HP vs. RPM & Torque vs. RPM

[Jim Wilson]

Could someone please explain why horsepower and torque always crossover at 5,200 RPM? I figured that since torque curves vary dramatically from engine to engine that this could not be the case. Is this only valid for NA four-stroke gas engines?

Thanks!

 

[Makara]

HP = (torque X RPM)/5252

 

[Big Medicine]

It's because HP is a bull$hit number that uses the same constant in the denominator for any automobile engine's power calculation.

If your TQ curve goes up early in the RPM range and stays up, you have a winner. No real need to multiply it by the RPM along the way & divide by a constant to understand it's ability to twist.

HP was established to quantify an engine's ability to do work over a given period of time. (not RPM range) It's really better suited for industrial engines that have a rated torque & speed based on design criteria instead of engines (like ours) who's design envelope includes the entire RPM range. Somewhere along the way, we got time & RPM confused & lost sight of what we were really trying to quantify.

HP= force*distance/time, or P.L.A.N./33,000

P is the peak firing pressure
L is the length of stroke
A is the area of the piston
N is the number of strokes per minute(divide by 2 for four stroke cycle)


$0.02

 

[Ulysses]

Torq = force = lb*ft

1 HP = 33000 lb*ft/minute

w = RPM

angular velocity = 2*PI*w = distance/time

Power = force*distance/time = Torque*(2*PI*w) = Torq * 2*PI * RPM = lb-ft * Revolutions/ 1minute

Conversion from Power to HP

Total amount of HP in power = Power/ 1HP =

(Torq * 2*PI * RPM)/33, 000

33,000/(2*PI) ~= 5252

Torq * RPM * (2*PI/33,000) = Torq * RPM/5252

 

[Torquemonster]

everyone is right

to answer your specific question - it applies to ALL internal combustion engines no matter what fuel they burn.

in simple terms - big torque is important for moving heavy things, big hp is important for moving anything QUICKLY.

5,000,000 lb/ft of torque will move a 554,000DWT supertanker to 16 knots - but it won't ever crack 12 seconds on the 1/4 mile until its HORSEPOWER per weight ratio reached the level required for that feat....

put another way - a Formula One engine which makes half the torque of a Viper would need a push start to get going in a Viper body - but once underway - it would destroy the V10 like it was a toy.... HP wins races.

That is the easiest way for people to understand it. In TECHNICAL terms it is more complicated than that as Big Medicine explained -

but in laymans terms - high low speed torque for pulling, high HP for speed events. Gearing creates the torque needed for the higher speed higher hp engines!

 

[Big Medicine]

They say it takes a horse one minute to move a 33,000 pound rock a distance of one foot.

I don't know what kind of horse they had.

What I was trying to convey is that two different engines with similar operating RPM ranges (zero-6,000 maybe?) & rpm limitations (7,000 maybe)that are used to propel vehicles of similar weight, HP tells you squat.

Why?

They match RPM for RPM & weight cancels out (unless you're talking a big %variance). The formula one car has a MUCH righer RPM limit, which is what helps "quantify" horsepower, whatever that really means. Get up a few more thousand RPMs & look at the equation expound upon that capability. Put two cars with the same RPM envelope, and it doesn't matter much. It all starts with a twist (TQ) and gets multiplied by RPM then divided by the same generic number.

Let's start some string theory.

 

[AJ]

hp is calculated from tq. as such (like somebody already mentioned) they always have the same value at 5252 rpms.

simply put, hp is the ability to maintain tq over higher rpms. if your engine breaths right, has a light "cranktrain" and valvetrain, it will maintain torque at high rpms thus achieving high hp.

hp in racing is said to be preferred because you can gear it accordingly (ie multiply torque) and maintain it over greater range of road speed.

or even simpler - if you have the peak hp and tq rating of an engine, tq shows how the vehicle accellerates below 5200 rpms, the hp- above that... remember, the race is won by the engine that can maintain higher torque at the wheels over a longer period of time.

alex
few cars

ps. i just saw the post above mine. hp tells you a lot. it tells you (as i wrote above) that at higher rpms, the more hp engine will win. if the race stays below 5200 rpms, the tq-er engine wins, above that- the higher hp wins.

 

[Ulysses]

Could someone please explain why horsepower and torque always crossover at 5,200 RPM?

To answer the question:
from my previous post with calculations and Makara's,

HP = Torq * RPM/5252

at 5252 RPMs, HP = Torq * 5252/5252, therefore HP = Torque.

HP and Torque do tell you a lot. BUT it only tells you about what the ENGINE is capable of doing.

You also have to factor in the weight of a vehicle, so power to weight ratios give a more clear picture.

True, other factors also play a major role, like RPM limitations. The curves of torque and HP are also important when trying to figure out if car A will win over car B. Peak HP/TQ numbers are not all foretelling, which is what I think Big Medicine is alluding to.

 

[Makara]

They say it takes a horse one minute to move a 33,000 pound rock a distance of one foot.

I don't know what kind of horse they had.

What I was trying to convey is that two different engines with similar operating RPM ranges (zero-6,000 maybe?) & rpm limitations (7,000 maybe)that are used to propel vehicles of similar weight, HP tells you squat.

Why?

