Help explaining horsepower and torque feel

[engineermike]

(a) Max acceleration in any given gear is maximized with the engine running at peak torque rpm.

(b) Max acceleration at any given speed is maximized by running the engine at peak HP rpm and gearing accordingly.

Perfectly explained!

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[ponie1992]

Acceleration is calculated by dividing the Force (Torque) by mass, if you want to argue against that, you need to take it up with the laws of physics.

Why does a car accelerate faster in first gear than fourth whilst the horsepower at the tyres hasn’t changed? That’s right, the exertion of more torque at the wheels.

You make more horsepower by creating more torque (for a given rpm).

Not "more" torque. Engine makes same torque in any gear at a given rpm. The trans manipulates the application of torque.
Also, the OP asked about the "feel" of hp and torque, not necessarily how it's calculated.

 

[ponie1992]

Hello; Here is my way of thinking about torque and power. My take is you have both. Here is a way to think about it.
I figure many on this forum have used a ratchet on a nut or bolt. Put the ratchet and socket on the nut and push on the end of the ratchet. Your arm is providing the power. If the nut rotates with the ratchet some of than power has turned into movement and since the movement is going to wind up being circular I think of it as torque. I am not absolutely sure the movement has to be in a circle but do suspect this is so. I do not mean there has to be a complete 360 degree spin, more that it can also be what I think of as an arc. So you have power and torque when using a wrench or screwdriver.

If the nut is very tight or frozen you can push harder and make more power and this will also result with more torque applied to he nut. (Think of a beam type torque wrench. When you tighten a nut with one the thin indicator beam stays in it's original and the handle bends. The pointer winds up over the numbers on the scale as the handle bends. More power yields more torque.)

If you are not able to break the frozen nut free and have used all the power available in your arm then you can try a ratchet with a longer handle. The longer handle acts as a lever and with the same arm(power) you can generate more torque on the nut. Still stuck, then get an even longer ratchet or power bar to get eve n more torque from the same power.
( Here is the analogy: the lengths of the ratchet compare with the gears in a transmission. For a three speed the short ratchet is like top gear. You do not try to drive off in high gear. Medium length ratchet is like second gear. You get decent acceleration in second gear once you are moving but even second gear is not always able to get you moving from a stop. The longest ratchet is like first gear. You can get a lot of torque to the wheels in first gear and get things moving from a stop.)

In a car as with in our arm we can have a range of power. The power in a car comes from the explosion of fuel in a cylinder that is a closed tight space. The explosion can push the piston which pushes on the crankshaft and causes it to spin. The power of that explosion determines how much force (torque) the crankshaft spins with. Explode more fuel and you make more power and if that power is harnessed to a spinning crankshaft you get more torque. Or you can stack more cylinders together with linkages and timing to get more power from the same sized explosions.
You can then pick a set of levers( gears) that suit your purpose and transfer that power/torque to the ground and drive away. Very low gears equal really long ratchet handle so the bulldozer or tractor can move a lot of weight with small power.
Or pick a set of gears that are also low so you can accelerate quickly for a few hundred feet until you run out of RPM's of the spinning crankshaft.
My favorite is a set of gears/levers with a couple of low range ones with some middle range ones and at least one that like the very short ratchet handle (overdrive).

My take is if you make something spin with power you will always have torque. Enough from me for now. I think my computer screen is running out of ink. There is more to be said. maybe later.

The energy you exert on the ratchet is force, not power.

 

[K4fxd]

Are you referring to me?

I was responding to what I thought was an engine with the same avg and peak HP with differing torque curves.

Think an 18,000 RPM 800 HP F1 engine VS an 800 HP supercharged 6500 RPM LS

 

[SplawnDarts]

So you are telling me that racers have had it wrong all these years?

We always geared so that when we shifted at peak HP we landed at peak torque.

There's no good reason to do that.

The first thing to think about is how you'd program a CVT for maximum acceleration if your car had one. You'd set it to run at the peak HP point and never move off it. Crank torque is irrelevant, as it always is.

