LordRipberger

Hey Guys,

I need help to try and get a very good understanding of horsepower and torque. (this will be long)

It is my current understanding that torque is the amount of force needed to turn the wheel of car. Whereas, horsepower is the time element of torque; the amount of HP is the result of the amount of time it takes the torque to turn the wheels to get the car from point A to point B. (probably why HP ratings are often accompanied by the time the car took to get from 0-60mph ---> example: 0-60 in 3.3 seconds)

Equation of HP in relation to torque:
HP = (Torque x RPM) / 5252

Based on this equation, any car with the a higher torque is going to have more horsepower at the same RPMs. However, when I think about cars, not every car is going to have the same HP at the same RPM level; a 4-cylinder car at 3000RPMs will not have the same amount of HP (or torque for that matter) as a 8-cylinder at 3000RPMs. This is where my issue really reveals itself.

Lets say it takes a 4-cylinder 3000RPMs to maintain 70MPH. I know a 8-cylinder does not need that many revolutions to maintain that speed (lets say, 2500RPMs). If we use the equation above, and both cars have the same torque, the 4-cylinder shows more horsepower.

Granted, I know a V8 would never have less torque than a 4-cylinder, but just for the sake of understanding the concept, I used extremes. (But I would say it is possible for a better engineered 4-cylinder to have more torque than a V6; so the conceptual question still remains.) With this in mind, is torque and HP calculated for each piston? So for a V8, the equation above would be used to calculate HP then multiplied by 8. Therefore, a 4-cylinder with the same torque (or more, within reason), would have less HP than a V8.

So in reality, a buyer would really want to know the torque comparison between two cars and not the HP?

Lastly, which car would be faster and why? (I am looking for off the line and for a distance race, lest say 5 mile stretch)
Car A --> V6 ---> 300lbs. torque --> 224HP
Car B --> V6 ---> 210lbs. torque --> 247 HP

Recon111

This will explain
http://www.jacobmcdonald.com/weblinks/hptq2.html

SpartaPerformance

I think you're looking at it totally wrong. You're assuming numbers and asking what would be faster and you're leaving out a bunch of variables. HP is a function of torque, your equation of (Trq * RPM)/5250 is correct but that applies to the whole power curve from 0 RPM to end of power band. You're trying to figure out what car is faster but what do the cars weigh? What is trans gearing? What is the power curve?

danzcool

I won't claim to be an expert in any way, but there are a few other things involved. for instance the BMW M3 (a few years ago, maybe still does or is better) had some 400 HP but somewhere in the low to mid 200s torque. The difference there between that and the 4.6 that was in the GT is the redline, the 05-10 hadc a redline at about 6500, where the BMW had a redline at something like 9000, so with a higherrevving engine it can develop more horsepower.

The other thing is that a max number doesn't mean much of anything when you look at the curve throughout the RPM range one car may have a peak horsepower of 400 at 6500RPMs, but it is a steep slope to get there, another may peak at 350 but be above 300 for 2/3 of the RPM band.
The other thing is the gearing, trucks tend to have crazy high torque numbers, but the transmission gears are so close that you have shift at lower speeds and you can't hit the high speeds, not as a limit of power, but because you have run out of gear.

JIM5.0

This is a tough one. But to explain the difference between power (HP) and torque and how there is a fallacy in going only by HP numbers, we need to hold all variables exactly equal to explain this.

Lets us assume every variable except the engine outputs are exactly same for two cars, both weighing the exact same, the drivers are weighing the same, both drivers are exactly of the same driving skill level, the transmission gears are exactly the same, the rear gears are exactly the same, STP (standard temp and pressure, sea level), both cars run the exact same track, and every other conceivable detail, including a bug that hits the windshield, in a drag race, the car with more area under the torque curve will win. Torque wins races, HP only gives you an idea of what torque the engine is making if you know the RPM.

The best thing to do is get a hold of a dyno graph. This tells you the important part: Where and how much torque the engine is making.
In our two identical cars above, lets say one car has a nice broad dome of a torque curve, say 300ft-lbf and it holds that torque throughout the entire RPM bandwidth, from 0 to redline. And lets say the other car makes a useless "spike" but it is a massive amount of a torque curve only at 5K RPM and throughout the rest of the bandwidth it only makes 100 for the rest of its entire bandwidth.

The car with the broad dome of a torque curve will have an advantage of the 500 ft-lbf "spike" engine. The reason: The 300ft-lbf engine has so much more area under the torque curve and off the line, it is not struggling as hard to get to the "power band" or the meant of its torque curve because it is already in this "power band" or meat of the torque curve.
The 500ft-lbf engine has to struggle with only 100ft-lbf until it reaches its magical 5000RPM mark to hit its max torque, and even worse, you have to have a very close ratio tranny to keep it at that 5000RPM because once you leave 5K RPM, you are screwed with only 100ft-lbf.

