Torque, Horse Power and Acceleration (2014)

motorson · Jan 13, 2014

Quite a bit has been said about horse power and torque numbers on this forum as they relate to our good old Nortons. I would like to raise a question and make some observations on here where my ideas can be tested in the good old beat them up, knock em down, chew em up and spit them out methods. It has been pointed out that an engine makes the same torque as horse power at 5250 rpm. (That is 5252 actually) At that RPM the torque and hp curve should cross on all engines since that number is a constant in the formula that defines what a horsepower is. You can't change what torque is. It is a measured thing in foot pounds or Newton meters or any other unit of measure of force and length of a lever arm you would want to use. But, horsepower is an arbitrary unit that we have all agreed is a product of a certain amount of torque and speed (rpm). (And is essentially the ability to do work.) The units could be changed if we all wanted them changed but still you would have all of us old guys reminding everybody about the old measurement for years to come.

The question I want to raise is that it is an engines ability to accelerate in a motorcycle that makes it feel powerful. Thus we enjoy the torque of our Nortons at lower rpms where hp is not at its highest and a lighter flywheel, while not increasing horsepower, does make an engine more able to accelerate. So, rather than being enamored with horsepower numbers shouldn't we be building engines and drive trains that are good at accelerating under us? I think that it can even be demonstrated on the track that it is not always the highest hp that wins a race but the ability to pull out of a corner and accelerate to the next one.

I am sort of keying off of a comment that mentioned a benefit of having maximum horsepower at 5500 rpm. My thought was, "well you would want to be able to accelerate through that rpm to your next shift for it to be a good thing."

Happy thumping, Dan.

comnoz · Jan 13, 2014

The torque and horsepower readings will cross on a dyno slip but they are not necessarily the same.

Torque, usually measured in ft lbs, is the amount of turning force on a crankshaft or sometimes a wheel. On a dyno slip it is a crankshaft figure.
A torque reading does not depend on RPM.

Horsepower is a measure of the ability to do a certain amount of work in a certain amount of time.
Torque or RPM readings will affect a horsepower reading. RPM has the largest affect.

You can think of it like this.
Take a AC electric motor rated at 1/2 horse at 1725 RPM. You can not stop it with your hand- it has a lot of torque.
Take an AC/DC motor that is rated at 1/2 horse at 10,000 RPM. You can easily stop it with your hand- it has very little torque.

Now take each of these motors and install them in a motorcycle with the appropriate gearing so they will drive the rear wheel at the same RPM.
They will both be able to produce the same acceleration due to the gearing.

It takes both- torque and RPM to go very fast. [torque at high RPM]

A Norton has lots of torque but not much RPM capability due to it's long stroke. The torque makes it feel strong but about the time it runs out of RPM a modern multi will just be coming into it's powerband and will leave it for dead.

Regardless, the torque of a Norton makes for a great ride at sub 100 mph speeds. To make it feel stronger you need to maximize the torque because substantially increasing the RPM is self-defeating with the high speed friction losses in a long stroke motor. Jim

Fast Eddie · Jan 13, 2014

Well, this should run for a while. I'll start off with a repost, then back away whilst the better minds pontificate...

Personally, I find that understanding the difference between torque and power becomes more difficult the more one thinks about it!

My dyno man explained it thus:

Torque gets you out of corners quickly... Power gets you to the end of the straights quickly!

He waffled on a bit more than this and kinda lost me, eg: "Torque is more important in the lower gears, whilst power is more important in the higher gears" It seems logical, as a rider, but I don't really understand why!

But this bit stuck, he claimed that if you: "change up a few hundred revs past peak power... and then land close to peak torque... then maintain WOT... repeat... you are going as fast as you can".

Dave Nourish said to me "you're only ever on full power for split seconds each lap. Its torque that wins races boy" (may not apply at Daytona tho)!

Discuss...

pommie john · Jan 13, 2014

I'll repost something I heard from a car race forum. The last bit is what we're interested in, but it's all amusing:

Understeer is when you hit the wall forwards
Oversteer is when you hit the wall backwards
Power is how hard you hit the wall
Torque is how far the wall moves.

B.Rad · Jan 13, 2014

Fast Eddie said:
Well, this should run for a while. I'll start off with a repost, then back away whilst the better minds pontificate...

