I know the basic differences but I would like to hear your opinions and explanations of horsepower vs. torque. Also the difference between speed and quickness, one is measured in mph and the other in elapsed time, right? As far as Horsepower and torque, is it true that in general 2-strokes produce more HP, but 4-strokes produce more torque? Is this because 4-strokes produce more power per stroke and 2-strokes have more power strokes? What produces torque? (compression and more cc's)and why are 4-strokes better at producing torque? Does HP = wheelspin and torque = traction? I know 2-strokes are better for the dunes and paddles because they produce more wheelspin. It seems like when climbing dunes, two strokes use speed and 4-strokes use power?? What does horsepower translate to when riding, speed?? What does torque translate to, acceleration, low-end power, traction?? Lastly, in general which accelerates faster off the line, is quicker, and has a higher top speed?? Lets say a 400cc 4-stroke vs. a 250cc 2-stroke racing without paddles and then with them??

Hope I gave you enough to write about! I am just really curious about this subject and no one so far has given me any solid answers on it.
Thanks to all who post
Norm

 

You ask some good questions!

Your inquiry goes to the heart of perhaps the most misunderstood and misued concepts in ATVdom. Doubtless you'll receive many conflicting responses.

I won't try to answer them all; even if I knew the answers, they'd require a looooong post to contain them. Reading in a couple of websites (I hope you receive an appropriate address or two in this topic) and some books will give you background useful for understanding the topics.

Horsepower is a unit of WORK, one horsepower is 550 ft.-lbs. per second; i.e., if your machine lifts a 550-lb. weight at the rate of one foot per second, it's putting out one horsepower. TORQUE is radial force, expressed in force X distance, as in lbs.-ft. If you apply one pound force to a wrench gripping a pipe at a distance of one foot from the axis of the pipe, you apply one lb.-ft. of torque.

Notice horsepower has time as an element (per second); torque does not.

The maximum torque an engine is capable of producing occurs at a certain rpm; a convenient formula can tell you what horsepower the engine produces at that point; however, the peak horsepower generally occurs at an rpm point higher than that of the torque peak.

The shape of the torque vs rpm and horsepower vs rpm curves are important in determining the engine operating characteristics. For example, an engine developing high torque at low rpm may be a better "puller" than an engine with even higher torque at a significantly higher rpm. Diesel engines, developing peak torque early with a relatively flat torque curve profile, illustrate this point.

Regarding two-stroke and four-stroke engines, two-strokes generally produce higher horsepower and peak torque per displacement, because they produce twice as many power strokes as their four-stroke counterparts at the same rpm. The shapes of the horsepower and torque curves of the two engines vary greatly; the two-strokes' being generally more "peaky."

Again, your questions are good, I hope my limited responses helped instead of confused the issue further.

Tree Farmer

 

While Tree Farmer does a nice job of explaining what torque and horsepower are in measured terms, he didn't answer the question, did he? C'mon Jim, you can do better than that.

OK - I asked that question to several engineers at work a few years ago. Many of them were gearheads and the best and most understandable way to explain what hp and torque are is this:

In a motor vehicle, hp can be seen as the top speed (assuming the engine is geared so as to not over or under rev). Torque is the accelleration.

In the real world, you can see why commercial trucks require lots of torque, they need to accellerate a load, while drag racing engines typically are tuned for high horsepower. But drag racing requires torque to accellerate you say. Correct. Except, the torque applied to the rear wheels is dependent on the overall gearing. Running steep (higher numerical) gearing will apply more rear wheel torque and therefore more accelleration to any vehicle.

Many factors come into play when applying power to the ground. Flywheel weight can either allow an engine to accellerate very quickly (more wheelspin with lower flywheel weight) or it can "tame" the power and "apply more power to the ground". A lot of people hear that term with a four stroke. They "apply the power to the ground" better. But why? The answer is because the only fire (or produce a power stroke) every other engine revolution. During the other revolution, the engine is "coasting". The only thing keeping the engine spinning is the inertia from the flywheel and other rotating components. With a two stroke, the power stroke happens every engine revolution. Ever wonder why a four stroke has a deeper sound that a two stroke? It's because it's only firing half the time and that produces a lower frequency and a lower (deeper) sound.

Now as far as four stroke vs two stroke torque is concerned, with equal engine displacement, the two stroke actually creates MORE peak torque, but it is over a more narrow rpm range. Four strokes typically develop torque very evenly across the rpm range, so they feel stronger at lower revs but taper off on the top end. Ever notice that a four strokes don't have a sudden rush of power? The power output is more even and "tame".

So which is better? That depends on the application. Slippery pea gravel on top of hardpack ground would be better for the tame power output of a four stroke. Less wheel spin is better. Sand is different. The more wheel spin, the better (at least with paddle tires). It's sort of like you're throwing sand behind you in order to move forward. More thrown sand, more accelleration.

Depending on the state of tune of the specific engine, either one can be made to be mild or wild. Given the typical engines found on atv's today, most all of the above applies, but there are always going to be exceptions.

 

Interesting post, SandmanBlue!

