I know that they do...I guess you and I disagree over how it should be explained.

I fail to see the difference in meaning between "how much work can be done in X amount of time" and "work per unit of time"...to me that sounds like different words saying the same thing...no?

You don't think that your just simply confusing people by saying that?? That is not what horsepower is in this context. The context we're speaking in, has a lot to do with how these terms are defined.

True, but this is just a more complicated way of saying what I've said a bunch of times before in this thread. I've said it in a more simplified manner to make it easier to understand. What you've said, isn't confusing to me, but it may be confusing to someone who doesn't have the basic concept down.

I am purposefully simplifying things...it doesn't mean what I'm saying isn't true...it's just that....if you take anything I've said out of context, it may not be true. The textbook definition of horsepower doesn't necessarily get the message across in this context. I don't believe anything I've said is very confusing after it's comprehended and I feel it's easily comprehended compaired to digging out the textbook.

Big trucks don't rev very high because their either diesels, or low RPM torque gas motors, the reason they don't have a lot of horsepower is because they don't have very high RPMs (lower RPMs means less horsepower because the work cannot be done as quickly). The gas engine in my 22 foot RV only revs to 4000 rpm, it doesn't like to be above 3500 rpm.

Not necessarily true, diesels physically cannot rev at high RPM because of the unique way diesels make combustion. At high RPM, they are just simply not able to work. Gail Banks is actually coming out with a new diesel design and his goal is to make a diesel run to 7000 RPM, that diesel will make a lot of torque and a lot of horsepower. You could put high performance light weight parts in a diesel, and it still won't run at high RPM because of the way they make combustion (no spark plugs, combustion happens from compression). A gas engine on the other hand, when built for torque/low RPM, focus on velocity of the air moving into the engine and therefore use smaller head ports, and a special cam. All gas engines that are setup to be torque monsters, can't flow a lot of air through the engine because of the size of the ports, and the way the cam is designed. That is why they can't rev high, because they can't flow enough air through the motor to do so. It is not because of the durability of the components inside (although if the parts inside stink, then of coarse it can't rev high...I hope you know what I mean).

Now we're getting complicated

 

"All gas engines that are setup to be torque monsters, can't flow a lot of air through the engine because of the size of the ports, and the way the cam is designed. That is why they can't rev high, because they can't flow enough air through the motor to do so. It is not because of the durability of the components inside (although if the parts inside stink, then of coarse it can't rev high...I hope you know what I mean)."

kinda like my good old 4-9 liter!!! exactly. so justin, you are saying if I put the small block chevy valves in my 4.9, i forget their sizes, i will loose my great low end torque?

 

Not to speak for anyone else, but…

Yea, that is pretty much true. Unless you are supercharging, high torque gas motors typically do not produce high horsepower or high RPM’s. If you build a motor to produce high horsepower the trade off is usually in low-end torque. Porting, larger valves, bigger cams, higher cam timing, longer intake runners, free flowing exhaust all produce more HP at the loss of some torque.

 

Not necessarily a loss of torque, but remember how I said engines make different torque outputs at different RPMs...what it does is move that peak torque point higher in the RPM band. So rather then the I-6's 2000 RPM torque peak....it would move up to maybe 3k instead. That means you'd have less torque down below 1500 rpm. I don't know much about modiftying the 300 specifically, so I don't know how changing the valves would actually affect your power output. I'm a small block guy but I did own a 300/6, and my dad had one. Nice engines. Torquey and pulls hard because they have a lot of torque but they're slow to accelerate because they only have about 160hp (low rpm redline is seen again). See how it's tieing together?

A torquey gas engine typically uses small ports on the heads. They do this for a good reason. When the piston comes down and makes a vacuum sucking that air in, the smaller ports make the velocity of the air coming in much higher than it would if the ports were larger. That extra velocity somehow helps your torque...I don't know exactly why the velocity helps but I know that's why they do it. Since the ports are smaller, they can't flow as much air at high RPM like the larger port heads do. You can only suck so much air through those ports.

Now on the larger port heads you'll see on engines that rev high...but remember they're also going to reach a point where their ports can only flow so much...thats when those larger ports see high air velocity going through...more torque...high in the RPMs.

 

GT, your first post was pretty much correct, but they have gone downhill since then. The only thing HP determines is top speed, torque determines how fast you will get to that speed. In other words, torque determines the rate of acceleration.

You have mentioned your snowmobile engine a few times in comparison to a Saturn Ion engine. They have the same hp, but different torque. You contend that the snowmobile engine can't accelerate the car as quickly as the car engine can, but this is incorrect. Both engines are capable of accelerating the car at the same rate, but the snowmobile engine needs more gear reduction to do it. The gear reduction is a crutch for an engine that doesn't make enough torque for the application.

The school bus example is severely flawed. The relatively small change in acceleration when loaded with kids has nothing to do with the engine. It has to do with a smaller change in torque to weight ratio. If you add 50 kids at 100 lbs a piece to a 10k lb school bus you have changed its weight by 50%. If you add 5k lbs to a 5k lb truck you have changed its weight by 100%. The acceleration of each vehicle would change a corresponding amount.

You can't build an engine with zero power and you can't build an engine with zero torque. I never said that you could and don't know where that came from. If you build two engines, one with 500 hp and 100 ft-lbs of torque and the other with 100 hp and 500 ft-lbs of torque, and gear them the same, the engine with more torque will accelerate faster than the engine with more HP. However, the engine with more torque will stop accelerating when it reaches a speed that requires 100 hp to maintain. That speed isn't very high. The engine with less torque will accelerate at a much slower rate, but will continue to accelerate past the speed reached by the high torque engine. If the high power engine was regeared to make better use of its high rpm power at lower vehicle speeds it could out accelerate the high torque engine because it has proportionately sacrificed rpm for torque.

