Substitute "power" for "torque". For any vehicle we can make the torque at the wheels whatever we like. Since we can make torque at the wheels whatever we like we're not so concerned about engine torque, the dominant factor will always be engine power. In reality engines said to have "torque down low" pull harder at lower RPM not because they produce greater torque at that RPM, but because they produce greater power at that low RPM than a peaky motor.

The algebra identities sited in the link prove all of this. Torque never appears in the equation because it doesn't matter.

Agreed. That statement accounts for the peak vehicle acceleration of one particular engine installed in the vehicle and is true of any engine.

The argument however isn't about where on the tach a single particular engine produces greatest vehicle acceleration, it's that given a choice of engines which one will accelerate a given vehicle at the highest rate? The answer is always the engine with the most power (and I use the term power to mean the area under the curve, not peak power).

 

You can make the acceleration g's whatever you like by using the gears, which is exactly what we do when we change between gears in our transmission. No matter the gearing if we drive it with more power we get greater acceleration. We can create torque (and thus acceleration g's) with a transmission but we can't make power with it. That's why we can't gear a car to do 250 mph or gear an engine to make 75 mpg.

 

I assume you're referring to "infinite gear ratios" here? If so, then yes, if you weren't limited by gear ratio availability, you could "make torque at the axles whatever you wanted".

However, you still couldn't totally compensate for a smaller displacement engine's lack of low end torque (or power, since you insist on using that term). You can only get it into its powerband more quickly at the expense of terminal speed and fuel economy in each gear. Look no further than the FD's gearing for an example of what's required to make a pint-sized engine perform well, even in a ~2,800 lb. car.

Really? Well, let's select whatever gearing is necessary to make terminal speed in each gear identical for a rotary-powered FD and an LS1-powered FD and modify both to produce precisely 350 RWHP at whatever rpm it naturally occurs at, and ensure the two vehicles weigh the same. Neither has a horsepower, gearing, or weight advantage over the other.

At that point, wouldn't you like to compare the torque curves of the two engines to see what's really going to happen when they're run head-to-head, or do you think you can predict the outcome from horsepower alone?

Actually, they pull harder at lower rpm because they're ingesting more air and fuel at low rpm, producing more torque. It's a function of displacement.

Not by name, no. But what do you think the Force is in Power = Force * Velocity?

What doesn't appear in the equation is horsepower. Don't make the common mistake of assuming that "power" is equivalent to horsepower. People like to use them interchangeably, and they're not.

You should have used it to mean "average torque".

"Build for torque and horsepower will follow."

 

And the gas pedal or adding or discarding ballast, since you seem intent on being silly.

True, but it does not change the fact that the shape of the acceleration curve will precisely mimic the shape of whatever torque curve is produced by your part-throttle, gear-changing subterfuge.

Can't we? Tell Bugatti.

 

Again true but I think you are confusing acceleration and rate of acceleration with speed and power. I don't want to get much into calculus because I had to study it every hour as a student just to comprehend it and it's been years. If I try to go too far I'm going to dig myself a hole You have to look at the derivatives of position. Velocity is the first derivative of position which is the rate of change of position. Acceleration is next which is the rate of change of velocity. Jerk follows which is the rate of change of acceleration. These are merely increasing rates, they don't tell you what the absolute value of the speed is. You seem to think that increasing rates of acceleration or jerk rule. They do not. What counts isn't the rate of change, but the absolute value at a specific point in time. That absolute value that counts is power. Over whatever instant or interval of time you choose to isolate, the engine that makes the most power within that interval will accelerate the car faster no matter where that engine happens to be on its torque curve. The torque curve merely tells us how relatively quickly that particular engine can accelerate from one point of its power band to another. That's useful information, but what counts in motivating the vehicle isn't the rate of change of the engine power, it's the absolute value of the power itself at that point in time.


I'll attempt an analogy

Let's say you and I walk into a room and find a pair of 300 pound weights on the floor. The task is to lift that weight from the floor and place it onto a shelf 5 feet above the floor. Since it's my analogy let's pretend I'm much stronger than you Since I'm stronger, I'm more powerful. Since I'm more powerful I can get that weight off the floor and onto the shelf more quickly than you can because being more powerful means I always get more work done in less time than it will take you. Each of us is lifting the same weight the same distance so we are doing the same amount of work, but the one of us with the most power will be able to accomplish the task more quickly. It's not about who can get the job done, it's about how long it takes to get the job done.

So we each walk up to a weight on the floor. I grasp the weight in front of me and with great effort I lift it 5 feet into the air and onto the shelf. You attempt to lift your weight from the floor but you can't; you're not powerful enough. No matter you say! I'll hang a block and tackle from the ceiling and I'll use its gain in mechanical advantage to allow my less powerful self to lift the weight up onto the shelf. So you rig your block and tackle. Let's say your block and tackle has a 3:1 reduction ratio. This means you only feel 100 pounds at the end of your rope rather than 300, but in order to lift the weight five feet into the air you have to in fact pull through 15 feet of rope. You pull through the 15 feet of rope and raise the weight 5 feet into the air. Job done.