A couple of fine points. I don't dispute the statement in any way; however, I'd appreciate a clarifiction of your statement, "Horsepower is top speed, and torque is acceleration."

Top speed of most vehicles occurs beyond the horsepower peak rpm; also, the optimum dragracing shift points are beyond the horsepower peak rpm. Further, I think in dragracing, where acceleration is a premium, the torque peak is crossed but once after launch; the remaining engine operation to the finish line occurs on the downhill side of the torque curve, beyond its peak (if the racer has enough closely-spaced gears).

So, I ask for help in visualizing "Horsepower is top speed, torque is acceleration."

You make many valid points, among them; regardless of the powerplant output, transferring that power to the ground, accommodating all kinds of variables, remains critical.

Finally, you're right--I didn't answer the questions!

Tree Farmer

 

Treefarmer is correct technically speaking on the differences. In less technical terms it can be explained as torque is power put to the ground, whereas horsepower is wheelspin. Basically 4-strokes us most of the power they make, whereas a 2-stroke looses a-lot to excess wheelspin. Which one is better depends on the conditions you ride in and your riding style. I used to ride 4-srokes, now I'm on 2-strokes and I actually preferr the 2-stroke for trails and racing because it seems to suit my riding style. Now I look back and realize that I actually rode the 4-stroke like a 2-stroke should be ridden, so I was actually hard on 4-strokes.

Hope this helps,

Sickman

 

I too have a hard time visualizing the difference. In order to simplify it so that it makes more sense, we must look at the application. In a vehicle with a gearbox, the applied torque varies with gear selection. Lower gearing (higher numerically) produces more torque and higher acceleration. Everyone has experienced the pull we receive from the handlebars when we downshift and increase throttle. That's a change in torque. However, hp is gear independent. It is not possible to change gearing and affect hp. When a vehicle reaches its top speed, it does so at a high rpm (where hp is highest). If torque was the determining factor in top speed, then the engine would be spinning at a lower rpm, where torque is greater.

A classic example of torque and hp is in the Harley-Davidson engine design. A 1200cc Harley only produces about 45 hp stock, but it produces close to 80 lbs.-ft of torque. When accelerating, the engine causes very rapid acceleration, but it runs out of hp, so top speed is limited to only around 110 mph. In contrast is a 750cc Japanese 4 cylinder engine. Peak hp is better than 110 and peak torque is only about 75 lbs.-ft. Top speed on this machine is better than 160 mph.

It's not a perfect definition, but it's as close as I can get.

 

The best example of torque I can think of would be the diesel engine, The work horse of our nation. The 99 Ford power-stroke diesel produces over 400 ft lb's of torque but only 175 hp.And thats how that truck pulling a heavy trailer can pass you going up hill on the highway.As your gas motor is falling off on hp and slowing down and heating up.

E=MC squared............Hmmmmmmmmmmm

Torque=energy in motion with stored energy exceling that motion.

Horsepower=energy in motion living in the moment with no stored energy, To excel past the moment.

Whhooo........Ok,,,,,Now.
First,To fairly compare a two stroke to a four stroke, The four stroke must be twice the size of the two stroke.Reason?The two stroke fires twice as many times as a four stroke.So to even the playing field use that formula.Example-250r vs 500ex would be equal.

Now to your last question. a400cc 4-stroke is allready under matched to a 250cc two stroke due to underdisplacement.

A 400EX against a 250R is unfare to begin with, BUT being that the 250R design is over ten years old. The new 400ex has made up some ground due to new tecnology in four strokes and suspention.
Let the shoot off begin. Off the line the R will take the hollshot by about a length,Because of the horsepower.(ie-instant Rpm power).With the EX close in tow , The EX now starts developing torque. (Energy in motion with stored energy exceling that motion)As it's RPMs increase it begins to pull putting a lot of energy to ground efficently. With no wheel spin, And starts to gain on the R.This experience is duplicated through every gear.
In the end stock to stock the R will take the ex by about a lenth.But that says a lot for the 400,being under matched by 100ccs.

Their are to many varibles to measure in general applcation.On any given day a 400EX could take a 250R or not.But as time marches forward and advances are made,The only thing you can be sure of is change.

Whooooo.................



[This message has been edited by trx430ex (edited 12-09-1999).]

 

Mabey a fair comparison would be '99 YZ250 vs. '99 YZ400?

 

Torque is like power and horsepower is torque at speed. That is a very simple definition. Listen to tree farmer on topics like this. It may take him a while to explain it, but he is normally right.

------------------
Andy Bassham *(1999 Arctic Cat 500 4x4, 1989 Honda 300)*

 

Sandmanblue,

I like your gearing analogy, however there is one point I have a question about. If I'm not mistaken, the HP produced by most engines increases as the RPM's go up - until a certain point is reached where HP peaks and begins to fall off. Therefore would it not be true that by downshifting, and thus increasing the RPM's, the HP would indeed be increased?

I was an English major in college, so I won't be hurt if you tell me I'm an idiot on this.

Regards,

Dennis