Article: Why Horsepower doesn't matter...

[Rogan]

For the last century, horsepower has been used to describe the power output of the internal combustion engine. The horsepower unit was created by James Watt in the 18th century. Its origin is based from how much power a horse could lift in foot pounds, 33,000 ft-lbs to be exact in one minute. The unit is derived from torque, which is the true measurement of the engine physical power production. What is strange about the units of horsepower is that it has no physical meaning. Its an arbitrary unit that has no real significance in describing the characteristic of the engine. For those that are curious to calculate horsepower:

      horsepower = (rpm/5252) * torque

From this equation you can see that horsepower is nothing more than a contrived unit that is based purely from torque and rpm. You’ll notice the number 5252 in the equation, this represents the point at which every dyno graph must intersect horsepower and torque. Its a mathematical relationship, both strange and interesting since horsepower is a function of torque and rpm. There has been much confusion and rumors across the internet about gaining more horsepower. In essence, gaining more horsepower is gaining torque. If you are after “peak” horsepower, you are interesting in carrying the torque curve as high in the rpm range without falling as possible. You can see from the equation that as the rpm’s increase, and the torque remains the same you get a higher horsepower number. What physically is happening is that the engine is able to produce enough torque to overcome frictional forces through the air, tires, etc. As you are able to keep the torque from falling off on the top end, you are able to maintain a steady torque curve that will “pull” the car through the mph you are trying to reach. So people who are after “peak” horsepower really want to extend their torque curves as far towards redline as possible, without letting the torque fall off. Check out some dyno graphs and see what I mean. Horsepower doesn't describe the true nature of how the engine performs, its the torque curve. From a tuners perspective, I don't tune off of the horsepower curves. The physical relevance towards the engine performance is arbitrary, since the torque is truly what is effected by the fuel, timing, breathing, etc of the engine. The horsepower is merely a concocted unit of measure, showing no true characteristics of the engine power output. A good tuner will only make changes from the torque curves, see what increase/decrease the curves show from the changes. So next time you are thinking horsepower, think “what would I want my torque curve to be”?

[CSM]

Good thoughts, but I think there is one thing to keep in mind.

A 8000 RPM engine that makes 700 ft-lb of torque will push something faster over a given distance than a 2500 RPM engine that makes 700 ft-lb of torque.

Torque is important, but the area under the curve of horsepower is what matters.

You can't move an object with torque. Torque does not imply movement. Torque is a static measurement. Power is a dynamic measurement.

"The horsepower is merely a concocted unit of measure, showing no true characteristics of the engine power output."

I disagree. When you are staying in the same RPM band that we are, increasing overall torque is going to make more power due to where torque typically peaks in the RPM band. Our engines are different than most "tuner" engines due to the small RPM band that we operate in.

I think you are right to tune that way, but limiting to only considering Torque is limiting in fully understanding the engine theory.

[Rogan]

You're right, it is something to keep in mind.. HOWEVER.. 8000rpm and 700lbft TQ moving faster than 2500rpm @ 700lbft Tq is a a variable. 8000rpm through 5.38:1 R/P on a 28" tire, and 2500rpm with a 2.79:1 R/P on a 28" tire are two different animals.I'm not against a horsepower #, but without Torque, HP cannot be calculated, period. That does not hold true for HP calculation's reliance on torque.Torque = Force x Distance

[CSM]

You're right, it is something to keep in mind.. HOWEVER.. 8000rpm and 700lbft TQ moving faster than 2500rpm @ 700lbft Tq is a a variable. 8000rpm through 5.38:1 R/P on a 28" tire, and 2500rpm with a 2.79:1 R/P on a 28" tire are two different animals. I'm not against a horsepower #, but without Torque, HP cannot be calculated, period. That does not hold true for HP calculation's reliance on torque. Torque = Force x Distance

Torque is FxR... but it is just a force. I can get a big enough cheater bar and make 700 ft lb of torque with my muscles. But, I can't make 700 ft-lb at any RPM. Horsepower is a rate, that is mathematically derived to describe the application of torque at a specific RPM. To fully understand how an engine makes energy, you have to look at both the torque and the RPM output, and when realized that... they decided to make the unit of Horsepower as the goofy unit of energy for us. kW = power = HP with conversions. My point is, that Torque alone will move no truck. RPM is required along with torque. Horsepower is a common unit used to describe the power that the engine is producing. (side note on gearing) Gear ratios don't change horsepower. They multiply torque up or down. Power is constant through gears. We feel the effect of gears reducing or raising the power output of the engine due to the change in engine power output at a different RPM in that new gear. We also feel differences due to it taking less power (energy) to go from 5-10mph in 1 second than it does to go from 55-60mph in 1 second. Not trying to be disagreeable here or anything... I hope you don't take this in that way.

[Rogan]

I don't take it that way.However, this article was more for those "who don't understand" the relation; apparently, you aren't in that group :)You and I would eventually funnel down to the same conclusion/result, we're just looking at the same thing from two different angles.As for the gear ratios, I know they don't change Tq or HP. My point was towards your reference of RPM = speed. On a 1:1 ratio, your example would be spot on, by a long shot. But with different ratios, that changes dramatically.

