dragula53

Hey folks,

I frequent rx7club and rx8club pretty regularly, and a topic came up over there about torque, and I figured I would toss it about and maybe one of you super gurus could clarify a couple things for me.

I don't know if any of ya all are familiar with jimlab over there, but he is rebuilding his rx7 from the ground up with a v8, an extremely clean job, and his car is beautiful.

He has informed the masses over there that acceleration is exactly equal to torque x gearing x contact patch. Absolutely nothing else matters (specifically horsepower). He further affirms that any 150 ft/lb of torque engine in the rx-8 would be equal in acceleration (assuming it can rev to 9000 rpm), as the reason an rx-8 can do 0-60 in 5.9 are a combination of favorable gearing and weight.

I am willing to concede that torque gives one a much more useable power band and that a launch from idle depends exactly upon torque, and also huge amounts of torque save abuse of driveline components from having to dump the clutch. But the way I understand it is that if I were to launch at anything over 5252 rpm, the revolutions of the engine would be more and more relevant to your acceleration (100 torque at 2,000 rpm accelerates faster than 100 torque at 1,000 rpm).

There are many sites out there that support jimlab's assertions to the letter, but somehow that doesn't make sense to me, as any clutch dumping launch in any car is faster than an acceleration from idle.

I am not trying to start a flame war one way or another, as I am really just technically curious and couldn't care less about import vs domestic or any crap like that, so none of that please.

That's it. Any thoughts?

foxman

http://www.yawpower.com/tqvshp.html

Have you seen this?

Spin9k

Horsepower, Torque, and Gearing
...and how they relate to one another.


This practical, easily understood explanation between a 350Z and an S2000 should help. Kudos to those Mazda6tech guys...

http://www.mazda6tech.com/index.php?...id=9&Itemid=74

BaronVonBigmeat

Three great articles to read from www.modulardepot.com are:

http://www.modulardepot.com/?show=articlesdet&aid=44
http://www.modulardepot.com/?show=articlesdet&aid=46
http://www.modulardepot.com/?show=articlesdet&aid=50

The Bottom Line: Horsepower is the number you want to look at when comparing engines. Torque (at the crank) means jack squat. It's not enough information, just like looking at RPM's alone.

Perhaps shelly's man can chime in, it's been too long since I had any engineering classes. But I'm fairly certain that the formula for acceleration that people quote when beating their chests about torque is for instantaneous acceleration. If you're running a .01 second race, sure, look at torque. But even then you have to realize that you can create nearly infinite amounts of torque at the wheels with gearing. So unless you're running a race where everyone hooks up their engines directly to the drive wheels, torque at the crank doesn't matter. Horsepower does. And remember, torque does not include a measurement of time, but horsepower does. Winning a race is all about doing a certain amount of work in the least amount of time possible.

(This is completely aside from arguments concerning peak HP versus average HP. A nice broad distrubution of HP across a wide range may very well be more useable in the real world than a higher but peaky HP.)

dragula53

Thanks fellas,

Those articles are exactly the way I understood it.

However, I could point out just as many sites that say torque is all that matters, and it would make sense as well. I could also point out that the diesel redlines at 3,000 rpm, so gearing benefits the F1 car because of it's extremely broad revolution range (and lighter weight), not because of it's greater horsepower. I argued these points exactly and walked away still in doubt.

and from that yawpower article:

(quote from jimlab)

I've read that article in various forms several times over the years. You should stop at...

"By now it should be clear that the acceleration rate of a vehicle is determined by the weight, and the force at the contact patch, which is the result of the torque output of the engine, and all the levers/gears between it and the ground."

All you need to calculate acceleration and ideal shift points for your car is the torque curve, weight, gearing, and tire size.


"So there it is. Horsepower is the determining factor in the rate of acceleration of any vehicle"

Gee, and right after he got done saying...

"By now it should be clear that the acceleration rate of a vehicle is determined by the weight, and the force at the contact patch, which is the result of the torque output of the engine, and all the levers/gears between it and the ground."

(end quote from jimlab)

here is the rx7club thread (It turned into a hp vs. torque debate around page 5 I think) :

http://www.rx7club.com/showthread.ph...&page=13&pp=15

and there are a gaggle of links on that particular page that support his assertion that torque is the only relevant factor.

I'm not trying to drag this into a multiple forum war either, but it was easier to link than to try and post every single link.

They do seem like really smart guys, which is why I am in doubt.

therm8

These hp vs torque discussions always get too deep for me. It results in what we call in the Navy..Nuking it out (nukes being nuclear trained people who are stereotyped, not entirely unfairly, as overthinking things too much). It seems to me that the whole point of an internal combustion engine is to produce torque(ie to make things turn). Horsepower is just a derived measurement from this produced torque.