Personally, I find that understanding the difference between torque and power becomes more difficult the more one thinks about it!
quote]
Exactly , Fast Eddie. A fascinating subject that should run for a while.. I hope so. I learn so much from these engineering threads. There is an unbelieveable amount of experience and knowledge here.
What about Jim Comstock. So much knowledge, but he can explain complex ideas in practical terms to the lay man. the explanation using electric motors, could not be any clearer.
looking forward to this. best wishes bradley

Hobot · Jan 13, 2014

Its all on a sliding scale of weight torque hp tire traction and stance and weight shift of the pilot-cycle. Torque is what accelerates and hp is how long the pull can be sustained. I'm spending-inventing to make above the 2nd best feature in Ms Peel. In general Commandos accel enough in 2nd to take on almost all comers into the 80's mph. Once ya ever do get more power by torque and hp that tire can take its rather addictive not to pull the trigger any time ya dare. I'm most happy with Combat wet sumping blood to the back of brain case to point vision narrows and dims keeping on it. I want Peel to develop about 1 lb ft of shaft torque per 4 lb of bike/pilot mass. This could take a nut case into fuel burn mass and excessive btu handling to get idea how long can get away with it. I'm going with lighter fly wheel so traction control is about solely under throttle control, as with great power response its the power cut delay that starts to matter most.

Rohan · Jan 13, 2014

Power is just a calculated number, from the torque at that rpm.
The two are inseparably linked.....
Some dynos can supposedly measure horsepower directly - but they have to be measuring torque (or acceleration) , and calculating the power ?

Torque, which happens EVERYWHERE in the rev range, ALWAYS, has been explained elsewhere as how well the cylinder fills with fuel/air mix at that particular rpm.
Thus at idle, where the intake is throttled right down , cylinder filling is at a minimum, and so is torque.
At the point of maximum torque, the cylinder filling is at its maximum in that engine - thats the maximum fuel/air that engine can cram in to the cylinder.

It has been said, perhaps not correctly, that the Manx norton was among the first of engines that achieved more than 100% cylinder filling at points in its rpm range.
This was achieved by tuned resonances in the inlet AND exhaust systems - such that more fuel/air mix was pulsed into the engine than would otherwise normally happen.

BTW, it may have been me that penned that 5250 rpm, not 5252 rpm.
I wondered if I'd written it incorrectly but didn't go back to check and correct it, sorry.
The NUMERICAL values of torque and horsepower correspond at 5252 rpm, which is why the plotted curves cross at those rpms.
By definition - so they have to...
If the graphs are scaled differently though, the graphs and curves can be quite different, and misleading....

Hobot · Jan 13, 2014

Most everyone seeks a flatish torque curve, but I've tasted what is like to have increasing torque curves to red line in two Nortons, couple outboards and cage V8's it allows constant 10%-ish tire spin just about all the way to top out. It gets harder and harder to break traction the faster ya go in a bee line on pavement and hill climbs. Also makes shifting optional depending on traction.

comnoz · Jan 13, 2014

pommie john said:
I'll repost something I heard from a car race forum. The last bit is what we're interested in, but it's all amusing:

Understeer is when you hit the wall forwards
Oversteer is when you hit the wall backwards
Power is how hard you hit the wall
Torque is how far the wall moves.

I like that. I think I will print it out for my shop wall. jim

comnoz · Jan 13, 2014

hobot said:
Most everyone seeks a flatish torque curve, but I've tasted what is like to have increasing torque curves to red line in two Nortons, couple outboards and cage V8's it allows constant 10%-ish tire spin just about all the way to top out. It gets harder and harder to break traction the faster ya go in a bee line on pavement and hill climbs. Also makes shifting optional depending on traction.

If you have an increasing torque curve all the way to redline then your redline must be really really low. Jim

acotrel · Jan 13, 2014

'Torque gets you out of corners quickly... Power gets you to the end of the straights quickly!'

Six speed close ratio box with high overall gearing. You might even need a very low first gear and make all the others much higher. Keep that heavy flywheel spinning. You cant beat a top end motor pulling high revs if the straights are too long. The rest depends on handling, and acceleration and there is often a lot of it. Lowering the gearing doesn't necessarily mean faster acceleration when you have a heavy flywheel. Commandos have such silly motors.

Fast Eddie · Jan 13, 2014

Rohan said:
Power is just a calculated number, from the torque at that rpm.
The two are inseparably linked.....
Some dynos can supposedly measure horsepower directly - but they have to be measuring torque (or acceleration) , and calculating the power ?

Torque, which happens EVERYWHERE in the rev range, ALWAYS, has been explained elsewhere as how well the cylinder fills with fuel/air mix at that particular rpm.
Thus at idle, where the intake is throttled right down , cylinder filling is at a minimum, and so is torque.
At the point of maximum torque, the cylinder filling is at its maximum in that engine - thats the maximum fuel/air that engine can cram in to the cylinder.

It has been said, perhaps not correctly, that the Manx norton was among the first of engines that achieved more than 100% cylinder filling at points in its rpm range.
This was achieved by tuned resonances in the inlet AND exhaust systems - such that more fuel/air mix was pulsed into the engine than would otherwise normally happen.

