Help explaining horsepower and torque feel

[SplawnDarts]

Errrr...

Yeah, I got to stop engaging with this. He's ignorant of basic physics.

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[Burkey]

That said, I’ve owned both combinations of torque and power...the SOTP difference to me is a higher torque, lower power setup hits hard then falls off. Every time you shift, it puts you back in the seat again. A higher hp, lower torque still hits hard if geared correctly, but never falls off with rpm.

That is probably the most succinct explanation I’ve seen so far. Good job.

 

[SplawnDarts]

That said, I’ve owned both combinations of torque and power...the SOTP difference to me is a higher torque, lower power setup hits hard then falls off. Every time you shift, it puts you back in the seat again. A higher hp, lower torque still hits hard if geared correctly, but never falls off with rpm.

Yeah, that's one thing that makes the mustang exceptional to drive for me. Since peak HP is at redline, you never really want to shift. You're just forced to by the rev limiter. Acceleration drops when you upshift. Where as with say a FE 427 Cobra, you're like 1000 RPM past max power at redline, and when you upshift you probably don't really make more power but it's close and you're kind of glad you shifted.

The mustang drives like a Lambo or something at least in that respect.

 

[Norm Peterson]

Peak acceleration for a given gear occurs at peak torque. Period.

True . . . but that's not the only truth here.

Peak acceleration at any given road speed occurs when your combination of drive tire size, axle gearing, and transmission gear ratio puts the engine at the rpm where peak HP is developed.

Dyno chart from a Camaro below to graphically illustrate for the OP how the peak accel happens early (in this case) but the driver will hold the gear regardless, cos he wants to capitalise on the torque multiplication of that gear, rather than shift up.

That's a useful chart. From it, I think you could take this away as a bottom line answer . . . 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

 

[Burkey]

True . . . but that's not the only truth here.

Peak acceleration at any given road speed occurs when your combination of drive tire size, axle gearing, and transmission gear ratio puts the engine at the rpm where peak HP is developed.



That's a useful chart. From it, I think you could take this away as a bottom line answer . . . 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

Replace peak horsepower with peak torque and you’d be correct. Unless peak horsepower is occurring at the same place as peak torque of course, in which case, yes, you’d be right.
In the example below, peak acceleration occurs at ~4750 rpm. Anything either side of that is slightly less.
Again, acceleration is the force (torque) divided by mass.
We continue to rev past the torque peak because the loss in engine torque is less than the loss at the tyres that an up-shift would incur. Once that ceases to be the case, we shift up.
This isn’t disputable. It’s this exact methodology that dictates optimum shift points.

 

[K4fxd]

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.

 

[Burkey]

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.

Not sure how you think that conflicts with anything that’s been said here.

 

[sk47]

Horsepower and torque kinda go hand & hand.

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.

 

[Norm Peterson]

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.

Ideally, you upshift slightly past peak HP rpm, assuming that there's a few more rpm available. You're still losing less torque than the upshift to the next gear would cost you in terms of ratio.


Norm

 

[Meatball]

Gearing can only multiply torque, not power.

One if my favorite examples is the M1 Abrams tank engine that produces 1500 hp and only 500 ftlb of torque. By the time it goes through numerous gear reducers it’s making 210,000 ftlb at the rear, ahem, tracks. But still only 1500 hp.

If you tell me the time it takes your car to accelerate from speed a to speed b and the weight, I can tell you the power it made. I only need the gear ratio to calculate the torque. Tell me which one is calculated from which then...

Sorry to pick on you engineermike but I especially respect your opinion. So two (related) questions:

1- If you plotted acceleration vs rpm for a given gear, would it track the HP or torque curves? Neglecting traction, wheelspin, etc.

2- Say you have a car with a 6.2L pushrod V8. You also have the same car with the same exact same tranny, diff, wheels, tires, etc. and curb weight (via sandbags) but with a DOHC 2.0L I4 with the same HP peak (at higher rpm) and much less torque at all rpms below the V8s redline. Let's say, in the same gear, you accelerate at WOT over say a 10mph span across the hp peak (will be a higher speed in the 2.0L), and let's say the curve shape is such that the avg hp is the same over that span. Same change in rpm since same gear. Neglect air resistance. Will the acceleration be the same (same time to accel by those 10mph) even though the 2.0L is making less torque, but at higher rpms, than the 6.2? This is probably an unnecessary follow up to question 1 but thought I'd ask anyway.