T.T.

This is the correct answer.

dragula53

I understand that horsepower is the amount of work being done. But you can say that it takes x horsepower to make car y travel z miles per hour. That's horsepower.

I guess I haven't been presented proof that acceleration is not just more torque transferred to the wheels by gearing.

You can say that at 30 miles per hour the car is generating a certain amount of horsepower because it is doing a certain amount of work and the engine is revolving at a certain rate. With 1 horsepower, I could go 30 miles per hour with the right gearing.

I think I will be confused until that point. Horsepower implies an exact number of revolutions and an exact amount of work done. not the amount of force applied and how fast you made it there.

T.T.

Well, the truth is, horsepower and torque are related by the engine rpms. One could say that they're kinda the same thing.
hp = tq*rpm/5252

shelleys_man_06

No they're not the same thing. Power, unlike torque, is a time derivative. Torque in general is either the applied force multiplied by the moment arm, the moment of inertia multiplied by the angular velocity, or the cross product between the applied force vector and the direction vector.

The equation you used T.T., notice how you have to multiply the torque by the angular velocity (rpm), divided by the constant 5252.

BTW, I've started a few mini-wars in that other forum regarding engineering and whatnot. There are some smart people there, and I got a lot of information, but most are sheep.

Well anyhoo, IMO, power is more important than torque, since like what Baron said, torque cannot determine the ultimate performance of a given engine. Given power, one can determine all sorts of parameters of an engine, such as BSFC, mass flow rate, mechanical efficiency and thermal efficiency, etc.

shelleys_man_06

I want to go back to dragula53's initial query about the notion of torque. If you break down the general form of what it is, force is mass times acceleration, so therefore

M=Fd=(ma)d

Rearranging,

a=M/(md)

Acceleration is directly proportional to torque. However, over the powerband, power is dominant factor on engine, and more importantly the car's performance. If you're talking solely about acceleration, then yes torque is important, especially when it comes to instantaneous acceleration. Kinetic energy is based on one-half velocity squared, which comes from an intgration, but that's not important. What is important is that power can only be related to velocity squared (not angular), or at least that's what I've been told.

Over the years, I've learned having low-end torque is tantamount to a more everyday-use engine. However, with proper gearing, among other things, cars with peak torque on or near the upper end of the engine's powerband can also be daily drivers. What to look for in torque and power curves, or course is the slope.

With that said, forget about what I said, and go drive.

T.T.

What I was trying to get across is that horsepower is directly related to torque. Of course they aren't the same thing, but horsepower is derived from torque. Horsepower is torque per unit of of time. As someone pointed out, since horsepower has the time unit it is important for acceleration.

gansan

Dragula, think of it this way:

When you say "acceleration is more torque transferred to the wheels by gearing", that is exactly what horsepower measures. Here is why:

You have a car with two gears in the transmission: One gear is 2:1 ratio, one is 4:1 ratio. The engine always makes 100 lbs-ft of torque.

Suppose the higher gear at 2:1 makes your car do 2000 rpm at 10 mph.
The second gear is twice the ratio at 4:1, so your car does 4000 rpm at 10 mph.

The higher gear takes your 100 lbs-ft of torque and multiplies it by 2 to make 200 lbs-ft of torque at the wheels. The engine is running at 2000 rpm and 100 lbs-ft of torque, so it's making 38.08 horsepower.

The lower gear takes your 100 lbs-ft of torque and multiplies it by 4 to make 400 lbs-ft of torque at the wheels. The engine is running at 4000 rpm and 100 lbs-ft of torque, so it's making 76.16 horsepower.

Look at these two paragraphs. When you have twice the torque at the wheels, you have twice the horsepower! That's not a coincidence, and that is why twice as much horsepower will accelerate you twice as hard.

dragula53

Ah.

That totally makes sense.

And I believe the remainder can be explained by the amount of time near peak horsepower. use-able rev range, that is.

Thanks fellas, interesting stuff.

trophymaker

I know one could derive this from the very clear explanations given above. I am glad there are so many engineers on this forum.

But I think I can make it more simple. I'm not sure if it matters which one is more important? An engine with 150lb/ft of torque can make 300hp with the right gearing and revs... an engine with 700lb/ft of torque can make 100hp with the right gearing and revs...

Horsepower is simply derived from and dependent upon torque, or the work being done.

I think you should be more worried about power-to-weight ratio.