If power is what accelerates a vehicle, why do we have several gears in our transmission? Everybody knows that a vehicle accelerates better in 1st gear than in 5th, but the top speed in 1st gear is lower. Power through a gear reduction is constant; this can be proven using the formula for HP that was given earlier. So why do lower gears accelerate better? Because they produce more torque at a lower rpm, that's why.

Instead of thinking about rotating forces, just think about it as linear forces. Torque really in nothing more than force on a lever. If you have an object that requires 550 lbs of force to move at a steady speed, it will require 1 hp to move it at a speed of 1 foot/second. It will not accelerate unless more force is applied. As soon as it begins to accelerate more power is consumed, but the force is what causes the acceleration.

Another point of view is that you one thing that can push with a force of 1100 lbs, but it's peak speed is only 10 ft/sec. Another thing can push with a force of only 275 lbs, but at a speed of 40 ft/sec. Both are capable of producing 20 hp, but the first thing is capable of accelerating the 550 lb object at the rate of 2 ft/s each second while the second thing can't even get it to move. Power means nothing if there is not enough force to accelerate the object. In an engine torque is the force.

 

I've tried to explain it to you and what your missing...but you don't get it. If you want to disagree, thats fine.

 

Well, I read every word of every post and enjoyed it all. There was patient explaination, lots of examples, disagreement and re-explaination and then it got complicated cause HP is complicated and I learned a lot. In a nutshell I learned that HP is the result you get from combining torque and rpm. It's a label with positive fuzzies sorta like coffee. But coffee beans..ugg, and hot water you never drink it! But mix them together properly and you get a great result just like combining torque and rpm in various ways to make the horsepower you want. Love em both and the stronger the better. Okay, here is a question. My horse and I (1100lbs) cover 50 miles in 8 hours and go up (and of course back down) 13,000 feet. Howmany horsepower is that? Seriously, can it be calculated?

 

Does any of this discussion of torque and horsepower explain why diesel pickup trucks are not geared for increased fuel mileage, as an option for those who do not need the pulling power of a diesel?

 

diesels get better fuel mileage with lower gearing(numerically higher). It takes less force for them to get moving.

 

I'm not missing anything, you are. HP does not equal acceleration. If you build a 500 hp 302 and a 500 hp 460, which would accelerate faster all other things being equal?

 

If all other things are indeed equal, the 302 will probably be quicker because it will weigh slightly less.

The V-twin Harley motors are low horsepower and high Torque. The Japanese sport bike (aka crotch-rockets) motors are high horsepower and low torque. Which do you think will accelerate faster?

 

And what'll happen if you swapped engines between the two bikes...

Silver Streak, your post #30 says it all. You do not understand horsepower, and I do not wish to argue over it with you. I tried to explain what was wrong with what you said in post #31, but it didn't get through to ya. Nickfordman understood what I was getting at, it may be the way I am explaining it to you...but either way you don't understand.

There is the evidence of your ignorance.

 

Do you know what a Harley weighs vs. a sport bike? Harley lists a new Softtail at 730 lbs; Suzuki lists a new 'Busa at 480; a difference of over 50%. The Busa also makes more torque than the HD does, but at much higher rpm. Swapping engines probably wouldn't help the HD much unless the gearing was changed to increase the torque at the rear wheel.

If you had a 500 hp 302 you would be doing very well to get 400 ft-lbs of torque out of it. A 500 hp 460 would probably make about 600 ft-lbs of torque. If you geared them the same the 460 would destroy the 302. The only way the 302 will win is if it is geared to take advantage of running at a higher rpm. Regearing it is a way to increase torque at the wheels.

If you aren't going to debate the issue, why do you continue to do so? You've said I keep giving you ammunition to prove me wrong, which might be true. The only problem is that you have neither a weapon to fire it from nor a target to aim it at. When you are going down the road and push the gas pedal, the force you feel pushing you back in the seat is torque, not horsepower.

 

Ha, Other than being ostracized by my biker buddies?

It would be interesting to try. Here is what I think would happen:

The Harley motor would perform about the same. While there would be some marginal improvement, I do not think it would be very noticeable. After all these bike run pretty much the same whether you are riding solo or double-up with fully loaded saddlebags.

The crotch rocket motor performance would drop slightly (especially at low speed/RPM’s) from the extra few hundred pounds. Again, I do not think the difference would be significant except at low RPM’s.

Ultimately the crotch rocket with two heavy riders is still going to be faster than the Harley with one thin rider and the weight differential is becoming less of an issue.

Back to the original topic… Torque by definition is an axial force causing rotation. Torque is determined by multiplying the applied force by the distance from the pivot point to the point where the force is applied. Think of my earlier example of using a two-foot breaker bar on the crank pulley of your truck. A 200-pound man could apply up to 400 foot-pounds of torque. This is a classic example of torque. However the application of 400 foot-pounds of torque does not guarantee that any work is being performed. Whether the truck moves or not the applied torque is identical. Horsepower on the other hand is a measurement of the amount of work being done. Only after the truck is moving can you calculate horsepower expended.

Since torque is the applied force multiplied by the distance from the pivot the same man could generate 600 foot-pound with a 3 foot breaker bar of 1,200 foot-pounds with a six-foot cheater. Big torque numbers but it still does not mean the truck is going anywhere far or fast because the guy (at 0.1 horsepower) can’t expend enough energy.

 

Here are a few links that possibly make my point beter than I can. Horsepower is all good and well, but it has to be converted to TORQUE through gearing to be of any use. Pay particular attention to part three of the first link.

http://www.slowgt.com/Tut.htm#HVT4

http://vettenet.org/torquehp.html

http://www.allpar.com/eek/hp-vs-torque.html