I lifted 300 pounds 5 feet into the air (300x500=1500) and from your end of the rope you lifted 100 hundred pounds over 15 feet (100x15=1500). Same amount of work in each case. However you had to stand there and go hand over hand for 15 feet of rope to accomplish the task and all I did was pick the weight up and sit it in place. The weight was very heavy to me so I struggled and strained and you noticed that while I struggled the weight accelerated slowly from the ground as I pulled it into the air. The rope you pulled through your hands accelerated at a much higher rate than my arms did in lifting the weight from the floor, but nonetheless it took you longer because what counts is the velocity of raising the weight itself, not how fast you could pull the rope through your hands. The velocity with which you moved the weight was much slower than myself even though the rate of acceleration you imparted to the rope was higher than the one I imparted the weight with my bare hands. Your rate of acceleration was higher but you were forced to in fact travel 3 times farther to get the same amount of work done, that's why your weight's velocity was lower. What's more is that I'm more powerful than you. I could walk over to your block and tackle and raise the weight faster than you can. No matter what way you invent to manipulate the weight I'm going to get it done faster than you every single time because I'm more powerful.

Couldn't you have just pulled the rope so fast through the block and tackle that your weight lifted from the floor as quickly as mine? No. If you were able to pull the rope so fast through your block and tackle that you could in fact lift the weight as quickly as I did you wouldn't have needed the block and tackle in the first place. If you could have done that it would mean you were in fact just as powerful as I and could have lifted the weight with your bare hands. Manipulating mechanical advantage cannot increase your power. It can increase your rate of acceleration, but at the expense of your speed. Also no matter what the mechanical advantage of the system is, if you drive it with more power it will go faster every time.



We can make the rate of acceleration at the tire contact patch whatever we like. Want the tire to accelerate faster? Use a bigger ring gear. Want it to accelerate faster than that? Use an even bigger ring gear. Want to accelerate it even faster than that? Use a huge ring gear. What moves the vehicle down the road isn't how fast we can accelerate the ring gear though, it's how many times we can turn the ring gear in a given interval of time. Every time I increase the acceleration of the axle by using a bigger ring gear I now have to travel farther around that gear to complete one axle revolution. We move the car down the road by turning the axle. If we use a huge ring gear we can get high acceleration at the axle, but the velocity of our vehicle will be slow because the pinion must travel a farther distance around the ring gear to equal one revolution of the axle. Torque at the axle is truly irrelevant as regards to speed, speed is how fast we can turn the axle in a given period of time and that is proportional to power.

If we have more power available to drive the axle we can turn that axle more often within a given period of time, no matter what the gearing (and thus torque) there is between the engine and tire. How fast the axle is accelerating does not determine the vehicle's velocity. How fast the axle is turning will determine the vehicle's velocity.

 

You should have isolated and bottled this statement and saved yourself the trouble.

I think you'll also find that I have never posted anything that disagrees with this statement, and in fact, that's precisely what I've been trying to get these other people to comprehend. Maximize the area under the torque curve and you will maximize acceleration.

 

Let's simplify this.

Let's say we have a vehicle with a CVT. In order to achieve maximum acceleration of the vehicle, should the CVT maintain engine RPM at peak torque or at peak power?

 

Neither. It should maintain engine rpm for peak efficiency and use the CVT to vary the effective output ratio to increase or decrease torque at the axles to meet demand.

Damon, do you think I'm stupid? I'm just curious.

 

I had real trouble with Myths 3 & 4, until I understood an unstated assumption. Jim's certainly correct that a car's absolute maximum acceleration (barring traction issues or driving a slow vehicle over a cliff) happens at peak torque in first gear. OTOH, the mythbuster's actually talking about maximizing acceleration at any given road speed. Poor writing IMO, but it made much more sense when I reread it with that assumption in mind.

Moving on...
So of these imaginary engines, the one with more torque is the higher performance option?

 

I think you didn't answer the question.

The demand is maximum vehicle acceleration. Should the CVT maintain the engine at maximum torque or maximum power?

 

Strangely, Paul van Valkenburgh got this one wrong a few years ago. Wish I could remember where I read it...

 

acceleration = power / (mass * velocity)

This should clear up some confusion. You accelerate slower in higher gears & equal rpms because your velocity is higher. Drag also plays a small part. Also notice that acceleration is a function of power, mass and velocity only.

If you use metric units - m/s^2 = W / (kg * m/s) - you can use this equation as-is. For standard units you need to multiply by some number I don't have off-hand.

 

Pounds of thrust = HP * 375 / MPH

 

Im still not sure what the point of the initial post is...