Lets do the calculations to see how this fallacy is played out, using the HP formula already mentioned above:
The 500ft-lbf "spike" engine is producing a max possible 476 Hp. Even if it can rev to 10,000 RPM, its HP 190HP because it is only making 100 ft-lbf!

Now the 300ft-lbf engine that is holding onto that 300ft-lbf at its redline, lets say at 6500RPM, will only be making 371HP max possible. But it has so much more of an advantage over the 500 ft-lbf "spike" engine because it is producing 200ft-lbf more torque everywhere on the curve, except at 5000RPM.

I know this is an extreme example, but to show something much more real-world, lets look at the 2010 BMW M3 4L v8 and compare it to the 5.0L Coyote.

The 5.0L Coyote is a torque monster compared to the 4L BMW and both make over 400 BHP at the crank (412 for the Coyote, I think 420 for the 4L BMW).
The 4L BMW never makes more torque than the Coyote,if I remember correctly, but the only reason why it makes a tad more BHP than the Coyote is because it revs higher.

And the reason why the BMW M3 keeps up with the more "torquier" Coyote is because its tranny and rear gear are geared lower such that you have to run at those higher RPMs to exploit the 4L's weaker torque.

Bottom line: You have to look at a dyno graph to know which engine is superior. And if you discover you have a weaker engine, gear down to exploit the engine by using torque multiplication (gear ratios).
And knowing where the meat of the torque resides on your engine's RPM bandwidth, choose gears such that you stay in the bandwidth of the meatiest part of the torque curve.

Mikado463

With all the many explanations provided I'll give you a good one that those of us that have drag raced for a long time use, as an easy analogy to understand..........

Torque off the line....HP through the traps !

LordRipberger

I think I am getting a better idea of how it all works. So I can see it in my head, your Coyote example would leave the Coyote winning in a short race because it has more torque at its disposal early on. Where as the BMW needs to get to higher RPMS before it can really keep up with the Coyote. Therefore, since there is more time associated with the BMW to reach the higher torque, the Coyote would initially pull away, but since the BMW can reach higher RPMs it would eventually pass the Coyote if the track were long enough, correct?

JIM5.0

That is absolutely correct. The Coyote is capable of a 6800 Redline (with a tune, some push it to 7500 RPM), but it starts to run out of steam above the 7K RPM mark, thus it does not retain its torque curve as good as the BMW 4L. Even though the 4L makes less torque, that torques stays on the car like white on rice.

I do not know how high the 4L BMW redline is, I think it is above 8K RPM.

LordRipberger

Jim5.0,
I am a huge fan of Corvettes, as I have wanted a 1981 since as long as I can remember. When I started paying attention to cars, the mustangs at the time didn't trip my trigger (90s mustangs). However, when it came time to buy a car, I bought a 2006 Mustang. But I still love my vettes.

I have a question pertaining to the New 2011s for both.

The new 2011 Z06 corvette produces 470lbs. of torque at 4800RPMs.
The new shelby gt500 produces 510lbs. But only 80% of it is generated between 1,750 and 6,250 rpm--from what I read on site. (does this mean that at 6250 rpms, the mustang is only putting out 408 lbs.?)

If the above is true, then the corvette would beat the stang off the line even though the corvette produces 505 HP and the GT500 produces 550 HP, correct? And possibly even the a longer race because of how quickly it gets up to speed. The gt500 might not be able to catch up, yes or no?

It was surprising for me to see the shelby get my HP than the Z06 at $25,000 cheaper. So I am trying to decide if Ford built a better engine. I know the corvette would win, simply because it is about 2ton lighter than the mustang, as the vette is made of plastic.

What I really want to find out is, if a brochure of car A says it has 600HP and car B has 550HP, does that mean that Car A will faster? I am looking for a typical answer, as gear ratio, aerodynamics, and the like, are never included on the brochure)

Another question goes toward fuel economy. Correct me if I am wrong, but gear ratios are designed to keep the RPMs higher so the car can maintain higher torque for longer periods of time, correct? So this would mean that these cars get terrible fuel economy because they are constantly driving around at high RPMs. (however, an auto always shifts around 3K but I really don't car about driving to the grocery store. I am just trying to get an idea of how it all fits together.)

LostBoyz

You aren't going to be able to look at a brochure and be able to say which car will be faster.

Rear gear ratio can alter the final drive ratio which will affect what RPM relates to the speed you are going. If you are looking for real fast acceleration and not much top speed you want a taller gear. You will get a couple mpg less on the freeway because you will be cruising at a higher rpm.