Let's ignore component failures and traction issues for the sake of this thread and assume an arbitrary, fixed vehicle weight.

I understand that torque is a measure of twisting force and horsepower is a measure of the ability to do work. That is, one horsepower can raise 550 pounds at a rate of one foot per second.

But how does each (as reported in dyno results) relate to how fast a car is and it's ability to accelerate? Why are both numbers reported? Could either value, by itself, be a reliable predictor of vehicle performance?

 

Torque relates directly to instantaneous acceleration. High horsepower is an indicator of high-rpm torque, which enables you to take advantage of gearing to increase your multiplied torque to the ground. In general, the car with more horsepower will be "faster" than the other over any reasonable time. You only need one (torque, or horsepower) curve from the dyno run since you can extract one from the other with RPM information.

 

A dyno measures torque PERIOD. The HP rating is extrapolated from accelerating the fixed mass to give a value of "work" performed over unit time. The reason HP rating is important is due to gearing and indicating where the peak torque occurs (why dyno runs at 1:1 are the most accurate). With regard to a car being fast, having the peak torque occur higher in the RPM range is better because you can take advantage of gear reduction.

Diesel engines have huge low end torque, but they aren't considered suitable for drag racing are they? Because they cant maintain the twisting force at a rate (RPM) high enough to support large HP values. They are great at maintaining low RPM inertia, but are poor accelerators.

 

looks like Mech beat me to this one.

 

As mentioned, Torque is the ability to accelerate at any given moment, whereas HP is the total amount of work that is being done at a given moment.

Torque will tell the exact acceleration rate at any given point, and HP will tell you the best range to drive in for maximum performance in a given gear. common misconception is that the car accelerates fastest at the peak HP point; not true, it will accelerate fastest at peak torque.

 

Someone tell me where I'm getting confused, but I've always thought of horsepower being *the big deal* so to speak. I understand the relationship between torque and horsepower, and the respective definitions. But horsepower is a function of revs and torque, the more revs the better.

For example, although the s2000 is quite a bit lighter than the g, the fact remains that it has little torque. Once one advances into the proper horsepower range is when the car comes alive.

If it were all about torque, F1 cars wouldn't rev to 19000 rpms.

 

Someone on this forum had in thier signature a quote from carroll Shelby. It said " Torque wins races and horsepower sells cars".

 

There's a saying:

I'll try to make this as simple as possible and not get real technical and put people to sleep Assuming both cars are the same and we're talking about 1/4 mile, power under the curve is key to the way a car will accelerate. A car that can maintain a high and broad torque curve over low-mid-high rpms is one that will accelerate the strongest. This is why a C6 Z06 is so damn potent with it's 505hp (450whp). It's powerband is extremely broad and there is power everywhere.

Back to the Supra example now. A 700whp Supra operates like a light switch, it's power is either on off. I call these cars dyno queens and to someone who doesn't understand power, they would automatically expect the 700whp Supra to beat the 500whp Supra simply because 700whp is more than 500whp. One must look at the dynos to see the truth. The single turbo car makes 150-200whp/wtq from 2000 to around 4500rpms, then all of a sudden the single turbo spools up hard and fast and the car instantly increases power by 250-300whp over the span of 500-700rpms. Above the 6000rpms the power keep coming on, as does torque. At 7000rpms or so, the fun is over. Then there's the 500whp twin turbo Supra. At ~2500rpms, the smaller turbo kicks in the and the power goes from 150-200whp to 280whp or so and the power steadily climbs. Then at around 4000-4500rpms, the larger turbo spools and the power increases again and continues to climb at a rapid pace till 7000rpms, but not with violence of the single turbo setup. When you sit down and crunch the numbers for power under curve (the racing powerband), you almost always see that both Supras make the same overall power. One delivers more power in a very quick and violent manor and the other delivers less power in a very linear fashion over a larger amount of operating rpm. This is why I'm never impressed with high hp turbo cars. Their powerbands suck and the cars are nearly undriveable on the street. I am a huge fan of broad power bands. The cars don't feel as fast because the power is so linear and thick, but they sure are easier to drive in all situations. They're not so dependant on a high rpm launch either which means they're more consistent off the line and far easier to launch.

IMO, the 5AT G35 is a great example of a car with great power under the curve. A lot of people are shocked when they find out that a 3,400lb car with only a V6 and only 215-220whp can pull 14.2-14.4 stock. Why can the 5AT G35 do this while most 3,300lb 99-04 Mustang GTs with 235-240whp and a lot more torque typically run 14.1-14.4? Look at the power delivery of both cars and you'll see why. The G35 delivers ~85% of it's torque from 2500-6000rpms and HP peaks at ~6,100rpms and holds steady until around ~6,400rpms. The 4.6 SOHC V8 in the Stang delivers about 50 more wtq from 2500-4000rpms, but then the torque starts to tail off quite a bit. HP comes comes on strong and easily surpasses the VQ35 from 2500-5000rpms. The power peaks 5,200rpms, holds steady for 200-300rpms and then it nose dives. At 5800rpms, the floor has fallen out and the fun is over.

