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View Poll Results: Which car would win in a drag race?
Car A will win because it has twice as much torque. 216 51.55%
Car B will win because the engine spins faster. 80 19.09%
The cars will tie. 123 29.36%
Voters: 419. You may not vote on this poll

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Old February 10th, 2006, 23:26   #1
jackbombay
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Default TQ vs. HP

You have 2 identical cars with the exception of the engine and the transmission, the engine and trans of both cars are the same weight though.

Car A has an engine that produces 420 lb/ft of torque at 2500 RPM for an output of 200 WHP and it has a CVT tranny that has no losses and it maintains a constant engine speed of 2500 RPM under all conditions.

Car B has an engine that produces 210 lb/ft of torque at 5000 RPM for an output of 200 WHP and it has a CVT tranny that has no losses and it maintains a constant engine speed of 5000 RPM under all conditions.

Which car would win in a drag race?
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Old February 11th, 2006, 00:36   #2
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Quote:
Originally Posted by jackbombay
You have 2 identical cars with the exception of the engine and the transmission, the engine and trans of both cars are the same weight though.

Car A has an engine that produces 420 lb/ft of torque at 2500 RPM for an output of 200 WHP and it has a CVT tranny that has no losses and it maintains a constant engine speed of 2500 RPM under all conditions.

Car B has an engine that produces 210 lb/ft of torque at 5000 RPM for an output of 200 WHP and it has a CVT tranny that has no losses and it maintains a constant engine speed of 5000 RPM under all conditions.

Which car would win in a drag race?
I would say car A because it reaches it's maximum HP earlier. In effect it gets a jump on the gasser...........er car B , which can't possibly recover as it never has more HP through the run than car A.
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Old February 11th, 2006, 02:05   #3
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With the CVT operating boundary conditions as you've stated, and assuming also that in both cars the CVT and rest of the drivetrain have exactly equal efficiency, which is defined as the power out of the driveshaft as a fraction of the power being produced at the crankshaft, then the answer is simple and it is FACT: They will accelerate at the SAME rate. Argument closed. This is a good question because on Thursday I am writing an exam in Automotive Engineering 1: Longitudinal Dynamics that will cover the exact same subject.

Edit: reading your question again, you've made the answer even more obvious and simple: you said the CVT has no losses, and you quoted 200 WHEEL HP for both cases.

The acceleration you feel in your car is equal to the excess accelerative force divided by the mass of the car, the term "excess" referring to what's left over after overcoming rolling resistance, aerodynamic drag, etc.

This excess force that propels the cars, between the road surface and the tire contact patch, is equal to the torque at the wheels divided by the dynamic radius of the driving wheels.

The torque at the wheels is equal to the power available at the wheels divided by the angular speed of the wheels.

If the CVT adjusts the ratio to maintain the same engine RPM as a function wheel rotation rate, then power (which is constant between the two cars) divided by angular speed at the wheels (also constant between the two cars), is also exactly the same.

The only things we are left to assume is that the acceleration occurs with no tire slip and that there isn't a time dependency from when the light turns green to when the conditions described in your question is reached.

This should clearly illustrate that at the end of the day, it's the FORCE (analogous to torque) at the WHEELS that matter, and if you follow the path back to through the drivetrain and ultimately to the engine, it can be be shown that it is a function of POWER (the arithmetic product of torque AND ROTATIONAL SPEED) rather than torque alone. It is a ridiculous notion that engine torque independent of RPM accelerates the vehicle.

IF the question were reworded and you set the gear ratios to be the same between the two cars and you stay in the same gear during the acceleration run, then you change everything, and then YES, engine TORQUE alone will determine the acceleration because now you've set the relationship of the rotational speeds of the wheels relative to the engine between the two cars to be constant, and then, the engine with the greater torque wins the race.

Aside: Practically speaking this race scenario can never happen from a standstill, becase the power at the wheels is exactly zero at a standstill, and yes, there is a finite time dependency of the engine and drivetrain reaching the conditions you described in the question. If you had such an ideal CVT that can lock the engine RPM regardless of wheel speed, and if you did the race from a rolling start from, say, 30 MPH, then the analysis I stated would be true.

However, for interest's sake let's look at another scenario where we have a conventional single speed, fixed ratio transmission, two cars identical in every way except on one, the engine has a characteristic of developing constant POWER throughout the RPM range, and the other developing constant torque. Both cars have the same peak HP value.

At any initial point where both cars are travelling at the same speed, the demanded power to overcome rolling resistance and drag is equal. Now both cars take-off to race each other. The car whose engine develops constant power will outaccelerate the one that develops constant torque.

