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View Full Version : Torque vs. Horsepower Often Confusing



JD
11-02-2014, 03:20 PM
Horsepower is essentially the rate of change of torque (over time), i.e. "acceleration".

Torque is basically the engine's capacity to hold speed, i.e. "hill climbing ability".

On Harleys, different cams will change where the torque and horsepower is maximized.

For people who like to ride loaded up and not have to downshift a lot, they prefer low end torque cams (short duration with low lobe overlap). They are able to hold speed without needing to downshift. Tradeoff is these cams tend to accelerate more slowly than a larger duration cam.

For people who like to twist the throttle and accelerate through the gears quickly, these people prefer a larger duration cam with more overlap. Tradeoff is these cams tend to lose a bit of low end torque, which means more downshifting to stay in the powerband.

Where people get confused is saying they are "low end torque junkies" and like to accelerate hard off the line. While torque and horsepower are related, what this is actually is usable horsepower, whereas torque is the ability really to hold speed under load.

A lot of people choose the wrong cam for their riding purposes, both too small and too large. If you like to rip through the gears and pass with some authority, look for a cam with more midrange horsepower at the expense of low end torque. A small cam might leave you feeling that your engine doesn't pull hard enough under heavy throttle. If you like to set the cruise and not touch the gears, and hard acceleration is not very important to you, a low end torque cam is what you want. Cruising RPM and gearing should be a consideration (5 vs 6-speed tranny, primary ratio, etc).

IamVince
11-02-2014, 04:43 PM
Thank you JD

Wild1Poet2
11-02-2014, 07:13 PM
Can anyone explain why, in general, diesels generate huge amounts of torque compared to comparable sized petrol engines? I know it as one of those facts of life but not sure why.

JD
11-02-2014, 07:27 PM
Diesels do tend to be very torquey, not sure exactly why. I suppose they are designed for the intended use, which historically has been moving heavy objects. For something like a work truck, they are often geared very high to take full advantage of using that torque. I recall light industrial trucks that you would typically start off in 2nd gear and only use 1st when fully loaded down to essentially get you up to 3-4 MPH before jumping into 2nd.

Robby
11-02-2014, 08:11 PM
As the truck application goes, diesels normally are longer stroke which produces more torque than high rev short stroke engines for a given CID, regardless of fuel. Torque is the ability to do work, horse power is how fast work can be done with that torque. At some point the work to be done (torque) is overcome by the load and the horses can't run any faster.

JD
11-02-2014, 08:31 PM
Let's oversimplify things and take gearing out of the equation. Think about a typical push lawn mower. Fire it up and it runs at one speed. Horsepower really isn't a consideration (and you will lately see engine capacity listed by manufacturers in torque and not horsepower). Get into thicker grass, weaker motors will start to labor, lose RPM, even stall. Higher torque motors will be able to go right through it without much if any negative effects. That is torque.

Wild1Poet2
11-03-2014, 07:52 PM
I've noticed that forced induction engines make a lot of torque too and they come in at low rpm's with a very flat torque curve all the way up to redline. The latest thing that we'll soon see on production cars is electric turbos.

JD
11-03-2014, 08:56 PM
Forced induction is a lot like higher compression, in the sense that it builds power throughout the rpm range. Turbos are way better than years ago when it comes to materials of construction and reducing "lag". Of course the downside is higher cost and typically lower engine life vs. the naturally aspirated equivalent.

Relic
11-07-2014, 07:59 AM
A turbo on a diesel does not necessarily reduce engine life. It depends on how much air pressure is pumped into the engine. Most tuning kits for turbo diesel engines really don't change mapping but increase the amount of air pressure pumped into the engine, thus reducing engine life. Most turbo diesel engines here in Germany can go 500,000 miles before overhaul.
I still can't figure out why the US has not embraced diesel technology. I have a 2.0 liter diesel in my SUV that produces more HP and torque than its gasoline powered counterpart, with fewer emissions (thanks in part to a particle filter and common rail design) and much better mileage. (I average 60 mpg and can drive at 150 mph).

wvfox
11-07-2014, 11:53 AM
As the truck application goes, diesels normally are longer stroke which produces more torque than high rev short stroke engines for a given CID, regardless of fuel. Torque is the ability to do work, horse power is how fast work can be done with that torque. At some point the work to be done (torque) is overcome by the load and the horses can't run any faster.
That's a great explanation. I always simplified it by saying horsepower gives you speed, torque gives you acceleration.