Torque vs Horsepower

Status
Not open for further replies.
Joined
Jan 21, 2011
Messages
2,679
Location
California
I understand what each means but I don't understand the mathematical equation that all the engineers use:

HP = (TQ x RPM)/5252

How could this equation be true for every engine? There are SO MANY different engine designs: different bore x stroke, different compression ratios, 2 large valves vs 4 small valves per cylinder, different camshaft profiles... all these factors will affect the output of torque vs HP at different RPMs and different throttle settings. Specifically I am thinking of the long-stroke Harley-Davidson motorcycle engines that produce loads of torque off-idle vs an equivalent displacement Honda 4-cylinder motorcycle engine that produces comparatively little torque off idle but way more peak HP than the Harley. In this scenario the Harley has high TQ at low RPM, which would generally equal a high HP number. And the Honda has a low TQ at a low RPM, which would generally equal a low HP number but the Honda ultimately produces more peak HP. So again it does not make sense to me.
What am I missing here? Someone please explain, preferably in the simplest terms possible.
Thank you!
 
As you pointed out, the horsepower is changing at every rpm. When we say an engine has N horsepower, that is where HP is at its maximum. In other words, calculate horsepower at every rpm and see where it's maximum. That number gets assigned to the engine as its horsepower.

https://www.youtube.com/watch?v=fgLNO3ThGD4
 
Last edited:
So do I want a car with a lot of torque or a lot of horsepower? Never could figure that out.
 
Originally Posted by walterjay
So do I want a car with a lot of torque or a lot of horsepower? Never could figure that out.



A dump truck has a lot of torque.

You need to look at the torque curve. Torque that diminishes after 2500 rpm or that doesn't hit its maximum until 6000 rpm aren't the best for a street car.

A dump truck can get away with a drop off in torque at 2500 rpm because it's moving heavy loads and not accelerating very quickly.

A race car can get away with max torque at 6000 rpm because it can stay at that rpm during a race more so than a street car.
 
Last edited:
Torque Rules but Horsepower is King! Simplest explanation is that torque pulls the load and Horsepower describes how fast the rpms increase at that load. In a drag race increasing rpm's more quickly than the other guy will get you down the track more quickly.
 
So my Hyundai engine produces 195 lbs of torque from 1,500 rpm through 4,500 rpm. The horsepower is 185 at high rpm. So what is giving me my power under normal driving conditions..torque or hp?
 
Originally Posted by walterjay
So my Hyundai engine produces 195 lbs of torque from 1,500 rpm through 4,500 rpm. The horsepower is 185 at high rpm. So what is giving me my power under normal driving conditions..torque or hp?


Torque.
 
Originally Posted by walterjay
So my Hyundai engine produces 195 lbs of torque from 1,500 rpm through 4,500 rpm. The horsepower is 185 at high rpm. So what is giving me my power under normal driving conditions..torque or hp?

Under normal driving conditions, your OOMPH comes from torque before the downshift, and HP after. As oversimplified as I can put it.
 
A typical 18 wheeler engine has 400 hp and 1500 ft lb torque. A Mustang GT 5.0 has 460 hp and 420 ft lb torque. If you could swap engines, what would be more fun to drive?
 
Think of it this way - torque is pure muscle, but horsepower is the ability to do work. The reason it is RPM dependent is because an engine that makes x ft/lbs at 4,000 rpms can do twice the amount of work as one that makes the same amount of torque at only 2,000 rpms. The higher-revving one can be geared down to make twice the torque as the lower-revving one at the same speed.

To use a real-world example, a Honda S2000 from the turn of the century made 240 hp from a tiny 2.0l engine at a ridiculous 8000+ rpm but only 153 ft/lbs of torque. An F150 of the same vintage with the 4.6 V8 made 220 hp at 4500 rpm and 290 ft/lbs at 3250 rpm. While the S2000 engine made more power, it would be stupid to try and tow anything with it because it would have to be geared super low and scream along at some crazy-high rpm burning lots of gas and probably wearing itself out in short order. Meanwhile the lower hp V8 is loafing along with your trailer at less than half the engine rpm like nothing is even back there.

Put another way, torque is that instant seat-of-the-pants feeling a big-engined muscle car gives you when you stab the throttle. Horsepower is that buzzy small-engined import in your rearview mirror with the huge wing that's somehow managing to keep up.
 
Last edited:
I love modern day 4cyl DI turbos because you get max torque(221 ft-lbs my Tiguan ) at 1500rpm -4500 typically so everyday driving is a pleasure.
 