BTW, it may have been me that penned that 5250 rpm, not 5252 rpm.
I wondered if I'd written it incorrectly but didn't go back to check and correct it, sorry.
The NUMERICAL values of torque and horsepower correspond at 5252 rpm, which is why the plotted curves cross at those rpms.
By definition - so they have to...
If the graphs are scaled differently though, the graphs and curves can be quite different, and misleading....

Are you sure about that Rohan?

I would have thought that Max cylinder filling gave max power. But as to where, and when, max torque is delivered to the wheel, is also effected by leverage (stroke) and inertia (flywheel) (and probably other things) which are not exclusively linked to cylinder filling...

Hobot · Jan 13, 2014

Maybe just sales propaganda like Norton published but makes me tingle none the less. There is a way to use torque to store energy in the frame if designed for it and it takes a Lot of engine torque to wind it up for release in turning.

Torque, Horse Power and Acceleration (2014)

Fast Eddie · Jan 13, 2014

hobot said:
Maybe just sales propaganda like Norton published but makes me tingle none the less. There is a way to use torque to store energy in the frame if designed for it and it takes a Lot of engine torque to wind it up for release in turning.

Ok, serious question here: are these blowers, or kits, still available?

Although, I do find the "30 day limited warranty" less than confidence inspiring...!

Rohan · Jan 13, 2014

Fast Eddie said:
Are you sure about that Rohan?

I would have thought that Max cylinder filling gave max power. But as to where, and when, max torque is delivered to the wheel, is also effected by leverage (stroke) and inertia (flywheel) (and probably other things) which are not exclusively linked to cylinder filling...

Of course I'm sure about it - its all straight out of the textbook.

You could tune an engine to have max torque at 2000 rpm and yet still have max power at say 4500 rpm.
As some older sidevalves indeed did.

Some of these current big cruiser engines (HD, Victory, Kwikasaki) make max torque quite low down in the rev range.
But some go on to make some quite big numbers.
Which is why they feel to have some serious mumbo, all loaded up with pillion and lots of gear, without needing to rev them...

comnoz · Jan 13, 2014

Here is a good illustration of torque vs horsepower. The next 1/8th mile would have been different.

https://www.youtube.com/watch?v=jXsoAdh5P5g

Rohan · Jan 13, 2014

hobot said:
Maybe just sales propaganda like Norton published but makes me tingle none the less. There is a way to use torque to store energy in the frame if designed for it and it takes a Lot of engine torque to wind it up for release in turning.

Thats a serious increase in hp with that Drouin there.
Quite a few lbs boost ?

We notice they quote the brochure hp for a Commando, not the actual hp.
Maybe its all just marketing talk...

We also notice they are always on ebay with bits missing, and the owner not knowing anything about them.
Add Drouin, add unreliablity ?!

Rohan · Jan 13, 2014

comnoz said:
Here is a good illustration of torque vs horsepower. The next 1/8th mile would have been different.

Remember that horsepower has to have torque to go with it - by definition.
So the 'horsepower' engine just doesn't have as much of the torque ... low down...

Fast Eddie · Jan 13, 2014

Rohan said:
Fast Eddie said:

Are you sure about that Rohan?

I would have thought that Max cylinder filling gave max power. But as to where, and when, max torque is delivered to the wheel, is also effected by leverage (stroke) and inertia (flywheel) (and probably other things) which are not exclusively linked to cylinder filling...

Click to expand...

Of course I'm sure about it - its all straight out of the textbook.

You could tune an engine to have max torque at 2000 rpm and yet still have max power at say 4500 rpm.
As some older sidevalves indeed did.

Some of these current big cruiser engines (HD, Victory, Kwikasaki) make max torque quite low down in the rev range.
But some go on to make some quite big numbers.
Which is why they feel to have some serious mumbo, all loaded up with pillion and lots of gear, without needing to rev them...

I understand that max torque and max power are / can be achieved a different rpm... Which kinda goes back to the OP...

My question was / is: is it really ONLY max cylinder filling that dictates max rear wheel torque? If so, does that mean that heavy flywheels and long strokes etc actually do not help increase rear wheel torque (contrary to popular belief)?

comnoz · Jan 13, 2014

Fast Eddie said:
Rohan said:

Fast Eddie said:

My question was / is: is it really ONLY max cylinder filling that dictates max rear wheel torque? If so, does that mean that heavy flywheels and long strokes etc actually do not help increase rear wheel torque (contrary to popular belief)?

Click to expand...

Click to expand...

Heavy flywheels will not increase the torque of the engine. Of course the energy that is stored in a rapidly spinning flywheel may certainly help get a good launch in a drag race. Jim

Torque, Horse Power and Acceleration (2014)

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