Torque, Horse Power and Acceleration (2014)

Here’s an interesting way of thinking about the difference between torque and power and, incidentally, about what is involved in a drag race start: -

If your clutch can stand it, and if you are skilled enough, the very best performance will be achieved by holding the throttle wide open and using the clutch to hold the engine rpm at the point at which maximum torque is produced.
Even before the bike starts to move, and with the clutch slipping 100%, maximum torque is being transmitted to the rear wheel. If that were not the case the engine would not be held at peak torque revs on full throttle; it would, instead be in the process of exploding or bouncing off the rev limiter.

So again, before the bike starts to move maximum torque is being applied to the rear wheel, but power as measured at the rear wheel is zero: because it isn’t moving and power = force x velocity (or torque x rate of rotation) and without the motion there is no power.

Measured at the crank the engine is developing close to maximum power, so where is the power going? The clutch. At the point at which the bike starts to move off the line the clutch is absorbing all the power that the engine can develop at the same time as it is transmitting maximum torque.

Gidday All.

Referring to Jim's post, high RPM on a long stroke causes lots of friction, heat, and other problems. therefore long strokes dont run high RPM, but produce lots of torque..

A hypothetical situation based on what I have understood from this topic. if a motor had tremendous torque, plenty available, but not many revs is it possible that the gearing could be speed increasing therefore allowing high speed at low revs . this would rely on the tremendous torque produced. Of course acceleration would be affected.

Practically could a motor be produced, ( a speed record engine perhaps where acceleration was not the main goal), that say had 4,500RPM redline but a top speed of 200 MPH.
In fact, is this what Bert Munroe did to his Indian?. Was his gearing ratio speed increasing or 1:1 at least. Did Bert's engine do 7,000 RPM. he certainly got the speed from a long stroke.. it seemed to have reasonable acceleration.
Bradley

I recently took the Seeley 850 to a ride day and had a lot of problems with it, so I didn't get it out onto the circuit. First I couldn't start it, however found a plug spanner and a couple of plugs and fired it up. I tried to ride it around the pits and found it t o be extremely high geared. Because I'd changed the sprockets while fitting the new 6 speed box, I thought that was the problem. The clutch was smoking, but the bike staggered off so I rode it down the next pit lane then turned back into the one I had come from. Like an idiot, I then gave it a big fistful and slipped the clutch. When the clutch took up the bike took off like a rocket, and I almost put it through the fence, even though I had about 100 metres to play with. I'm used to bikes accelerating fast - this one almost beat me. I took it home and discussed what happened with a friend. He said what I thought - that the bike couldn't pull top gear from almost a standing start. I started counting rotation of the clutch and final drive, and found that the gear change was upside down. I think my 850 motor pulled 5th gear from walking speed and the bike accelerated like buggery. It is very deceiving.

The formula for HP is: Hp=torque x RPM/5252. (The 5252 is 33000/2 x pi.) So, we can see from this formula that all engines will have the exact same number of horsepower as ft/lb of torque at 5252 rpm. Below 5252 rpm the horsepower will be-(by definition)-lower than torque and above 5252 rpm the horsepower will be higher than the torque number-(also by definition.) It is all because we humans have assigned a value to horsepower and a formula for its calculation. As I mentioned earlier, we cannot mess with the torque numbers because whatever units you use, it is simply a measured quantity. (Obviously, not all engines can attain 5252 rpm but the formula holds and "if" they could then the torque and hp would be the same number at that speed.)
Horse power may be the ability to hold RPM under a load but does not necessarily mean an engine can accelerate. I don’t see too many of us being interested in a slipper clutch or a torque converter drive so an engine that will build rpm quickly under load is important for the numbers and the seat of the pants feel. Gearing matched to engine performance is a critical thing. I would think that the ideal ratios would be such that you run up to an RPM where acceleration begins to fall off and shift so that the engine is back at a speed where it can ferociously accelerate again.

You are right.
I have always looked at horsepower as nothing but an advertising tool. If I want to see how a vehicle is going to perform and feel in the real world, a torque @ rpm graph means a lot more to me. Jim

I'm used to bikes accelerating fast - this one almost beat me.

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LOL now ya know my pissed off shock on having 2 Norton twins that could run right out from under me it not seemingly over preparing first. Alan, I know two others that can relate to your top gear explosion, they had both put lot of money in engines both installed in extended hard tail chopper frames and never got beat by just dropping clutch at WOT and handing on. This was in the later 70's early 80's. They also told me how they lost them and had nothing to do with anything breaking under the power loads.

