque discussions, that always twists some brains into noodles is...

The M1 Abrams tank produces 1,500 hp and . . . wait for it. . . 395 ft-lb of torque at peak.

I, with a 10 ft cheater bar, can produce 4,000 ft-lb. I can produce much more with a longer bar. Archimedes said, "Give me a long enough lever, and I can move the world."

Torque can be manipulated using gears and levers, whereas hp can not. And, yes, you can manipulate hp by "simply" raising engine speeds, but this requires things like long duration camshafts, high flow cylinder heads, headers, etc... which are proven to also make the cars faster, not by coincidence.

Think about the fact that you are forced to upshift to higher gears that have less mechanical advantage (i.e. 1:1 vs 3.06:1). If you manage to make 400 ft-lb of torque, but modify your engine in such a way that you can maintain 400 ft-lb at twice the rpm, then you could hold each gear twice as long. Imagine how much faster your car would be if you could shift out of 1st gear at 70 mph instead of 35 while maintaining the same ratios. Well...the result of keeping the same torque at double the engine speed is double the hp.

1989 305 TBI made 223 ft-lb at 4000 rpm (170 hp)
If you can make 223 ft-lb at 8000 rpm, you get 340 hp
If you manage to make 223 ft-lb at 12000 rpm, you get 510 hp
Try spinning a 305 TBI to 12000 rpm!

Mike

Thanks for the input guys, but I found the information I was looking for. I think there was a little confusion in how I asked the question, sorry about that. In other words I am not as confused as you may think lol I just did not do a good job expressing my question.

Actually if you know the weight and the power you can use several equations to find the acceleration very easily.

If you only have a static torque value though you can not figure anything out at all as far as acceleration.

Yes you can like I said earlier
Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass

I can did up an Excel spread sheet that we did on a stock car if you would like to see it. It was actually pretty accurate, within 0.1s in a 1/4 according to Car and Driver. However we did account for drag and rolling resistance. I think it was a Jetta not a real cool car but that what the teacher wanted us to do.

Yes you can like I said earlier
Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass

I can did up an Excel spread sheet that we did on a stock car if you would like to see it. It was actually pretty accurate, within 0.1s in a 1/4 according to Car and Driver. However we did account for drag and rolling resistance. I think it was a Jetta not a real cool car but that what the teacher wanted us to do.

No you can't.

One car has an engine that makes 400 ft pounds of tq and weighs 3000 pounds.

One car has an engine that makes 500 ft pounds of tq and weighs 3000 pounds.

Which is faster?

You can't tell only knowing what tq they put out.

No you can't.

One car has an engine that makes 400 ft pounds of tq and weighs 3000 pounds.

One car has an engine that makes 500 ft pounds of tq and weighs 3000 pounds.

Which is faster?

You can't tell only knowing what tq they put out.

Haha no I mean like using 50 different data points. I looked and I don't have that Excel spreadsheet, but I will see if my teacher still has it. If so I will post it up for you to look at, it is pretty cool. Actually all the engine data we had was redline, max torque @ a certain speed, and max power @ a certain speed. From there we plug them into some crazy equation and solved for coefficeints then made a simulated torque curve. I know that torque curve wasn't perfect, but suprisingly it got the job done.

Haha no I mean like using 50 different data points. I looked and I don't have that Excel spreadsheet, but I will see if my teacher still has it. If so I will post it up for you to look at, it is pretty cool. Actually all the engine data we had was redline, max torque @ a certain speed, and max power @ a certain speed. From there we plug them into some crazy equation and solved for coefficeints then made a simulated torque curve. I know that torque curve wasn't perfect, but suprisingly it got the job done.

If you have Tq at a certain speed it is HP which again can tell you something.

Tq alone has no impact on performance unless it is referenced at a linear or angular speed which again makes it horsepower.

If you need any RPM data then the spreddsheet you are looking at used HP to find acceleration.

Go back to basics...

assuming T is constant:

W = T.Θ

P = dW/dt = d(T.Θ)/dt = Θ.dT/dt + T.dΘ/dt = T.ω

P expresses the time rate of work done... i.e. the rate of torque producing angular movement...
it "summarizes" T and ω... it is a handy "tool".

Compare 100 lbft at 100 rev/s and at 200 rev/s.

Edit: found a better symbol for angular displacement.

If you have Tq at a certain speed it is HP which again can tell you something.

Tq alone has no impact on performance unless it is referenced at a linear or angular speed which again makes it horsepower.

