HP & torque document

[T.W.R]

A pretty concise article on Horsepower & Torque

howdamnhard asked to see it, so here it is for all

[howdamnhard]

Cheers , thanks T.W.R. a good read.

[TripleZee Dyno]

I agree with the writer regarding “low end torque” “high end horsepower” and the way some road testers waffle on about “waves of torque” “massive low end torque” and other bs, especially when testing the latest offering from Harley. No offence to Harley, they actually make pretty good bikes, but considering a GSX1400 or ZZR1200 (not to mention ZX14 and Busa) all put out more “low end torque” than a 96” Harley, it says more about the testers/writers than about the bikes.
I don’t agree with his assertion that HP is what makes your bike go, accelerate etc. and “Torque does none of the above” You cant have one without the other. Even he says that.
Using a chassis dyno we measure roller speed and rear wheel torque and from those we derive HP, measure engine rpm, and derive engine torque with
T(engine)=HPx5252/rpm(engine) and this is what gets put on your graph

Rear wheel hp and engine hp are essentially the same (not withstanding transmission losses etc)
Rear wheel torque and engine torque are different, because we have a gearbox, primary etc between the engine and rear wheel.
Example
At the engine
10,000 rpm
76 HP
40 ft/lbs
4:1 reduction gearbox etc
At the rear wheel we get
76 HP
2500rpm
160 ftlbs
HP is the same but torque has increased.
And that’s why your bike accelerates better in lower gears
Why you change down when going up a hill (some of us)
Because it is rear wheel torque that actually pushes us along, determines how the bike accelerates etc.
To accelerate harder we need more rear wheel torque, increase the HP or change the gear ratio.
Not wanting to be too picky but, his idea about VE is more wrong than right, and the “torque figure for a twin……is likely to be higher due to the longer stroke” is just perpetuating a common myth that long stroke motors produce more torque because they have a “long” stroke

I guess the flowery hyperbole is a bit like chrome, sounds and looks flash but actually means sweet FA
rock on

[xwhatsit]

Not wanting to be too picky but, his idea about VE is more wrong than right, and the “torque figure for a twin……is likely to be higher due to the longer stroke” is just perpetuating a common myth that long stroke motors produce more torque because they have a “long” stroke

Well that's actually true. Given a motor that produces the same force within the cylinder (i.e. the force pushing down the piston is the same), the motor with a longer stroke will produce more torque. For precisely the reason illustrated with diagrams in the article -- a longer stroke means a longer distance from crank centre to crankpin; so there's a bigger leverage. We all know the longer the lever, the more torque is applied.

But the side effect of that is you need more movement to produce that torque. So the piston has to move further... means trouble getting the engine to rev higher. So develops less peak power compared to how much torque it's producing, as while you might be able to put out more torque due to more leverage on the crank, you can't put out that torque as often (because redline is lower), so the rate of work is less.

Of course what you say is true in the sense that every engine is different, so many different variables. So you're right in that just because it's a long stroke motor doesn't mean it's going to produce wads of torque... which is the usual bullshit the magazines and know-it-alls at the pub like to spout.


Good article, thanks for scanning it in TWR.

[TripleZee Dyno]

Well that's actually true. Given a motor that produces the same force within the cylinder (i.e. the force pushing down the piston is the same), the motor with a longer stroke will produce more torque. For precisely the reason illustrated with diagrams in the article -- a longer stroke means a longer distance from crank centre to crankpin; so there's a bigger leverage. We all know the longer the lever, the more torque is applied.

But the side effect of that is you need more movement to produce that torque. So the piston has to move further... means trouble getting the engine to rev higher. So develops less peak power compared to how much torque it's producing, as while you might be able to put out more torque due to more leverage on the crank, you can't put out that torque as often (because redline is lower), so the rate of work is less.

Of course what you say is true in the sense that every engine is different, so many different variables. So you're right in that just because it's a long stroke motor doesn't mean it's going to produce wads of torque... which is the usual bullshit the magazines and know-it-alls at the pub like to spout.

Good article, thanks for scanning it in TWR.

Well I will have to disagree with you. Its not actually true. The assumption that the downward force from the piston is the same for both long stroke and short stroke engines is wrong. For the same cylinder pressure a short stroke piston will provide more downward force. If you work it all out and you will see that the torque applied to the crank will be about the same. I am talking about 2 engines, same size, same cylinder pressure, one short stroke, one long stroke, both engines with their best rod ratio.
cheers

[LBD]

Well actually...conrod length plays a large part in the game...the longer the conrod, the less force wasted in sideways thrust to the cylinder wall and more downward thrust on the crank.

And if the PFP is the same in a long or short conrod engine the torque would be greater in the long rod engine than the short, if all other factors were the same...

However the longer the rods, the hevear the rods the more the rotating mass, the less rpm available the less HP could be produced...

[xwhatsit]

For the same cylinder pressure a short stroke piston will provide more downward force.

Could you explain that to a simpleton like me?

