Hello everyone and welcome you've probably heard the analogy before perhaps seen it written on someone's t-shirt that goes something like this
horsepower is how fast you hit the wall torque is how far you take the wall with you, and this analogy is inaccurate
it's poor. So the next time you see this written on the internet somewhere
I want you to type back nah dude science and then share a link to this video
because we're going to be talking about why the analogy is wrong so first off starting with
horsepower is how fast you at the wall this part is actually pretty accurate so my revised version of the analogy is
horsepower is how fast you hit the wall analogies are dumb
and so we're gonna explain why the first part is accurate why the second part is not
so horsepower is the rate at which work is done
so let's say you have this car right here. It's got a pulley
there's a string connected to a mass
which is connected to the spoiler of the car, so the car is going to accelerate and it's going to pull this mass up
well the rate of accomplishing work means how fast you move this mask upwards and so if this car has more power
it accomplishes work faster it moves this from here to here
faster the more power it has the faster this mass moves from here to here
so if you have a car and it's sitting there stationary, and it's about hit a wall
that's in front of it
the rate at which it accomplishes work is based on its power so the more power it has the faster
It's going to run into that wall from a dead stop
so that's that's the first part of the analogy and that part is you know it's fine
it's not wonderful
but it's fine and so the second part is
torque is how far you take the wall with you and this part
just doesn't make any logical sense so we can all agree that the greater the force
that you push on a wall the further it will move
so that's that's pretty straightforward
so force=(mass)(acceleration) in this case mass being the mass of our car
hitting the wall and of course the mass of the wall and
the acceleration relating to how fast our car is traveling so acceleration is the change in velocity over the change in time
so we can say that force is equal to mass times our final speed minus zero
we're going to stop because we're hitting the wall divided by the time that it takes to do that
so you can see that the force required,
which is ultimately gonna dictate how far this travels is a function of how much does this car weigh and
what is the speed at which it hits the wall and the speed at which it hits the wall is
based on you know from the first part of our analogy horsepower is
how fast you hit the wall so horsepower is how fast you hit the wall
horsepower plus the mass of the car is how far you take the wall with you, not a great analogy again,
but at least accurate to say now does all of this mean that torque is meaningless well absolutely not
horsepower is a function of both torque and rpm, so it's dependent upon torque to have good horsepower
and so if we look at a chart here
we're gonna explain
why does low end torque mean you have better acceleration so we're gonna have two different engines one of them is a gasoline engine
it's got a perfectly flat torque curve up to
5252 RPM with a hundred pound feet of torque and one of them
which it's producing 100 horsepower at that and then one of them is a diesel engine
it's got 200 pound feet of torque
it stops revving at 2,600 rpm where it's producing a hundred horsepower so both of them 100 horsepower
except the diesel engine has twice the torque
so now let's say we want to accelerate and both of these cars have the same gear ratios and
both of them are at 1,000 rpm, and then we floor it well. What's going to happen?
well the diesel engine has twice the torque and so it's going to start to accelerate faster, but as you can see
horsepower is a function of torque so at that 1,000 rpm the diesel engine also has twice the power of the gasoline engine in
hence it's going to accelerate faster. Now it will start to accelerate and then it will reach its peak rpm
2626 rpm at which point the diesel engine is going to have to shift gears and the gasoline engine won't
so the diesel engine is going to adjust its gear ratio lose some torque
well that gasoline engine is going to stay within that gear ratio and keep accelerating all the way to
5252 rpm so ultimately they'll be very similar in speed
but if you were to put them in the same gear ratio at a thousand rpm and floor it
the diesel is going to accelerate faster because it has more torque and more horsepower at that rpm so high low-end torque
also means high low end power and power is what's super important.
Okay now one last comparison to help prove a point, so we've got a Ford F-250 diesel which produces 440 horsepower and
925 pound-feet of torque so an absurd amount of torque versus a Honda s2000 2008 with
237 horsepower and 162 pound-feet of torque so pathetic amount of torque a
mildly impressive amount of horsepower for a small engine the ford F-250 diesel weighing 8300 pounds and the Honda
S2000 weighing 2800 pounds so all of these figures here I got from Car and Driver
they did two different tests on these vehicles um and I'm just taking their numbers from their actual tests
so if you look at the weight to horsepower ratio of the ford
F-250 it's 18.9 pounds per horsepower, which is not good
it means you've got a lot of weight and not that much power to pull it along
versus the Honda s2000 11.8 pounds per horsepower
now this isn't wonderful, but it's certainly better than 18.9 so a nice big improvement there
versus if you look the weight to torque ratio
so we've got 9 pounds per pound foot of torque because this thing has an absurd amount of torque versus the s2000 s pathetic
17.3 pounds of weight
per pound foot of torque so pretty poor now, which one of these is more important your power-to-weight ratio
or your your weight to torque ratio and so to illustrate this looking at Car and Driver's zero to sixty times
the Ford F-250 did it in 6.9 seconds the 2008 Honda s2000 did it in
5.7 seconds so really driving home the point here that the power-to-weight ratio is what is important for acceleration,
it's not the you know weight to torque ratio as illustrated right here
and we learned that at the beginning of this video because we said you know the distance that this mass moves ultimately comes down to
how much power does this car have and how much does it weigh?
so power-to-weight being the critical ratio for determining a vehicle's potential acceleration
plenty of other variables that go along with it
but this is a nice one that indicates whether a vehicle is going to be quick or not okay,
and then the question comes up. Well, why is this one better at towing? Why is it better at towing than this?
why wouldn't you just want, you know something like that to tow? Well let's think about this for a moment?
let's say both of these vehicles have to tow a
5,000 pound trailer and besides the fact that the Honda s2000 doesn't have the cooling or the transmission or the brakes or the heavy-duty
components the frame anything to put up with towing besides all of that from a power standpoint
we're adding 5,000 pounds of mass that's nearly tripling the weight of the s2000 where it's only adding
wou know a little bit more than 50 percent of the weight for the Ford F-250 so relatively speaking
this power-to-weight ratio is going to be
dramatically changed by a factor of three because we're nearly tripling the weight whereas this power-to-weight ratio
you know a factor of 50 plus percent because we're only adding
5,000 pounds which is relatively less compared to how much this already weighs so its power-to-weight ratio will be better than the s2000 when towing
5,000 pounds and as a result it's going to be able to accelerate quicker with that additional mass
but ultimately the thing to learn here is you know the analogy is silly?
the one that kind of makes sense I believe it's Randy Pobst that says torque is what presses you back in your seat
horsepower is what keeps you there, and while that is true
it doesn't teach you what horsepower and torque are which is why I made a specific video
if you care to watch, or would like to learn about that exactly describes what is torque
what is horsepower and how they relate? Because they are related so thank you all for watching,
and if you have any questions or comments, please feel free to leave them below
and if you have a buddy that keeps saying horsepower is how fast you hit the wall
torque is how far you take the wall with them perhaps consider sharing this video with them. Thanks guys.