If you need any RPM data then the spreddsheet you are looking at used HP to find acceleration.

Thank you but I know what power and torque are. That spreadsheet I used DID NOT use power to calculate acceleration. I only used engine speed to reference the particular torque data for that instant and to calculate the vehicle's velocity. I know I easily could have taken power data and divided out the engine speed to get it in term of torque, but I am just saying you do not need to actually figure a power #s to calculate acceleration.

,Chase

Thank you but I know what power and torque are. That spreadsheet I used DID NOT use power to calculate acceleration. I only used engine speed to reference the particular torque data for that instant and to calculate the vehicle's velocity. I know I easily could have taken power data and divided out the engine speed to get it in term of torque, but I am just saying you do not need to actually figure a power #s to calculate acceleration.

,Chase

If you are referecing torque at a rotational speed you are talking HP. You don't seem to understand that. Acceleration involves and is in reference to time and so you need rpm to se how much work you can do in a certain time. This is a HP scenario plain and simple. That's why in Physics you learn about power because it is what gets work done at the rate you are looking for. We care about the time involved!

If you can do 20 revolutions of a crank at 50 foot pounds of work in a minute and I did 40 revolutions of a the same crank at 50 foot pounds of work in a minute then I could do twice the work you did in a minute and I would also be producing twice the power as you did. I could use a different gear twice as low and create twice the tq at the wheels at the same wheel speed as you could since I make double the power even though we were both supplying the same input torque.

Torque on it's own doesn't mean anything at all!

Thank you but I know what power and torque are. That spreadsheet I used DID NOT use power to calculate acceleration. I only used engine speed to reference the particular torque data for that instant and to calculate the vehicle's velocity. I know I easily could have taken power data and divided out the engine speed to get it in term of torque, but I am just saying you do not need to actually figure a power #s to calculate acceleration.

,Chase

Yes, but the torque applied to the rear wheels decreases as speed increases, no? If the torque stayed the same as speed increased, hp would be infinite.

Yes, but the torque applied to the rear wheels decreases as speed increases, no? If the torque stayed the same as speed increased, hp would be infinite.

No I actually used a torque curve for the calculation, so the torque was not constant.

No I actually used a torque curve for the calculation, so the torque was not constant.

Did you factor in that the gear ratio is progressively taller as you increase speed and shift to higher gears?

Did you factor in that the gear ratio is progressively taller as you increase speed and shift to higher gears?

Yeah bud it is a pretty awesome spreadsheet. We even accounted for drag and rolling resistance with respect to the cars velocity.

this thread is kinda pointless...

you claim all this stuff you do in your upper level engineering course... yet dont understand power? Accounting for drag and rolling resistance in a spreadsheet your professor put together is hardly impressive. We have tried to assist in your learing yet it doesnt seem to work

Power = torque x rpm ... you used power in your professors spreadsheet:bang::bang::bang:

did you use torque? yes

did you use rpm? yes

Does that mean power? yes!!

Yeah bud it is a pretty awesome spreadsheet. We even accounted for drag and rolling resistance with respect to the cars velocity.

Ok well if it includes the fact that the gear ratio is steeper as speed increases, thereby reducing torque delivered to the tires, then it automatically includes hp.

this thread is kinda pointless...

you claim all this stuff you do in your upper level engineering course... yet dont understand power? Accounting for drag and rolling resistance in a spreadsheet your professor put together is hardly impressive. We have tried to assist in your learing yet it doesnt seem to work

Power = torque x rpm ... you used power in your professors spreadsheet:bang::bang::bang:



This thread is pointless. I said from the beginning that P=Tw, but I never did that in my spread sheet. I am not the one being hard headed hear I am just saying that I never multiplied my engine torque by my engine speed.

I am sorry I just assumed that this was engineering related since in one of my upper level engineering classes we spent a chapter or two on simulating acceleration.Is there a chaper on "Bullshitting About Moot Points" in the book? Take it to a general forum. This is not Advanced Engineering by a long shot.

This thread is pointless. I said from the beginning that P=Tw, but I never did that in my spread sheet. [I]I am not the one being hard headed hear I am just saying that I never multiplied my engine torque by my engine speed.

You didn't have to. You held RPM constant so torque was representative fo HP


Simple math.

P=T*2(pi)*rpm

If RPM is held constant, then 2(pi)*rpm is just a multiplier

How can you not understand that?


I am an electrical engineer and I remember be confused over this back in school before understanding rotational kinetics. You don't fully understand that torque is a rotational force on a shaft. This is different than linear force applied to an object.