Say you don't have a CVT, but rather a transmission with discrete gears. Now how do you use it? If the gear ratios are set by someone else, you compare your HP at the current operating point to your HP at the new point if you shift. If the new point is higher HP (and within redline) you shift. Again, torque has nothing whatsoever to do with it.

This gets a little more complicated when your shifts take time, but the principle is still the same.


If you get to pick your gear ratios, you pick them with two things in mind:
1) Making the gears closer (so you can stay closer to peak HP) at the speeds the car will spend the most time
2) Not making the gears so close that shift time becomes a big loss of overall power

In all these deliberations, you never once consider peak crank torque. It has nothing to do with anything.

 

[TopJimmyCooks]

To feel the difference: HP will make your engine spin up and the car accelerate when you want to pass a Karen in the left lane, and you need to swing around her on the right side. Torque is what allows you to pull her from a ditch, with minimal throttle, when she thought “I have an SUV so I can drive as fast as I want in the snow“.

 

[K4fxd]

In the Tiger we had at least 4 transmissions and at least that many center sections, all geared differently.

 

[SplawnDarts]

You missed my point entirely.

(a) Max acceleration in any given gear is maximized with the engine running at peak torque rpm.

(b) Max acceleration at any given speed is maximized by running the engine at peak HP rpm and gearing accordingly. It's just another case of gaining more from staying in the lower gear than you lose from torque fall-off.

(a) and (b) are not two ways of saying the same thing. They're distinctly separate.

This is physically correct. But let's be clear - (b) is the problem you're trying to solve when you're trying to accelerate your car. You are always currently at some speed. It is by maximizing HP (using whatever gear and RPM is required to do that) at that speed that you maximize acceleration. Cranshaft torque is a completely useless piece of information when trying to solve that problem. All you need to know is the engine HP at the various operating points available to you, and pick the gear that gives you the highest HP (ignoring shift time losses of course).

 

[engineermike]

Harbor freight cordless impacts rated at 1200 ftlb of torque are $200. Time to replace my supercharged coyote, double the torque, and knock about 700 lb off the front end!

Torque can be made while stationary. The instant something moves, it requires power to continue accelerating. Our cars move.

 

[Burkey]

You missed my point entirely.

(a) Max acceleration in any given gear is maximized with the engine running at peak torque rpm.

(b) Max acceleration at any given speed is maximized by running the engine at peak HP rpm and gearing accordingly. It's just another case of gaining more from staying in the lower gear than you lose from torque fall-off.

(a) and (b) are not two ways of saying the same thing. They're distinctly separate.



OK, let's run with that . . . yes, a = F / M

But force 'F' is not torque (forces and torques aren't even expressable in the same units). The force you're talking about is the total rearward tire force, measured or calculated at the contact patches.

To get to those contact patch forces, you have to divide drive axle torque (let's assume that wheelspin does not occur) by the drive tire rolling radius. But axle torque is engine torque times transmission gear ratio times axle ratio. You're glossing over all those things in boldface, which brings some risk of losing people who may be trying to follow along. I've been running this sort of analysis for longer than you're likely to guess, and I'm having to translate more than I should have to.

I've got about enough battery life left to post a couple of screenshots you may find interesting. It's part of my due diligence for choosing a different transmission but it illustrates what's going on reasonably well.

The spreadsheet is my own.


Norm

I think we might be misunderstanding each other somewhat.
As your chart below quite clearly shows, peak acceleration occurs at peak torque, regardless of the gear selected. From a “seat of the pants“ perspective, this is what we feel.
Yes, we continue to rev the engine beyond that point to capitalise on the torque multiplication of the current gear rather than piss it all away with the reduction of the next gear selected as you well understand, but the simple harsh reality is that the car is literally losing that “push in the back” as you do so.