Thanks in adv

 

[K4fxd]

Assuming the gearing is optimized for each engine, as my mentor Smokey said. "400 horse power is 400 horsepower. It don't matter if its a one banger or a V16"

 

[Rusherific]

It seems most commonly what happens is people get engine torque and wheel torque after gearing confused. Torque is quite literally the accelerating force that makes your butt go faster. The problem is that it is not engine torque, it's wheel torque, that makes buttgofast. To convert engine torque to wheel torque, you need gears, and the lower your gearing, the more you can multiply your engine torque. A high torque, low RPM motor must be geared higher than a high HP/high RPM motor for a given speed, so your wheel torque winds up being lower than a motor that can use a lower gear at that same speed and thus be multiplying its engine torque more. THAT is what horsepower measures. It's not any more complicated than that. In other words, horsepower is a measure of how usable an engine's torque is. 100 ft/lbs at 100rpm (1.9hp) is way way less useful in real life than 100ft/lbs at 8000rpm (152hp). That's what horsepower measures, that's it. The counterintuitive part is that you're accelerating the same with 100 ft/lbs at 100rpm as you are with 100ft/lbs at 8k rpm. The ONLY difference is that no one cares how fast you accelerate at 100rpm, you're not going anywhere.

Well then, some might say that horsepower is what accelerates a car! Wrong! Horsepower is simply how you get wheel torque from crankshaft torque, which are not the same thing because gears. High horsepower for a given torque means nothing on its own, any more than torque without RPM means nothing on its own.

Let's look at an example drag race between two cars with the same engine, same gearing all the way through to the wheels, but car #1 has a lower redline and thus lower peak HP than car #2. Let's say at 60mph, both cars in 2nd gear with 3:1 ratios, at 4k RPM making peak torque. Right now, the higher HP car has 0 advantage whatsoever, both are accelerating beyond 60mph *exactly the same rate*. Car #2's extra horsepower means nothing here. BUT, the real world is more complicated than that. In real life, given a higher RPM engine, you can gear it lower such that 2nd gear might be more like 3.5:1, and be making more wheel torque now than the other car at that 60mph. The other possibility is that even with the same gearing, the 1st car will have to shift sooner, at say 70mph, and thus be into 3rd gear before car #2 which is still in 2nd at 70. In this way, HP is what matters more in the real world (given ideal circumstances, which is hardly always), while also being kind of an abstract concept. What it means is that you have the potential to multiply your engine torque more than a lower HP motor--you're using more of your engine torque for longer/at higher speeds, which means on average higher wheel torque at a given speed than a lower HP car. That's all there is to it.

At the end of the day, when we throw around peak HP and torque numbers for motors it is a gross shorthand for how good or how powerful it actually is in the real world. What actually matters is the power band--how well it holds power across the entire RPM range. This is the entire area underneath a dyno trace, the larger the area the more accelerating you're doing from the bottom to the top of any given gear. You can have a 400 ft/lb 600 hp motor with a dyno trace like this ____/\_ and the exact same rated motor with a dyno trace like this /````````````` guess which one is 10x faster on the road?

 

[K4fxd]

In your example with both cars being geared for the power band, and assuming prefect traction, the race would be equal.

 

[Rusherific]

In your example with both cars being geared for the power band, and assuming prefect traction, the race would be equal.

Are you referring to me? My drag race example was showing how two equal, flat torque motors (not equal HP) but one with higher redline (and thus higher HP rating) would most definitely be faster. There's no way around that, whether they have the same gears, or optimized (the higher HP would have shorter gears then).

 

[Norm Peterson]

Replace peak horsepower with peak torque and you’d be correct. Unless peak horsepower is occurring at the same place as peak torque of course, in which case, yes, you’d be right.
In the example below, peak acceleration occurs at ~4750 rpm. Anything either side of that is slightly less.

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.

Again, acceleration is the force (torque) divided by mass.

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

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