[TripleZee Dyno]

Could you explain that to a simpleton like me?

no worries
assuming 2 engines same size one long and one short stroke the short stroke engine will have a bigger piston, more surface area. eg a 100 x 63 and 79 x 101 both 500cc. The 100mm bore will have around 12 sqin, the 79 mm bore around 7.5sqin. If you had say 300psi in the cylinder the force on the pistons would be 3650 lb and 2280lb. Assume max mechanical advantage for both ie best rod ratio, and you will end up with 453 ftlbs of torque at the crank in both cases.
450 ft lbs is a peak figure, lucky if it lasts more than 10-15 degrees of the power stroke so once you average it out over 720 degrees its a heap less.
cheers

[Fooman]

no worries
assuming 2 engines same size one long and one short stroke the short stroke engine will have a bigger piston, more surface area. eg a 100 x 63 and 79 x 101 both 500cc. The 100mm bore will have around 12 sqin, the 79 mm bore around 7.5sqin. If you had say 300psi in the cylinder the force on the pistons would be 3650 lb and 2280lb. Assume max mechanical advantage for both ie best rod ratio, and you will end up with 453 ftlbs of torque at the crank in both cases.
450 ft lbs is a peak figure, lucky if it lasts more than 10-15 degrees of the power stroke so once you average it out over 720 degrees its a heap less.
cheers

And this is the same reason twins have more torque than 4's of the same capacity - the pistons and/or the stroke are larger, leading to a higher torque. But, as they are larger, both piston speeds (related to stroke) and reciprocating masses (piston size) are larger, limiting engine speed, and therefore the maximum rate of work (i.e. power).

Cheers,
FM

[Fooman]

You cant have one without the other. Even he says that.

And he would be wrong!

On a purely fundamental approach, you can have power without torque, just not rotational/shaft power - reaction motors (e.g. rockets) have power, based on the purely linear thrust, and the velocity at which that thrust is developed (power = force times velocity as well as torque times rotational velocity).

Cheers,
FM

[johan]

And this is the same reason twins have more torque than 4's of the same capacity - the pistons and/or the stroke are larger, leading to a higher torque. But, as they are larger, both piston speeds (related to stroke) and reciprocating masses (piston size) are larger, limiting engine speed, and therefore the maximum rate of work (i.e. power).

Cheers,
FM

I don't think a twin has more torque than an IL4 with the same displacement.
Triples last posts sort of proved that, but math is not my strong side.

Please correct me if I'm wrong.

Good posts, cheers!

[TripleZee Dyno]

Well actually...conrod length plays a large part in the game...the longer the conrod, the less force wasted in sideways thrust to the cylinder wall and more downward thrust on the crank.

And if the PFP is the same in a long or short conrod engine the torque would be greater in the long rod engine than the short, if all other factors were the same...

However the longer the rods, the hevear the rods the more the rotating mass, the less rpm available the less HP could be produced...

Rod length does have an important role to play and rod length or lack of it is why long stroke motors can put out “low rpm torque” From a practical manufacturing point of view long stroke motors are built with relatively low rod ratios, ie short rods. The advantage of a short rod is the mechanical advantage (piston to crankpin) happens earlier in the stroke, eg a 103” HD stroker has a rod ratio of 1.2:1 and maximum mechanical advantage occurs at 67 deg ATDC so more pressure is turned into useful work. The disadvantage is friction and because frictional losses rise exponentially with rpm it doesn’t take long before the losses overtake the gains.
Having said that a 103”dragbike I worked on many years ago turned 7000rpm.
The big advantage the short stroke motor has is it can have a high rod ratio, ie long rods. Maximum mechanical advantage occurs at say 85 deg ATDC but as soon as you have a few revs going the lower frictional losses are more of a bonus than the mechanical advantage of a short rod. Lower avg piston speeds mean less power robbing inertia, means make them lighter means more revs means more power.etc etc
Its all a big trade off. Short rods accelerate the piston to and from TDC which helps with cylinder filling. Long rods "dwell" the piston at TDC which helps with the power stroke. A lot of it is to do with manufacturing techniques and materials as to what they can do or not do.
cheers

[TripleZee Dyno]

And he would be wrong!

On a purely fundamental approach, you can have power without torque, just not rotational/shaft power - reaction motors (e.g. rockets) have power, based on the purely linear thrust, and the velocity at which that thrust is developed (power = force times velocity as well as torque times rotational velocity).

Cheers,
FM

Thanks, that will save a lot of confusion if we get onto rocket powered powered bikes!!

[xwhatsit]

no worries
assuming 2 engines same size one long and one short stroke the short stroke engine will have a bigger piston, more surface area. eg a 100 x 63 and 79 x 101 both 500cc. The 100mm bore will have around 12 sqin, the 79 mm bore around 7.5sqin. If you had say 300psi in the cylinder the force on the pistons would be 3650 lb and 2280lb. Assume max mechanical advantage for both ie best rod ratio, and you will end up with 453 ftlbs of torque at the crank in both cases.
450 ft lbs is a peak figure, lucky if it lasts more than 10-15 degrees of the power stroke so once you average it out over 720 degrees its a heap less.
cheers

Ahhh right I see where you're coming from.

The way I was talking about it is where you have the same bore, same cylinder pressure. Just longer stroke. In which case you would have more torque -- not because of higher capacity causing more fuel/air being squashed and burned, but simply because there is a longer `lever' acting on the crank centre.

So we were both talking about the same thing except I didn't explain myself clearly

In the article it says you can have torque without power -- the case cited is where you apply torque to a stuck/rusted bolt and it doesn't come undone (no work is done, nothing moves -- nonetheless you are applying torque).

[TripleZee Dyno]

And this is the same reason twins have more torque than 4's of the same capacity - the pistons and/or the stroke are larger, leading to a higher torque. But, as they are larger, both piston speeds (related to stroke) and reciprocating masses (piston size) are larger, limiting engine speed, and therefore the maximum rate of work (i.e. power).

Cheers,
FM

the individual cylinders of the twin would produce twice the tq compared to the individual cylinders of the 4 basically because they are twice as big, but the 4 has twice as many tq 'pulses' so overall they will end up the same.
As you say piston speeds etc affect the twin more when it comes to revs. So the tuning will be optimised to suit the operating range and intended purpose of the engine.
cheers