The question being asked by the OP was specifically asking for the difference in “feel” between horsepower and torque. I‘m simply trying to highlight to him that peak acceleration for a given gear will occur at peak torque, whilst horsepower will allow him to hold onto a shorter gear for a longer time period, which is what allows you to cover distance more quickly, AKA, racing.
EDIT:
Your graph also shows us how drastically that “seat of the pants” feel diminishes each time we swap to a taller gear and why that’s relevant to covering distance quickly.

 

[Norm Peterson]

I think we might be misunderstanding each other somewhat.

Evidently.

As your chart below quite clearly shows, peak acceleration occurs at peak torque, regardless of the gear selected. From a “seat of the pants“ perspective, this is what we feel.
Yes, we continue to rev the engine beyond that point to capitalise on the torque multiplication of the current gear rather than piss it all away with the reduction of the next gear selected as you well understand, but the simple harsh reality is that the car is literally losing that “push in the back” as you do so.

The question being asked by the OP was specifically asking for the difference in “feel” between horsepower and torque. I‘m simply trying to highlight to him that peak acceleration for a given gear will occur at peak torque

There's no getting around the fact that peak acceleration is only a momentary snapshot of what is going on. Using peak conditions to describe "feel" is a bit too simplistic. Then again, I've been running these acceleration sims for nearly as long as the Mustang has existed so I am going to be looking a bit deeper into what's happening.


Especially for Meatball - acceleration mirrors the torque curve (more or less) rather than the HP curve, and that's what gives you that overall "seat of the pants" feel. If you're paying enough attention, you can feel the acceleration swell as you approach peak torque rpm from below, and taper off once you're past it. That's really the answer to OP's question, where he's asking whether it's the HP you feel or the torque.


Norm

 

[Norm Peterson]

This is physically correct. But let's be clear - (b) is the problem you're trying to solve when you're trying to accelerate your car. You are always currently at some speed. It is by maximizing HP (using whatever gear and RPM is required to do that) at that speed that you maximize acceleration. Cranshaft torque is a completely useless piece of information when trying to solve that problem. All you need to know is the engine HP at the various operating points available to you, and pick the gear that gives you the highest HP (ignoring shift time losses of course).

Like I said in post #53,

HP basically tells you where you need to keep the engine rpms for best acceleration, but after that it's the combination of the torque curve, gearing, tire size, car weight, and quite a number of other things that ends up giving you what you actually feel as acceleration.

Norm

 

[Norm Peterson]

Not "more" torque. Engine makes same torque in any gear at a given rpm. The trans manipulates the application of torque.
Also, the OP asked about the "feel" of hp and torque, not necessarily how it's calculated.

Once you know how to calculate acceleration, you understand it better. Guaranteed.


Norm

 

[Norm Peterson]

The first thing to think about is how you'd program a CVT for maximum acceleration if your car had one. You'd set it to run at the peak HP point and never move off it. Crank torque is irrelevant, as it always is.

Ummm . . . peak HP happens when the torque being developed by the engine falls off faster than the rpms are rising.

A CVT is a rather unique situation in that it can allow the engine to provide maximum acceleration potential at all road speeds. That's if the transmission programming dials back the reduction ratio as fast as possible without dragging the engine rpm down.

Say you don't have a CVT, but rather a transmission with discrete gears. Now how do you use it? If the gear ratios are set by someone else, you compare your HP at the current operating point to your HP at the new point if you shift. If the new point is higher HP (and within redline) you shift. Again, torque has nothing whatsoever to do with it.

This gets a little more complicated when your shifts take time, but the principle is still the same.

<snip>

In all these deliberations, you never once consider peak crank torque. It has nothing to do with anything.

That's still a rather simplistic approach to understanding it all. I dare suggest that a purely HP-based approach to actually calculating the acceleration would have a rather tough time dealing with rotational inertias (which siphon off significant amounts of torque to be accelerated rotationally, consistent with road speed).


Norm

 

[Coyotes55086]

I just want to thank everyone who contributed to this thread . Not only has it helped me really get a much much better clarity in my head about how torque and horsepower work and coincide , but also I think it's been a great conversation and debate for everyone

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