 

It's all about weight and operating rpm with those cars. They run small lightweight engines to keep the weight down obviously. The only way to get power out of a small engine is to increase it's breathing ability and rev-ability. More rpm equals more HP. Since these cars sustain high mph during the race, they keep the car running within a very focused power band. In a drag race, a 500whp/500wtq 1,800lb F1 car would be faster than a 700whp/300wtq 1,800lb F1 car. Out on a high speed course, the 700whp car would do better. Of course, there's always an odd ball though. Audi is currently racing a twin-turbo diesel that make peak power at around 6000rpms in case that typically has cars that make 50% more HP at 4000-6000rpms higher. he difference is the turbo diesel makes nearly twice the torque and most importantly uses less fuel.

 

In the end, horsepower is a catch-all number that is easier to convey information on how much work an engine can do. It is the best range to drive the car through for maximum performance because its use includes things like gearing.

i.e. if u were to shift earlier than peak HP range, you will end up at a point in the next gear with less effective torque at the wheels with the weaker next gear, even if the engine is making more torque at an earlier rpm.

but technically just wanted to point out a car does accelerate fastest at peak torque, not peak HP (all else equal) because I often see the terms get misused.

 

fuel vs. diesel ?!???

I would say is similar to

HP vs torque


take a look at audi lemans race with a diesel engine.... diesels have a LOT of torque ... therefore bmw and mercedes diesels are very fast in these days....

 

I was going to reference audi's torque monster(r10) as a counterpoint to myself in my original post; but thought if would make my post too hard to follow.

Let's look at the hp/torque comparison another way. In a 1/4 race, if one had a car with a CVT at a constant rev range, would the car travel faster if the revs were at torque peak or power peak(discounting things such as launching)? I would assume power peak, simply because of "puts."

I can't remember the book that I was reading, but the author pointed out that there are basically four ways for to make a car go faster. One way is to change the displacement. Another is to change the volume of air(FI). Another is to reduce weight/increase traction. And another is to change the amount of work the engine is doing over a certain period of time: revs.

If one had two identical engines, and they both had identical torque curves, the engine with the higher redline *should* produce more horsepower(assuming the torque doesn't fall precipitously after torque peak). The car with the higher redline would accelerate faster. One reason because it could stay in the lower gear longer. But is that the only reason?

 

If both engines had identical torque curves, then they'd both make the same power since HP is derived from TQ (HP=TORQUE * RPM / 5252).

In a sense, yes. Again, it’s all dependant on the power curve. Now we’re also getting into gearing or torque multiplication. A high torque car with a super broad power curve isn’t remotely as dependant on gearing as a one that has a peaky and short power band. Consider this, you have a 600awhp Evo and a 600whp Nova. Both cars roughly weigh the same (3,300-3,400lbs). Both cars also make the same HP and they both come close to making the same torque; however, the power band in the Evo is short and swift. The Nova’s power is broad and linear and there’s simply a lot more useable power across the rev range. The Evo is equipped with 6 gears, 5 of which will be used in the ¼ mile race. The Nova on the other hand is equipped with a 3-speed auto or even possibly a 2-speed Powerglide. With all the Nova’s useable torque, the longer gears allow the car to ride it’s power band for longer periods whereas the Evo needs additional torque multiplication to make up for it’s lack of lower rpm power and to keep the motor at full boil within a very small power band.

 

Yep. This is why I'm never impressed by mods that only improve power from 6000-6600rpms on these cars. I'm far more concerned about mods that elevate the entire power band, just not the upper range. The truth is a G35 that makes additional 15whp more from 6200-6600rpms will no quicker because that power really can't be utilized in the 1/4 mile. The amount of time spent in that rev range in gears 1, 2, and 3 is totally insignificant to taking advantage of the power. Now if the entire curve was elevated 15whp, the car would be measureably quicker. When you see dynos showing an intake gaining 3-5whp from 6000-6600rpms, don't be impressed. The car will be no quicker.

 

I respectfully disagree, at least for my car. 1st and 2nd go by pretty quickly, but I spend a good amount of time in 3rd above 6k. And I usually finish 4th at about 6200(indicated), or so. If I could gain 10 to 15hp beyond 5.5k, then that would help me considerably.