LiL BenNy

so i was talking with my friend the other day.. i always though the more HP the car has the faster it could go (MPH not time) and then torque is how quick the car is... this got me to think is that correct? prally not lol.. and if torque is how to get the car faster how come everyone worries about HP rather then torque.. and wasnt there a FI kit with like 40 hp add but makes like 300 torque? wouldnt that create the best results... can someone explain... thanks...

globi

https://www.rx8club.com/general-automotive-49/horsepower-torque-49469/page2/

rotarygod

Read this. It will explain everything.

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

LiL BenNy

thanks for the links!

RX8_Buckeye

While this link does provide a lot of useful information and gives a decent high-level overview of power and torque, there are definitely some flaws and certain statements are very misleading. The author definitely does not have a solid understanding of the many factors that affect the acceleration capability of a vehicle. His article comes to the conclusion that "horsepower is the determining factor in the rate of acceleration of any vehicle". The reason he comes to this ridiculous conclusion is that he fails to make any distinction between instantaneous acceleration and actual acceleration (as in an actual acceleration vs. time trace). It is very easy to show that his conclusion is incorrect. Consider two identical vehicles, one with a 300 hp/300 ft-lb engine, and one with a 300 hp/200 ft-lb engine. According to the author's argument, both vehicles should have the same acceleration capability because they have the same horsepower (see quote above). Unfortunately for him, any idiot knows that the car with 300 ft-lb of torque will easily out-accelerate the car with 200 ft-lb of torque, even if horsepower is equal.

Another problem I have with the article is the following paragraph:
First of all, torquey motors like a Cummins diesel ARE intended for low-RPM grunt and no one in his/her right mind would ever gear it for 200 MPH. The author, for some reason, feels that this is a ridiculous statement. The article is discussing torque and power as they relate to ACCELERATION, so why not provide a fair comparison? For example, the diesel should be geared for a top speed of 100 mph while the F1 engine remains geared for 200 mph. In this case, the instantaneous acceleration curves of the two vehicles will be much closer. To take an extreme example, the diesel engine could be geared for a top speed of 40 mph. In that case, the author's own equations could be used to show that the diesel would have a higher rate of instantaneous acceleration than the F1 car (default gearing) over the range of 0 to 40 mph.

BaronVonBigmeat

You could gear the RX-8 to have the same rear wheel torque as a Viper. Then you'd have the same instantaneous acceleration as the Viper too. But unless you're racing him on a 1/256th mile track or something, instantaneous acceleration doesn't mean anything.

The only thing Yaw's article may have left out IIRC is a discussion of average horsepower over a certain RPM range. An engine with high peak HP might lose to another engine with more broadly distributed HP but a lower peak number.

Oh and also

http://www.bankspower.com/Banks_Side...inder-news.cfm

cough cough

RX8_Buckeye

I'm not sure whether you're agreeing or disagreeing with me, but that's the point I'm trying to make--Yaw's article only considers instantaneous acceleration, therefore the equations provided are not sufficient for determining which of 2 vehicles will have a faster 0-60mph or 1/4-mile time. If he were to give a different example, say a stock RX-8 vs. a stock Infiniti G35, I guarantee that the traces of instantaneous acceleration vs. vehicle speed will fail to tell the whole story. It's a lot more involved than the article makes it seem.


If you honestly think that this is the ONLY thing that the article left out regarding power, torque, and acceleration capability, then there's a lot you don't understand. I could show this article to any competent mechanical engineer in the automotive industry and he/she would reach the same conclusion. The article is more of a misguided sales pitch for horsepower than anything else.

And just to clarify, I agree with the focal point of the article--that horespower in general is a better indicator of acceleration capability than torque; however, there's a lot more to the story.

rotarygod

Actually Paul Yaw is probably one of the single smartest people when it comes to how things actually work. FWIW, He is a mechanical engineer so don't think he doesn't know less than you do because you are one. If something he wrote is to be considered wrong, it's more likely that what he was actually saying wasn't totally understood by the reader. In this case that is absolutely true. His entire article is very valid. He has to make some generalizations for arguments sake but as is very typical on internet forums, someone always has to come along because every single little detail in every possible circumstance. We aren't PC here. You don't need a disclaimer to figure out every little assumption. Obviously no one would gear a low end car in such a way as to race a Formula 1 car. He knows that. You have to even out the playing field to get a fair comparison. He did that very well and he's right. Torque is not a measure of work and therefore is irrelevant in determining acceleration. Horsepower is a measure of work. The people that tend to argue this seem to forget that they are mathematically related but that still doesn't change the fact that torque does no work. Horsepower does. That's the whole point of his article. He is not accounting for other things such as traction, aerodynamics, road condition etc. That's just nitpicking at that point.