[CSM]

I don't take it that way. However, this article was more for those "who don't understand" the relation; apparently, you aren't in that group You and I would eventually funnel down to the same conclusion/result, we're just looking at the same thing from two different angles. As for the gear ratios, I know they don't change Tq or HP. My point was towards your reference of RPM = speed. On a 1:1 ratio, your example would be spot on, by a long shot. But with different ratios, that changes dramatically.

Eh... Somewhat. This is a tough discussion, and most engineers don't fully understand the physics behind it. This is a moth to a flame topic of mine and has been for years. The whole torque vs power discussion sucks me in. It bugs me when people call horsepower contrived as if it were an inferior unit! (yeah, I am a nerd.) Horsepower totally matters. A lot of what we do with properly setting up a harmony of turbos, gears, cams, fueling and timing are not "common sense" issues and are a lot more abstract. I do like where you are going though. Hopefully the discussion is helpful. Having a rigorous, yet as straightforward as possible explanation of torque/hp would be good considering the performance side of the site.

[Mopar1973Man]

Kind of a simple saying I heard years ago."Horsepower is how fast you can get to the wall""Torque is how far you can push/pull the wall"I've always considered HP/TQ a hand in hand measure measure. But with our truck (working trucks) the torque is more important than horsepower is really. The torque is what get the loads moving...(Not a science nerd in this topics)

[Rogan]

A 'play' on what Mike said, is what we used to say in drag racing circles (yes, I used to drag race turbo cars).."Torque gets you E.T. Horsepower gets you 'mile-an-hour'."

[CSM]

Here is an old forum and thread from my past... mostly empty of real good minds now but it was once frequented by the real big names in Winston Cup and NHRA engine building. Some of the archives there can be downright amazing technically. It was quite a discussion. I don't want to get into all this but its a good read that has hashed out most of the points we might make. This thread is more in regards to racing of spark ignition engines. http://speedtalk.com/forum/viewtopic.php?f=1&t=7809

[Rogan]

Eh... Somewhat. This is a tough discussion, and most engineers don't fully understand the physics behind it.

This is a moth to a flame topic of mine and has been for years. The whole torque vs power discussion sucks me in. It bugs me when people call horsepower contrived as if it were an inferior unit! (yeah, I am a nerd.) Horsepower totally matters.

A lot of what we do with properly setting up a harmony of turbos, gears, cams, fueling and timing are not "common sense" issues and are a lot more abstract.

I do like where you are going though. Hopefully the discussion is helpful. Having a rigorous, yet as straightforward as possible explanation of torque/hp would be good considering the performance side of the site.

I like the debate-side of things.

I want you to, first and foremost, understand that I do not think horsepower doesn't matter. It all has relevance.

However, in my past tuning history (having built many high HP/TQ turbo cars) during the "tuning" process of the ECM on the dyno and on the street, I focused on 3 major things (in order) Air/Fuel ratio, knock, and torque curve. The Horsepower always followed, but chasing HP numbers was a waste of time. Sure, I could tune my 575whp Subaru WRX to make 600+hp, but the torque curve looked like a roller coaster's initial hill climb; worthless. I'll see if I can find an old dyno plot for this car at 575 peak hp; the torque plot was insanely flat for about 2/3 of the pull. The car stayed in the 400s of lbft Tq from around 3500ish, all the way to about 5800rpm. As well, the AFRs sat near rock-solid of 11.5:1 (used 50/50 water/meth injection to compensate for the "leaner" AFRs.) It stayed above 300lbft to about 6500, then petered out above that to the 7650rpm redline I set for the engine. **

Why did it peter out? My GT35R turbo had too small A/R turbine housing (.63 AR) and ran out of cojones.

**I built it to spin safely to 9000, and did hit 8500 a couple times. If you've never been in a car spinning 8500-9000rpm, it's a bit un-nerving

[CSM]

Yeah, I agree with you and you are right in how you went about the Subaru... (sounds like a beast by the way!) And the unnerving part sounds scary because isn't that engine transverse? So when it blows rods parts go fore and aft? You are right that peaky horsepower numbers mean nothing other than bragging rights. Torque, and broad torque curves across the RPM band are what makes power. You get it. I guess I read a bit into the original post about horsepower being irrelevant. If I get some time this week on my night shifts though, expect some questions on the turbo articles you posted. I am curious! This is a fun discussion too, I enjoy this stuff though I don't have the time to do it much. I still owe ISX some stuff on compression ratio but I haven't had time to dig up anything yet. Thanks for the discussion! We agree. I am turning in.

[AH64ID]

Good discussion, but simply put HP does matter. As it has been said you have to factor in rpm, and even if torque stays the same as rpms increase so does hp. My stock motor is rated at 610 ft/lbs at 1600 rpms and at least 555 from 1400-2900. The torque curve on the OEM chart is flat, and shows 610 ft/lbs at 2200 rpms, but there will be 70 more hp than at 1600 rpms. Considering the same truck, weight, gearing, and trans gear one could easily pull hills at 2200 rpms that they could not pull at 1600 rpms. Torque is the same, but HP is VERY important to the ability to pull a hill. Torque gets the load moving, and hp keeps it moving. Likewise if you gear a 300hp honda 4cyl and a 300hp Cummins so both are turning the rated rpm they are both putting the same amount of power to the ground in terms of torque, and both could sustain the same load at that speed.