The reason I bring up this analogy is that the engine developing constant power can be very roughly analogous to a Diesel engine (TDI, if you look at the RPM range between 3000-4000 RPM or so), while the one with constant torque again analogous to a gasser (e.g. 1.8T with an RPM range of perfectly constant maximum torque). Road tests support the fact the the in-gear acceleration of the TDI is greater than that of the 1.8T when both engines have roughly the same peak HP.
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Old February 11th, 2006, 04:05   #4
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TDIMeister, I was in Cologne, Frankfurt and Munich a couple times in the past week and I knew I should have looked you up and taken you out for a beer! You are studying WAY too hard!


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Old February 11th, 2006, 04:23   #5
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Dude, if you're around after March 9th, when I will finish the last of my exams, definitely look me up.

Or you should try to plan to be in Cologne on February 23rd, when they will be celebrating Karneval, which is the equivalent festival to New Orleans' Mardi Gras (more accurately, Karneval occurs on the Thursday before Lent, which sets off nearly a week of festivites ending with Fat Tuesday -- a.k.a. Mardi Gras).

There will be revelers in costumes and lots of beer and partying involved. I will be there So don't you worry too much about me, in spite of my studies I still know how to take some time to have fun.

I'll get in touch with you offline.
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Old February 11th, 2006, 04:51   #6
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I have approx 189whp and my mates chipped 1.8T has approx the same i would imagine but hasnt been tested as yet.

My obversations are that my car feels quicker and i watched his speedo going up to the 100mph mark and compared to mine, it seems my cars speedo moves quicker to the 100mph than my mates 1.8T, but yet to verify his power. My TDI has longer gear ratios but i think a 1.8T can hold longer to the gears for a higher speed in each gear but its pointless to go too far as the power peak has come and gone, so probably wont be much in it.
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Old February 11th, 2006, 05:06   #7
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car a
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Old February 11th, 2006, 08:30   #8
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Quote:
Originally Posted by TDIMeister
They will accelerate at the SAME rate. Argument closed. This is a good question because on Thursday I am writing an exam in Automotive Engineering 1: Longitudinal Dynamics that will cover the exact same subject. <snipped all the brainiac stuff>
Figured I was wrong, though "tie" was my 2nd choice.
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Old February 11th, 2006, 09:20   #9
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Quote:
Originally Posted by TDIMeister
They will accelerate at the SAME rate. Argument closed.
That's what I have long believed, now its official

I started this thread in the hope that you or someone with a little bit more knowledge than I would chime in as there are often misconceptions about torque vs. HP here on tdiclub. A couple of them would include, "HP maintains speed and torque accelerates the car." as if they were independant of each other, or "If he has 150 HP in his turbocharged civic and you have 200 lb/ft (at 2000 RPM ) in your chipped TDI you TDI will be faster." Because 200 is a bigger number than 150? Wrong.


Quote:
Originally Posted by TDIMeister
Edit: reading your question again, you've made the answer even more obvious and simple: you said the CVT has no losses, and you quoted 200 WHEEL HP for both cases.
I was wanting to illustrate that torque numbers mean nothing without an associated RPM, which is necessary so you can determine the HP which is the determining factor as far as engine output is related to acceleration.
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Old February 11th, 2006, 09:29   #10
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Quote:
Originally Posted by jackbombay
That's what I have long believed, now its official

I started this thread in the hope that you or someone with a little bit more knowledge than I would chime in as there are often misconceptions about torque vs. HP here on tdiclub. A couple of them would include, "HP maintains speed and torque accelerates the car." as if they were independant of each other, or "If he has 150 HP in his turbocharged civic and you have 200 lb/ft (at 2000 RPM ) in your chipped TDI you TDI will be faster." Because 200 is a bigger number than 150? Wrong.
I dont know tons about this, but I wouldnt think your point proved that a 150HP civic would beat a 200 lb/ft and ??? hp.. Obviously torque plays a factor in a street race, with cars that have gears and accelerate through an RPM range, rather then sitting at 2000rpm constantly...
I think a car with 200hp and 420 ft/lb of torque would beat a car with 200hp and 210 ft/lb in a real world situation....
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Old February 11th, 2006, 09:33   #11
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TDImeister, STOP IT! My head hurts and I'm bleeding from my nose and ears!

Great stuff even though I had to reread it a couple of times before I got it.

Now to really change it up, throw a fart can on one and a wing on the other!
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Old February 11th, 2006, 09:38   #12
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Quote:
Originally Posted by eetsoot
TDImeister, STOP IT! My head hurts and I'm bleeding from my nose and ears!

Great stuff even though I had to reread it a couple of times before I got it.