The argument that a 300hp/300torque car will out accelerate a 300hp/200torque car is obvious. However, it isn't because one has more torque. First of all the car with the higher torque has a greater average power even though the peak horsepower is still the same. As Paul himself says, and is again correct about, average power wins races. Not peak power. The 300hp/300torque car has more average horsepower than the 300/200 car. Remember that if you have more torque at one spot, you also have more horsepower there as well due to the fixed mathematical relationship between the two. Since torque is only resistance to twist and not a measure of work, it isn't making you faster. The added horsepower there is what is making you faster. Like I said, he was misunderstood. His point still holds true. Torque is just like boost pressure. Neither tells you anything important on their own.

RX8_Buckeye

rotarygod, I understood everything that was covered in the article. I never claimed that any of the math was wrong, simply that the math he provided does not fully support his conclusion that "horsepower is the determining factor in the rate of acceleration of any vehicle".

Why do you believe that I'm arguing little details? I'm arguing that the math he provides in the article is inadequate for predicting how fast a vehicle will accelerate.

How is it fair to gear a torquey diesel engine for a top speed of 200 MPH and compare its acceleration capability to that of an F1 engine? The F1 car has incredibly steep gearing, so why can't we "even the playing field" by giving steep gearing to the diesel engine as well?

This is simply wrong. The computer simulations that automakers use to predict 0-60 mph times employ the engine torque vs. rpm curve as the main input. This, combined with gearing, shift points, aerodynamics, parasitic losses, rolling resistance, vehicle weight, etc. are the inputs that the simulation uses to calculate the trace of acceleration versus time. There is no simple formula to calculate this trace, despite what you might think. The instantaneous acceleration versus vehicle speed traces included in Yaw's article should not be confused with an actual representation of how fast a vehicle can accelerate.

Horsepower is derived from torque, not the other way around. We're arguing semantics here. Yes, the 300hp/300torque car has more average horsepower BECAUSE IT HAS MORE TORQUE OVER A BROADER ENGINE SPEED RANGE. All you did was twist my words around in an attempt to support your assertion that torque is meaningless.

Hmmm, that's funny. I scanned the entire article again and I didn't find a single mention of the concept of "average power".

globi

Actually if the gearboxes in cars were CVTs than all what would matter was peak power. :D
http://cvt.com.sapo.pt/performances/performances.htm

If the CVTs gearboxes wouldn't have been prohibited in 1994, the F1 race cars would probably all drive with CVTs.
The Williams F1 team built a race car with a CVT 1993 which was banned the following year.

Regarding the whole torque and horsepower discussions/writings I'd simply comment: "Brevity is the soul of wit."

RX8_Buckeye

I didn't mean to start a cut-throat argument about this. I may have worded some things a little too strongly and I apologize for that. As I said before, I just feel that some of the content of the Yaw article is misleading. I think rotarygod brought up a good point when he started talking about average horsepower over an RPM range. We get too caught up in peak torque/peak horsepower figures, when probably what is more important is the shape of the torque/power curves. A 220 hp engine with a broad peak could probably accelerate a vehicle faster than a 240 hp engine with a narrow power peak. This is why it can be misleading to associate peak power/torque figures with the acceleration capability of a vehicle.

BaronVonBigmeat

The point of the article was basically "which numbers do we need to look at when evaluating an engine, horsepower or torque?" and not "what makes a car accelerate faster?". I think it's reasonable to say that "all other factors being equal--weight, tires, aerodynamics, driver, drivetrain losses-- HP is the number to look at" was strongly implied.

Sure, you can do that. And that torquey diesel with the short gears will accelerate hard too. But you will have lost a lot of top speed. You can have either:

A) Loads of torque @ slow RPM's (good acceleration/slow top speed)
B) Very little torque @ crazy RPM's (poor acceleration/high top speed)

But if you want both the acceleration AND top speed of the F1 car, you will have to increase power . There is no other way around it.

Notice the "vs. rpm" part? Without it, you cannot determine acceleration, and any comparison of two engines based on just torque is meaningless.

Now if only those engineers could come up with some sort of mathematical expression that included both torque and RPM's.....hmmmm.....

Yaw's article was a bit long, let's look at this one that comes to the same conclusions. This one was written by a Mustang guy btw:

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

RX8_Buckeye

Baron, I agree with anything you said. You hit the nail on the head when you said: I interpreted the article as trying to explain "what makes a car accelerate faster". That's why I pointed out that the math he provides does not support the conclusion.