AbusiveWombat

You're example is a bit confusing and your conclusion is incorrect. Horsepower stays the same regardless of gear ratio. Here's a similer explaination:

We're all familiar with the formula:
hp = tq * rpm / 5252

So lets take your two speed car with a 1st gear: 4:1 and 2nd gear: 2:1

To keep things simple lets say that the car has a perfectly flat torque and generates 100 ft-lbs. It's redline is 8000 rpms.

So it's maximum crank horsepower is:
hp = 100 * 8000 / 5252 = 152.3 hp

Now lets solve for horsepower at the wheels:

1st gear:
ratio: 4:1 (4 revolutions of engine for every 1 revolution of wheel)
torque at wheels: 100 * 4 = 400 ft-lbs (gear ratio muliplies torque)
rpms: 8000 / 4 = 2000 (the gear ratio reduces the rpms. When measuring at the wheels you go by wheel rpms not engine rpms)

hp in 1st = tq@wheels * rpm / 5252 = 400 * (8000 / 4) / 5252 = 152.3 hp

2nd gear:
ratio: 2:1 (2 revolutions of engine for every 1 revolution of wheel)
torque at wheels: 100 * 2 = 200 ft-lbs
rpms: 8000 / 2 = 4000
hp in 2nd = tq@wheels * rpm / 5252 = 200 * (8000 / 2) / 5252 = 152.3 hp

Horsepower at the wheels always remains the same regardless of gear ratios. The torque at the wheels changes but not the horsepower. This is particularly why it's horsepower and not torque that wins races. Knowing the horsepower and weight of a car can give you a good idea how fast it is, knowing the torque and weight doesn't tell you anything.

gansan

Wombat,

You're doing the same math as me but varying the speed of the car where I vary the rpm while speed is constant. My math is explaining why at a single speed, different power results in different torque at the wheels. There's nothing wrong with my math, and nothing wrong with my conclusion. We are just adjusting different variables. I would chalk it up to the fact that different people conceptualize in different ways.

BaronVonBigmeat

Errr well....it's more like this:

First, we have to realize that you cannot create power out of thin air. The second law of thermodynamics and all. So think of a mountain bike...put it in first gear, and suddenly it's much easier to get through grass and mud. But you don't get something for nothing, right? The tradeoff is reduced RPM's. Your wheels are twisting nice and hard, but they are turning very slow. Once you get back on pavement, you kick it up several gears, which gets you very poor torque, but more RPM's. But in either case, the guy riding the bike didn't make any more power. He'll have to lift weights or use steroids to do that.

Having said that, let's imagine this engine:

100 ft.-lbs. of torque (distributed nice and broad)
5000 RPM redline

So the power is: (100 x 5000)/5252 = 95 HP

With the 2:1 reduction, you're right--the torque jumps up to 200 ft. lbs. But the wheel RPM's drop down to half--2500 RPM. (200 X 2500)/5252 = 95 HP.

With 4:1 reduction, you get 400 ft. lbs., but only 1250 ft.-lbs. Which again gives us the same 95 HP.

Now if we were really nuts, we could put a set of granny gears on this thing so short that our rear wheel torque is the same as the rear wheel torque on a Dodge Viper! And with the right tires, suspension, and equal weight....we could launch as hard too! But we'd only be nose-and-nose with the Viper for the first split second or so. That's because our little econobox has topped out it's first gear at 5 MPH and the motor is screaming. We traded RPM's for torque. The Viper didn't have to make that trade, it gets to have equal torque AND more RPM's--in other words, more power. It keeps right on pulling, and we have to shift into second, which reduces our torque.

Another misconception that some people have--you need lots of engine torque for towing. Torque-biased engines are desirable in current engine designs because torque motors (diesels) are fuel efficient and because low engine speeds = longer lasting engine. However, if someone invented an engine that beat the fuel efficiency of a diesel, had the same maintenance/ownership costs, but made it's power at 20,000 - 25,000 RPM's...that's what all the trucks would be using before long. With insane gear reduction of course.

EDIT: I forgot something. You have to consider throttle response, which is not the same as power. You can make gigantic power but still have poor throttle response--see jet engines. A turbine might be able to meet the criteria listed above (more efficient than diesels, high RPM's, etc. ) but from what I recall reading, the throttle response is poor. This of course doesn't matter to electrical generators and jumbo jets, which is where the turbine thrives.

neit_jnf

Anybody ever thought that a force or torque can be applied without moving anything??? (i.e. trying to get out a stuck nut or bolt, or trying to push a wall, there's applied torque or force without doing a thing)

shelleys_man_06

Of course. That's in earlier chapters of mechanics books, and the mechanics part of physics. That's Newton's 3rd Law. You can apply a force or torque to any object, and you'll always get an equal and opposite reaction. It only seems that you aren't moving it because the force you're applying to the system is equal to force being applied by the system. I can express this in mathematical terms.