Now to really change it up, throw a fart can on one and a wing on the other!
What about racing stickers
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Old February 11th, 2006, 10:07   #13
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Quote:
Originally Posted by ULELUZE
I dont know tons about this, but I wouldnt think your point proved that a 150HP civic would beat a 200 lb/ft and ??? hp.. Obviously torque plays a factor in a street race, with cars that have gears and accelerate through an RPM range, rather then sitting at 2000rpm constantly...
I think a car with 200hp and 420 ft/lb of torque would beat a car with 200hp and 210 ft/lb in a real world situation....
The question that was posed was a very hypothetical and specific case that would not be achieved in the real world for so many reasons, some of which were discussed.

In the real world two cars with the exact same peak numbers could have very different characteristics, especially if we're talking about a side-by-side drag race, there's no doubt about that, but would require knowledge of a lot more parameters, like the shape of the torque curve, gearing, traction considerations, and the RPM range.

What jackbombay's post correctly tried to point out was, "that torque numbers mean nothing without an associated RPM." I would add that what accelerates you and your car along is indeed torque, as everyone intuitively understands correctly, but that is torque at the wheels, and when referring back to a common measurable parameter at the engine, is the instantaneous POWER because of the effect of RPM and gear reduction. Kapeesh?!

This is a very interesting thread to me as I've long wanted to do some debunking of the widespread misunderstanding and misconception of torque and horsepower. Jack beat me to it but in light of the study that I'm currently doing, this thread is quite timely.

I'd like to hear what the other mech. engineers on this board have to say.
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Old February 11th, 2006, 10:22   #14
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Quote:
Originally Posted by ULELUZE
I think a car with 200hp and 420 ft/lb of torque would beat a car with 200hp and 210 ft/lb in a real world situation....
This scenario would really depend on the power curves...


Take an F1 car for example, 3.0L V10 (2005 spec) produces up to 950HP but only 300lbft of torque (torque figures not unlike what some people have with our TDIs). 950HP occurs right around 18,000 RPM. So it doesn't have much torque but it sure can accelerate, why? because it's geared really low. Our TDIs accelerate pretty quick in first, second, and if you're chipped third gear as well. So since I know that I can go a maximum of 70MPH in 3rd gear, I'll use that one.

At 4700 RPM my engine has pretty much given up in third gear at 70MPH, let's take that F1 car and drop that engine in on our same tranny... So it's still turning 4700 RPM in 3rd at 70MPH. But then engine hasn't even gotten into it's torque range yet, so at 9400RPM it's starting to get some legs and accelerate quicker than our TDI, but it's still in third gear going 140MPH now! So this would be about the start of it's torque curve and the engine is really taking off. So at 14,100RPM the engine is on it's torque plateau and it's still in 3rd gear and now going 210MPH!!! But wait, it's max HP is around 17,000 RPM so we can do this one more time: 18,800 RPM, third gear, car is going 280MPH!!!!!

So what can we learn from that?? A car with a very high usable RPM range can be geared much, much lower than a car with a very low usable RPM range (like our TDIs). It's all about gear reduction, really. Ignoring traction for a moment, you get the highest rate of acceleration in 1st gear, no matter what. So the more reduced you can make your gears the quicker you can accelerate. That's the basis of HP vs. torque.

But since the original question dealt with CVTs it changes things a little. I think what confused some people about the question is the fact that the CVTs will NOT be at the same gear ratio for the same speed if the engines are kept at the point of peak power. With the same gear ratio, car B would have a higher speed. But at the same speed, car B would have the lower gear ratio allowing it to accelerate at the same speed even though it has a lower torque figure.
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Old February 11th, 2006, 10:39   #15
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Quote:
Originally Posted by ULELUZE
I dont know tons about this, but I wouldnt think your point proved that a 150HP civic would beat a 200 lb/ft and ??? hp..

Yes the civic would win easily, I'll edit the post to clear that up.

Quote:
Originally Posted by ULELUZE
Obviously torque plays a factor in a street race, with cars that have gears and accelerate through an RPM range, rather then sitting at 2000rpm constantly...
Torque as an independent unit plays no factor in anything. For now I'll assume you meant "Low end torque" when you said torque. First off any racer that is familiar with their car will launch it at either a) close to the RPM that the engine makes max HP as HP accelerates the car, or b) the RPM that produces just a little bit less power than will make the tires spin. Either way it is irrelevant if the power is low torque at high RPM or high torque at low RPM because the HP is the same and that is what determines the acceleration.

As far as changing RPM as you accelerate is concerned, yes, that is a factor in a cars acceleration, but has little to do with "Torque makes my car fast." It has to do with "the area under the HP curve", of a dyno graph to be exact, look at the following two hypothetical dyno graphs with the associated shift points for the cars, which would be faster? I drew them with the intent of making the area under the curve for each gear, or average output for each gear, equal, 150 HP. So while car A has "175 HP" car B with 150 is just as fast, this is why Diesels with less peak HP can be faster than gassers that have a little more peak HP than them, because the area under the curve of the diesel is greater.


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