They match RPM for RPM & weight cancels out (unless you're talking a big %variance). The formula one car has a MUCH righer RPM limit, which is what helps "quantify" horsepower, whatever that really means. Get up a few more thousand RPMs & look at the equation expound upon that capability. Put two cars with the same RPM envelope, and it doesn't matter much. It all starts with a twist (TQ) and gets multiplied by RPM then divided by the same generic number.

Let's start some string theory.

The number was estimated by watching mules move coal from a mine. Since horses are larger, Watt figured that they would move one and a half times as much so the mule's estimated 22,000 lb/ft per hour was turned into 33,000 lb/ft per hour.

I don't know why you dislike power figures so much. Power is power. Torque without rate tells you squat.

 

[Big Medicine]

Right, they were trying to quantify the amount of work over a given period of time. Great for horses & industrial engines that run constant torque, variable speed applications for a designed load. Makes it easy to predict fuel consumption.

Not so great comparing an ls1 to a v1o. How much do their RPMs differ? The denominator is the same, 5252. So what is the deciding factor?

Torque.

I don't dislike power figures, they just do more harm than good when you are talking about Vipers & Vettes & whatever else. (UNLESS you significantly INCREASE the operating envelope- read RPM- through modifications)Weights are similar. Nothing complicated. Set up a spreadsheet, fill in the measured torque values for any given RPM, and you'll get your HP value. It will be the same as your torque, though, so why bother with all the math?

 

[Ulysses]

Well, the HP curve let's you know at what RPM your engine is working at it's peak and when the amount of energy it produces starts to tail off.

Your Torque numbers may still be increasing at this point, but the rate of change of the torque with respect to RPM will be slower.

 

[Big Medicine]

If memory serves me correctly, the crossover point is near the edge of the operating envelope for most production vehicles, and both curves (HP & TQ) typically have a negative slope. I agree HP curve helps with analysis for efficiently getting work done; I disagree that it really provides anything useful on a V-10 or V-8 with their similar, limited RPM ranges. I'd say when the TQ curve goes negative, shift. On a production vehicle, you're near the end of your rope anyway.

I know I'm simplistic, but that's all that really applies to our vehicles of choice. All my stumping here is for naught when we start talking high reving motors with lots of gears & a target sweet spot.

 

[Makara]

Well the power will tell you how fast the a car will accellerate with an optimal gearing scenerio such as a CVT. Of course, unless you are a total idiot, you have to understand that with almost all transmissions sold today, we can not drive with optimal gearing at all times but then again, we don't drive around at a torque peak at all times either. The power curve is much more important to look at when evaluating engines but that is just far too much information for most people in most situations. As it stands, a max torque rating and rpm as well as a max power rating and rpm should give a pretty good ballpark estimate to what an engine can do.

 

[Makara]

I disagree that it really provides anything useful on a V-10 or V-8 with their similar, limited RPM ranges. I'd say when the TQ curve goes negative, shift.

When gearing is taken into consideration, by shifting early, you very well might be putting less torque to the rear wheels by reducing the benifits of gearing and the torque that a high rate of speed will provide at the rear wheels.

 

[Big Medicine]

Whatever...

"you very well might be putting less torque...."

"Mights" are what grows on a chicken's a$$.

Meanwhile back at the ranch,

If anyone doubts, all they need to do is make a chart in excel like I have above, go get some actual measurements throughout the rpm range of two comparable engines, and fill the data blanks in. (e.g., formula one engines with 12,000 RPM limts are not comparable to V-8's with a 7,000 RPM redline.)You should be able to discern that, for example, when comparing the new LS-7 to the existing V-10 platform, when you measure torque you can multiply it by 100 and divide by two then divide by 50 and you will have the same number as your horsepower.

It will also be the same value as your torque. You can multiply & divide them by whatever you wish, as long as they are the same numbers. What's the point?

None of this has anything to do with acceleration, which is entirely dependent on the geometry & weight of the vehicle that the engine is intended to propel.

And that's all I got to say about that. Over Roger; Roger Under.

 

[Ulysses]

Big Medicine,

You've made your point, and it is duly noted. Thanks for the great discussion. It's been one of the best that I've had on this board in a while.

 

[Big Medicine]

I'm having fun with it guys; please don't take me too seriously.

 

[Makara]

Whatever...

"you very well might be putting less torque...."

"Mights" are what grows on a chicken's a$$.

You seem to be forgetting the huge advantage that keeping the engine in a lower gear will bring you. Shifting into a gear that multiplies the torque less, while going to a part of the engine's range where it makes less power, just to pick up more torque which is at a lower rate and multiplied less is going to accellerate you slower than sticking into the gear. I said might because it is dependent on the difference in gearing, the power curve, and where you are shifting on that curve.

 

[Big Medicine]

Touche.

 

[2MANYTOYS]

You guys are giving me a headache with all these damn formula's

 

[Jim Wilson]

Thanks for all the replies, I think I finally understand the HP/Torque thing. But I'm with 2MANYTOYS on this one........time for a beer!

Torquemonster put it in terms that I could understand.

 

[2MANYTOYS]

Thanks for all the replies, I think I finally understand the HP/Torque thing. But I'm with 2MANYTOYS on this one........time for a beer!

Torquemonster put it in terms that I could understand.

Just finished my beer now I think I understand all these formulas