Suppose you are pushing against a wall, where the mass of the wall, M, is much greater than m, the mass of yourself. Since, F=ma, such that, your acceleration A >> a, the acceleration of the wall,

mA=Ma

I think this is called dynamic equilibrium.

Same thing applies to torques.

neit_jnf

I know that, I was trying to get people to understand that torque alone is not really what gives us acceleration. we need time, rpm, etc thus Horsepower or work done.

dragula53

This is all easily understandable, where the confusion lies is:
Is 150 foot pounds of torque at 5,000 rpm better or worse than 300 foot pounds at 2500 rpm?
Horsepower is equal, but will acceleration be the same?

Assuming equal weight, there are scenarios where you will be going exactly the same distance in the same amount of time with the correct gearing. And horsepower will be equal.

What a high torque advocate said to me is that with 150 ft/lbs of torque, you may start with exactly the same horsepower by crunching your clutch, but you will bog your engine down because not as much force is being exerted.

RX8_Buckeye

dragula: listen to me. There is no simple answer to this horsepower vs. torque debate! In order to mathematically predict the 0-60 mph acceleration time for a vehicle, you would need to create a time-domain numerical integration model with lookup tables or analytical expressions representing the hp and torque curves, gear ratios & shift points, and aerodynamic and frictional drag (including driveline losses). This is how automotive companies predict the performance numbers for a new model before a prototype is even built! It is just not possible to say "low RPM torque beats high RPM horsepower" or "horsepower wins races, not torque". There are way too many factors involved.

That said, I can clear up a few things that have been mentioned in this thread. Baron and Shelleys_Man already said it, but I'll emphasize this point. That equation for acceleration represents INSTANTANEOUS acceleration. It has absolutely nothing to do with how fast a car does 0-60 mph. As I mentioned before, there are many factors that affect 0-60 mph time.

Thank him for stating the obvious to everyone. But actually he's not being specific enough. The engine would have to have the EXACT torque curve of the RENESIS. If the engine has the same peak torque at the same RPM as the Renesis, but its torque vs. rpm curve differs somewhat at other engine speeds, acceleration will not be equal. And I almost forgot: it would also have to weight the same.

Hmmm, I don't really think this is a misconception. Torque, not power, IS the important quantity when it comes to towing. I disagree with your comment about the 20,000 RPM motor being preferable to the low-RPM motor, even if it were cheaper and more efficient. You said it yourself: it would require insane gearing. Try manufacturing and packaging a 20:1 differential, or a transmission with a similar ratio, at a reasonable cost. It's just not possible. For these reasons it's much easier and more preferable to get the torque from the engine, rather than playing around with gearing. I know you could respond by saying, "if the 20,000 rpm motor were cheap, and if the insane gearing could be packaged, and if...". The fact is that the high-revving motor and insane gearing are not, and will never be, practical for production vehicles. Thus, the original statement is true: you need lots of engine torque for towing.

Deslock

Just happened across this thread... here's the simple answer:

1. Acceleration for any given situation increases if wheel torque is increased
   
2. Max wheel torque in any one gear happens at the engine RPM corresponding to max engine torque
   
3. Max wheel torque at any given vehicle speed happens at the engine RPM corresponding to max engine power

#1 should be intuitive; #2 and #3 are from:

• WheelTorque = EngineTorque * SystemEfficiency * Gearing
• WheelTorque = EnginePower * SystemEfficiency / WheelSpeed

You need to know your vehicle's gearing in order for engine torque to have any meaning. Engine power, on the other hand, tells you your wheel torque at any vehicle speed, regardless of gearing and engine torque.

Dismissing power as unimportant and claiming only engine torque and gearing need to be known is misguided since the whole point of the concept of power is to represent force and velocity (or as in this case, torque and angular speed) with one term (which can then be easily used to determine what kind of mechanical advantage can be achieved through output/input - aka gearing - ratios).

Also, a couple side notes:

• torque is a concept while hp is a unit so the frame of the discussion should be torque vs power or ft-lbs vs hp
• 5252 RPM has no conceptual significance; it's just the RPM at which the units of ft-lbs and hp cancel each other out

globi

At some point I also got involved in another forum regarding that always repeating Power & Torque topic and that's how I would describe Torque and Power: Also, I started to wonder why some people get so confused about this and thought this could be an explanation:Regarding jimlabs information:
The contact patch has nothing to do with acceleration. It's just an indicator whether the tires can carry the torque at the wheels or not.
Then jimlab is missing the mass of the car and he's missing the engine speed. Unless you know at what engine speed (rpm) the engine produces its torque, the torque information itself is useless.