Yeah engine torque without the context of gearing, wheel size or whatnot is nonsensical, to the layperson its obviously a good thing.

I just wish everyone would realize its about the POWER BAND and the most you can expect from a car motor is an asymptote of accleration over different road speeds. You cant get constant thrust in more than one gear, and if you try to get it in one long gear you could get more at a lower gear, or with a CVT, or with a steam/electric motor.

Regardless, a LS1 has a bigger powerband and more off the line power than a 13b, which is why people like to swap them in.

So WTF are we even talking about anyway?

 

Not V8's vs. Rotary

 

OH HEAVENS NO.

But that was the only relevant example I can think of, so I used it... sorry thats how it came out.

This debate might be a little different if Mazda's rear ends were easier to swap gears in, becuase you could gear it to accelerate just as hard as a FD, but for a much longer time, for example.

 

Having completed at least an undergrad engineering degree, surely you remember that it is a requirement when solving any problem to state your assumptions...........

Your information above is correct...... ASSUMING continual gearing adjustment to place the motor rpm at the HP peak at all times.

We drive cars with transmissions that have a finite number of gear ratios. Your equation is only valid with a CVT (continuously variable transmission) putting the power to the wheels.

 

In every gear but 1st you go faster in the gear making higher power than the other gears... with roadspeed as constant and the only variable being gearing (and thusly) engine RPM.

Torque vs Power is still a stupid thing to talk about anyway. Its POWER BAND, appropriate gearing, and feasability.

Bigger motors have bigger powerbands, are easier to gear for, and are more feasable to accomplish than the motorcycle/vtec 'rev to the moon' sort of thing, which is why theyre generally a better option... not some sort of power vs torque thing.

Oh, and if you have higher torque, you have higher power anyway

 

The answer is that to achieve maximum acceleration of the vehicle we maintain engine RPM at maximum power. Maintaining engine RPM at maximum torque or anything other than maximum power will result in slower acceleration. Why? Because the engine has the ability to do more work at maximum power than it does at maximum torque. It's that simple.

 

Nope, don't assume the equation's input is the engine's peak power output. This is not a 1/4-mile calculator! To calculate the acceleration at any given road speed & gear, you just use the instantaneous power output for the resulting RPM.

 

^ Yep.

Everyone is tripping up over a few fatal errors.

The vehicle will achieve maximum acceleration at peak axle torque. Peak axle torque does not occur at peak engine torque. Peak axle torque occurs at peak engine power! Power drives the axle, not torque! Gears can multiply engine torque at the expense of RPM.

It's being harped on that the rate of acceleration is greatest at the engine torque peak. Look at g graphs for proof of this etc. This is meaningless because it neglects time; it focuses on an instant. Velocity is the product of acceleration AND time. You can have a huge acceleration but if it's only for an instant you're not going anywhere fast. For a car you can resolve this to the more time a gear allows the engine to spend at peak power the faster the vehicle will accelerate.

Force and torque are not work. Work takes place over a distance traveled which means it takes place over time. Force and torque have no time element so they do no work. This is irrefutable.

Work = Force (Distance)

Power= Work / Time

We can also resolve Power to:

Power= Force (Distance) / Time

From the above you can see that power MUST involve all three factors. In order to have power you must have a force which travels a distance over a period of time. Neglect any of the 3 and you get zero power.

 

Not true. look at diesel engines...

1500 ft/lbs with only 300 hp.

 

Nope. Fatal error. You left out time.

Caterpillar engines make huge torque and yet have far less power than comparitively pint sized F1 engines which makes hardly any torque. How is that possible? The F1 engine makes little torque but it is able to makes its little torque for a much longer period of time (ability to run at much higher RPM). The enormous increase in RPM allows the F1 engine to be more powerful than the Caterpillar engine even though the Cat has a large torque advantage. The F1 engine will even out accelerate the Cat because the F1 engine is more powerful. Engine torque doesn't matter. Engine power matters.

To make the illustration easy let's take two engines with equal and ruler flat torque and horsepower curves. Exact same torque, exact same peak horsepower. One engine redlines at 5000 rpm and one engine redlines at 8000 rpm. Do the engines make equal power? No. The one that redlines at 8000 rpm is more powerful. How can it be more powerful if it produces no more torque or peak horsepower than the other engine? Because the 8000 rpm engine can produce its torque for a longer period of time than the 5000 rpm engine can. You cannot speak of power without including time.

You could even build an engine with less torque than another and yet have the less torquey engine be far more powerful by turning it at higher rpm (see the Caterpillar vs F1 example).

 

I'm not sure what you're trying to describe here, because a couple of your sentences seem contradictory. Mind drawing a picture to compare the engines?

And I welcome your comments on post #134.

 

That and the F1 car weighs about 1500lbs while the cat is about 30 tons