So this is just something that I cant figure out, and have been wondering about for a while.

Since horsepower is a product of torque, when you get the dyno results from your motor, shouldn't the torque and horsepower lines go up and down with each other? It just seems that in all the dyno test sheets I've seen posted the torque will fall down around 4500rpm or so, and then the motor will have it's max horsepower around 6500.

Wouldnt it make sense that the peak horsepower should be at 4500 in that motor then, since horsepower is the product of torque?

Thank you for any constructive replies

 

HP = torque x RPM/5252.11

So if torque was constant at all RPMs, then HP would rise with RPM. i.e. double the RPM, you'd also double the HP.

But instead, torque begins to fall off at some RPM; when it starts to fall off fast enough, HP quits increasing, and begins to decrease.

Since torque is what accelerates the car, that's why it's worse than pointless to try to rev a motor beyond - or even in some cases, up to - its peak HP RPM.

 

Agree with you 100% here.

I'll have to stop you here... Let me preface by saying I know you have a lot more experience in this field than I do, but the my arguments seem to make sense to me so please hear me out. It's better to stay in gear past the horsepower peak so that you can take advantage of the lower gear ratio you're in. Horsepower accounts for this fact, which is why horsepower is more important than torque. Only when the torque to the wheels in the first gear you are in is crosses what it would be after an upshift do you want to change gears.

 

That makes sense. Now I just have to wonder why people sing about horsepower all day long, when infact it should be torque that they're working for.

 

I'll drive your car, and it will go faster than it does when you drive it.

Don't bet against that. Done it too many times. Usually by shifting at lower RPM (not getting caught up in the excitement of the moment) than the car's owner does. Sometimes as much as 1000 RPM lower.

HP = the time rate of doing work; i.e. how hard your engine is pushing, measured vs RPM. When the torque falls off, it isn't pushing as hard. Get the engine back down to where it has some torque (push) again, it will begin to accelerate the car more.

 

This is true. But it's also true that torque to the wheels goes down with an upshift because of the change in gear ratio. The numbers seem to point to horsepower but in the real world things are different.

 

Work is force times distance. Horsepower is the rate at which work is being done. Acceleration is the rate at which speed (technically, velocity) is changing.

If all you're interested in is doing work, maximize torque. If you're interested in doing a given amount of work in the least amount of time, maximize horsepower.

The "force", roughly speaking, is the weight of your vehicle (remember, this is speaking roughly). The distance, for the sake of arguement, is 1/4 of a mile. The "work" never changes - force times distance. Guess what the units of torque are? Force times distance. Torque moves the weight the distances.

However, if you want to put time into the picture, now you're talking power. Unless you have a transmission that keeps your engine at max power from start to finish line, you have to shift. That's where shift points come in, not the "HP vs. torque" stuff.

 

Actually, in theoretical physics WORK (power) is defined as force times distance. The only reason torque is in units that appear to be force times distance is that torque is rotational force, and radius has to be taken into account. Torque is still just raw force.

 

Hell, I'll throw in my two bones even though I'm not invited...

When I have to explain it simply, I usually say that Torque is the amount of work you can do, and HP is how quick you can do it.
As for further explanation I usually preface it by making absolutely sure the person understands that HP is a calculated number while TQ is an actual measured force. (Which is also why the curves do not stay parallel and why it always crosses at 5252 RPM.)

Many builders subscribe to different theories when building for power. I happen to subscribe to the same one John Lingenfelter does... build for TQ and HP will follow.

 

A human being can apply 150+ ftlbs of torque through a breaker bar. If you tied this breaker bar to the driveshaft of a civic, yes you would get the car to move, but not very quickly because you only have one man-power. If torque were the whole game, you would be able to produce better performance than the 100 ft-lb engine the car comes with. But that isn't the case. It's obvious to me that torque is a number that can simply be modified by gear ratio, whereas horsepower is raw energy being applied to the car to move it forward. Lingenfelter's phrase could also be taken another way. "build for RPM and HP will follow" But lingenfelter tuned chevy's, not hondas.

 

Umm... no, it's calculated from the actual measured force. Calculated. A numerical representation only. Once that is figured out, you're ahead of the game.

Building for RPM is building for torque (just in a different rpm range). And flow potential and balance and... Torque is the actual force produced by an engine. Saying you are building for horsepower is just saying you are building a free flowing motor with high RPM torque production in mind. :shrug:

 

Calculated on a dynanometer yes... If you rigged an engine to do something like produce electricity, you would get a direct power value. The fact that it's conventionally calculated from torque doesn't mean anything. And in my RPM example I was suggesting an engine you modify to increase revs without making more peak torque.

http://www.google.com/search?hl=en&l...ue&btnG=Search

 

It would produce watts. Again... Your engine produces TQ, not HP.

I assume you wanted me to read through this drivel...

http://www.houseofthud.com/cartech/t...horsepower.htm

Entertaining and a decent explanation. You'll note that that guy ends his yapping with a recap of TQ, not HP. He just used HP to define his RPM range of TQ production.

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The second one is more drivel from someone that has no idea how to write...

http://vettenet.org/torquehp.html

Makes me ashamed to own a 'vette. He even prefaces his rant on HP with the following...

Gee, seems that TQ really is that important. He just doesn't realize or understand what he is writing, or the significance of his used 'quote'.

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Then the third one on your list, when boiled down, sounds shockingly similar to my posts, just the looong version...

http://www.mustangsandmore.com/ubb/D...orqueVsHP.html

His writing style may be lacking a tad, but of the three it is the best. At least he doesn't make me ashamed of owning a Mustang. :shrug:

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So why are you posting things that back my argument? You are trying to bring gearing into an argument about TQ v HP. A few have already tried to point out the issue is a moot point, but you seem to want to promulgate the myth.:shrug:

 

Who's theory would that be?

The units for work and power are not the same. Power always has time involved, work does not.

 

That's well put and exactly the way I've always understood it. Torque rules!!

 

You're right, I got my units mixed up.

As for the comment about watts, 1hp=746 of them, so you basically proved my point.

Torque is rotational force, don't let the units confuse you. It's true that the more torque you have at the wheels, the more you will accelerate. My point is, and the point of the linked search I provided, is that there is little correlation between torque the the wheels and engine torque, given the proper gearing.

Suppose you take 2 cars where one makes a flat torque curved of 200 ftlbs from 0-2000rpm and then another that makes a flat torque curve of 100 ftlbs from 0-4000rpm, and geared them both to redline at the same vehicle velocity. The second car will have twice the final drive ratio and hence twice the torque multiplier to the wheels. Both cars produce 76 horswepower, and both will have equivalent performance. If torque "ruled" the second car wouldn't even stand a chance. Ever wonder why F1 cars are so fast? If torque were so all-important, the thirdgen TPI own the roads. But that isn't the case. In fact almost every performance mod out there seems to geared towards raising the RPM of your powerband.

 

Suppose we not reinvent the wheel https://www.thirdgen.org/techbb2/sho...hreadid=213506

That's why there's a FAQ forum and function.