For acceleration, horsepower is king. For our drag racing engines, the cam, torque converter, trans gears, rear gear, tire size, and rotating weight/MOI are all catered around the horsepower curve. On the high rpm big blocks, they make peak horsepower at 9000-9200 rpm. They'll flash the converter at ~8000 rpm, sweep the horsepower curve, and shift at ~9500 rpm, falling to ~8500 rpm, and sweeping again through each gear, crossing the finish line at ~9500 rpm.

We couldn't care less about the torque curve. We don't even look at it. For that matter, we don't even look at the peak horsepower number. The average horsepower across the usable rpm range is what matters.
 
Originally Posted by DGXR
I understand what each means but I don't understand the mathematical equation that all the engineers use:

HP = (TQ x RPM)/5252

How could this equation be true for every engine? There are SO MANY different engine designs: different bore x stroke, different compression ratios, 2 large valves vs 4 small valves per cylinder, different camshaft profiles... all these factors will affect the output of torque vs HP at different RPMs and different throttle settings. Specifically I am thinking of the long-stroke Harley-Davidson motorcycle engines that produce loads of torque off-idle vs an equivalent displacement Honda 4-cylinder motorcycle engine that produces comparatively little torque off idle but way more peak HP than the Harley. In this scenario the Harley has high TQ at low RPM, which would generally equal a high HP number. And the Honda has a low TQ at a low RPM, which would generally equal a low HP number but the Honda ultimately produces more peak HP. So again it does not make sense to me.
What am I missing here? Someone please explain, preferably in the simplest terms possible.
Thank you!


In the simplest terms without using much math...

A long time ago, folks settled on the idea that a horse could pull a rope connected to a pulley and lift a 550 lb weight at constant rate of 1 foot per second. Since there's 60 seconds per minute, that converts to an equivalent of 33,000 pounds per minute because 550 x 60 = 33,000. This calculation is for things moving in a straight line. Engines have shafts that rotate so, conversions must be done to convert linear speed to rotational speed. The circumference of a round shaft is 2 x Pi x radius (Pi = 3.1416).

When all is said and done, the 5252 comes from 2 x Pi / 33,000 (because 2 x 3.1416 / 33,000 is roughly equal to 5252). Think of it as a conversion factor from linear motion per second to rotational motion per minute.

Thus: Horsepower ends-up equaling (Torque x RPM) / 5252.


Taking it a little further, Torque = Force x Distance. ( Example: Think of a 1 foot long wrench with 5 lbs of weight at the end. The torque will be 5 ft lbs).

Horsepower is the rate at which you can apply torque and is: (Force x Distance) / Time.

In other words, Horsepower = Torque / Time.

To convert from linear to rotational speeds the conversions must be applied and you get:

Horsepower = Torque x RPM / 5252

This is just a logical explanation that does not show the math to convert linear motion per second to rotational motion per minute.
 
Originally Posted by RDY4WAR
For acceleration, horsepower is king. For our drag racing engines, the cam, torque converter, trans gears, rear gear, tire size, and rotating weight/MOI are all catered around the horsepower curve. On the high rpm big blocks, they make peak horsepower at 9000-9200 rpm. They'll flash the converter at ~8000 rpm, sweep the horsepower curve, and shift at ~9500 rpm, falling to ~8500 rpm, and sweeping again through each gear, crossing the finish line at ~9500 rpm.

We couldn't care less about the torque curve. We don't even look at it. For that matter, we don't even look at the peak horsepower number. The average horsepower across the usable rpm range is what matters.



So true !!
I read an article about Ernie Elliott and his motors back in the 80's when Bill was killing the big tracks
He said TQ gets ya rolling
HP wins on the straightaways ðŸ‘ðŸ‘!!

Always enjoy your post
Thanks !!ðŸ‘
 
Last edited:
Technically, energy is the ability to do work and power (i.e. horsepower) is the rate of doing work. Torque and energy and work can all be expressed in the same basic SI units. As RayCJ points out, for power, it is per unit time.
 
If you removed the engine from your manual trans car, and applied a hundred foot long breaker bar, you could apply tremendous torque...it, as others have stated, will move almost anything.

Power is the speed aspect...how quickly can you spin the breaker bar to achieve your desired speed.

Power and gearing can provide effective torque, so an 18 wheeler with a 460hp gasser could be geared to do what's needed, the engine wouldn't last very long delivering that power for hours at a time.
 
Status
Not open for further replies.
Back
Top