pHEnomIC

I know what torque and hp are by definition but what I am wondering is how does more rpm = more torque on most combustion engines. Does the faster burning of more gas stack up the power? It just puzzles me a little bit because i think of this analogy. Say there is a bolt that requires 100 ft/lbs of torque to bust it loose and your ratchet can only give 80 ft/lbs. No matter how much you power the ratchet for, it wont stack up to the 100 required.

I don't know if this makes sense but can you explain to me what goes on exactly and possibly even explain to me what i am thinking because Im not too sure it is correct either.

SStrokerAce

What do you mean more RPM = more TQ..... there is a limit to that. A small block V8 in 300+ cube area has a hard time making peak TQ over 5000rpm and 6000rpm is usually the limit.

Bret

pHEnomIC

Well yeah I know there is a limit but how does spinning faster make more power in the range that it is in. I would think that spinning faster doesnt mean more power if you use the analogy with the impact ratchet.

Buttercup

Torque is a force. Power is potential work. Work is force over distance. If the bolt simply doesn't move, you are not doing ANY work but you are producing a force.

Now, if the bolt required a constant 80 lb*ft to cause motion to back it out, the faster you can spin it, the more power you are making (using).

You can never forget about that magical dimension of time... it can make huge differences The faster an engine spins, the more power pulses per second, the more times torque is applied to the crank, the more work it can do in the same amount of time than if it were going slower.

Think about this. Power to the rear wheels is more or less constant for a given drivetrain and conditions. Torque to the rear wheels can be changed by gearing (and wheel tire diameter, etc.) but power will be the same.

PUR_SSYN

The title of your post makes sense, but you're losing the train of thought in your post.

The ability for an engine to make torque at high RPM is quite complex; it's mostly
about how much air and fuel can be taken into the combustion chamber per intake stroke.

As the other gentleman is describing, an engine will encounter pumping loses
due to friction, valve train losses (valve bounce), etc. as RPM rises.

Making power on the other hand is a different story. If RPM can increase rapidly
with little loss of torque...then power will continue to increase.

Since power is product of work completed over time, you can plot the toque
over time and calculate the point of where power peaks and begins to roll off.

Try this link for some more information:

http://vettenet.org/torquehp.html

Stephen 87 IROC

http://www.hardtail.com/techtips/hpandtorque.html

rskrause

One way to clarify this is to try to answer a questioned posed as follows. Would you rather have a motor that made 300 ft.lbs. peak torque at 3,000 rpm or at 6,000rpm? Why? Which would make more hp?

Rich

Paul87

6,000rpm because you will spend more time there when racing? From what I understand it is better to make torque at higher rpm

MyShibbyZ28

From a racing stand point it is better to make torque all the way through the band, but if need be you would want the torque to start as low as possible, with a higher torque peak. If you turn 6000 rpm and only make max at 3000, and then it drops off, what is the point of the extra 3000? What is the point of working harder when no power is being produced? That's my thought of it.

Anywho, back to the original question. If you try moving a very heavy wheel with your hand, what is easier. Spinning the wheel at low speeds, or high speeds? The faster you get the easier it is to move the wheel, and the harder it is to stop it also. Eventually you'll reach a point though where limits are reach, and it doesn't matter how fast you spin the wheel, you will not make any more power. That's why more rpm = more torque. Also explains why torque eventually drops off.

The ratchet/bolt analogy can't be compared to an engine. Using your analogy, you are stating you need constant pressure of 100 ft/lbs to brake loose a 100ftlb bolt. That is true, but an engine is running in a circle. So if you want to compare them you need to change the "constant 100ft/lb pressure" part to "100ft/lbs in motion". 80 ft/lbs in motion (i.e. dropped or spun) can brake loose 100ft/lbs at the right speed. Motion is like a torque multiplier if you want to think of it that way.

pHEnomIC

Thanks guys, i understand it now. I knew that my train of thought was bad, i couldnt really described what i was thinking but, i do understand now.

PUR_SSYN

Are you stating that it's useless to spin the motor beyond the torque peak?



That's actually backwards to what you should be thinking. What about
spinning the wheel faster from it's current rotation? Don't you think it would
take more power to spin the mass at 5000 RPM than it would take to spin at 2000 RPM?

MyShibbyZ28

I did not say that. But in general, not always, it is useless to work harder for less. Cars is an exception. You need that RPM for speed and to keep the car moving.

Let me better explain this.

A spinning wheel is like an engine. In some places it is not though. Spinning a plain wheel at a higher rpm than the current will always make more power the faster it turns. An engine is the same way, but it has limiting factors. Those factors are mainly how much air can be taken in, and how much gas can be supplied. If those factors prevent it from making more power at 3000 rpm, then that's the peak.

Now if we can only find a way to take away those limiting factors....

WS6T3RROR

shibby i'm not slamming you, but a spinning wheel is not like an engine. a spinning wheel has momentum (angular) and produces no power. it is also not any easier to accelerate a wheel no matter what the rpm's as long as the change in velocity is the same (ignoring friction and drag), the same goes for something in linear motion. linear momentum (and angular) is a measure of the energy an object carries the formula for that is mass x velocity. no matter what the starting velocity is the increase in enegry will be the same.

for an example lets use a 1lb object.

at 5fps its momentum is 5
increase the velocity to 10 and it has 10
so the change in energy there is obviously 5

take the same object starting at 100fps and the momentum is 100
then bump the speed to 105fps and the momentum is 105
again the change in energy is 5

there are equivalent equations for angular momentum but often times thats a hard bump for folks to climb over quickly.

on the where do you want the peak tq at 3000 vs 6000rpms that rich asked. you want it at 6000 because you can use "twice" the gear and effectively double the force thats accelerating the car at the same velocity.

SageofKnight

Torque is the force and RPMs are the force applied. So if you make X torque do you want it applied 3000 or 6000 times?

The thing is, generally it's physically harder to move the same amount of air 6000 times than 3000 times. For example 200(ftlbs)*3000(rpm)=100ftlb*6000(rpm) but in this example you are doing the same amount of work overall.

But say you can make 200ftlbs at 3000 and 190ftlbs at 6000, you may be making less torque but you're making only slightly less twice as fast.

This is what HP is for. Measuring your torque over time.

MyShibbyZ28

How so? A wheel spins, so does an engine. Spins around the crankshaft.

Are you saying an engine doesn't have momentum? And also, how does a spinning wheel have no power? It takes power to get it moving, but it also has to protrude that same power. So in a sense it has power. If a disc is spinning in space, you can obviously stop it if something is not driving it. But if something is driving it, you need to over come the power of the driver, since the power is being transfered. I guess that is really what I was getting at.

I wasn't really trying to get at all the acceleration stuff.