The way Norton twins seem to deliver power to me is they can put out more torque below the rpm curve than other engines/per displacement until runs out of crank tolerance. The fastest rev up engines I've ever had were both 750's with rather lightened flywheels, both tended to got BLATTT!-Redlined, vs varrRROOMMMM of in line 4's. One of Peel's past times was red line races sitting still, Peel from 2000 to 7 or 8000 and inlines 4000 to 10-11, 000. Peel was so responsive I could not snap full throttle and back fast enough to avoid needle in red zone. The P!! was more violent-quicker to 9grand as would hit it before could hit WOT unloaded. I don't know if ya ever shot too big a calibre that tends to make you flinch and trigger shy, P!! was like that in gear. Peel's surprising response led to her power demise and can report that the hogged out early Combat ports come into their own a bit above the red zone with 2S cam. ugh. In practice Peel in 2nd going 60's would spin tire accelerating into turn and trip out if I leaned any off vertical barely avoiding fish tailing in entry bee line acceleration, so developed a way to to use that for effortless steering response. Only possible with torque hits as horsepower rpm takes too long to develop.

Its takes ~100 hp for 500 lb pilot/cycle or 1 hp/5lb for 10 sec 1/4 mile.
http://www.ajdesigner.com/fl_horsepower ... d_time.php

comnoz said:

You are right.
I have always looked at horsepower as nothing but an advertising tool. If I want to see how a vehicle is going to perform and feel in the real world, a torque @ rpm graph means a lot more to me. Jim

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You can't get a detailed impression of what a bike might be like to ride if you only know its maximum horsepower: it could be a screamer or a stump puller. But if you are comparing similar bikes being used for a similar purpose e.g those eligible for a particular race class, then maximum horsepower tells you more about the potential of the bike than any other single number can. It's a safer bet to choose to race the bike that produces ten more horsepower than the one that makes 10 more ft/lbs of torque. Even if you know the engine speed at which the torque is being produced you still can't guarantee that it ends up making more power.

ggryder said:

It's a safer bet to choose to race the bike that produces ten more horsepower than the one that makes 10 more ft/lbs of torque. Even if you know the engine speed at which the torque is being produced you still can't guarantee that it ends up making more power.

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Perhaps a real world example here of this may help.
In the early 1950s, Guzzi had two varieties of its flat single 350 GP bike - a long stroke and a short stroke.
The short stroke made a few hp more, and the long stroke was a stump puller.
They raced the short stroke on the long fast tracks, and the stump puller on the short slow tracks.
To good effect - they each won plenty of races, and Guzzi took the World Champeensheep that year.
On paper though, the short stroke should have won everywhere...
http://www.motorcyclespecs.co.za/Classi ... 201953.jpg

However, the story gets a little murkier, some years earlier as it happens.
JAP in the late 1920s could supply you with a few varieties of 1000cc v-twin engine - built in Tottenham London.
The short stroke JAP 1000cc v-twin was the stump puller, and the long stroke 1000cc engine was the race winning screamer.
Figure that one out - Its all in the tuning.
Cams and Carbs and Compression do make a difference....
https://farm9.staticflickr.com/8005/729 ... 65df_b.jpg

And a Norton is a screamer if you are thinking about piston speeds.

A Hyabusa at 10,000 RPM has a piston speed of 68 FPS [very high for a modern engine].

A Norton at 7200 RPM is moving it's pistons at 70 FPS. Jim

Yet another fantastic thread! Thank you for the enlightenment, gentlemen.

This is how I describe Torque & HP; Hold a Norton fork tube out straight in front of you like a sword with your weakest hand, have a strong friend grasp the other end and attempt to twist it as you resist. What you feel is torque but no horsepower is produced until the tube slips in your grip, then a little work is being done. This example would be high torque/low horsepower. Next, modify the fork tube so you can chuck it up in a 1/4' drill motor Have your strong friend man the drill and spin it up to speed in your loosely gripped weaker hand. Close your hand on the tube and it will stop in spite of the higher HP produced by the drill because torque is low. This example would be low torque/high horsepower.

Biscuit, that's one of the most elegant revealing explanations I've ever read. I'd like to share it around www and credit if ya like. Voltage/amperage = watts too.

This is how I describe Torque & HP; Hold a Norton fork tube out straight in front of you like a sword with your weakest hand, have a strong friend grasp the other end and attempt to twist it as you resist. What you feel is torque but no horsepower is produced until the tube slips in your grip, then a little work is being done. This example would be high torque/low horsepower. Next, modify the fork tube so you can chuck it up in a 1/4' drill motor Have your strong friend man the drill and spin it up to speed in your loosely gripped weaker hand. Close your hand on the tube and it will stop in spite of the higher HP produced by the drill because torque is low. This example would be low torque/high horsepower.

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This is one my favorite nothing special Commando deomo's of torque vs hp ya can feel in your guts muscles tension meter. For such a clunker sounds good on open pipes. Skip to 1:50 to skip over the ugly parts.
[video]http://www.youtube.com/watch?v=fkYpWySknw8[/video]

What make of engine is this 500 dragster, looks rather Nortonish and only shifts once.

[video]http://www.youtube.com/watch?v=h1x8CMqtzj0[/video]

This is math intense simpleton graphing of torque vs hp in two similar cars with dramatically different engine character and gear ratio testing to show it don't matter much.

[video]http://www.youtube.com/watch?v=CxK0x7AE3s8[/video]

hobot said:

What make of engine is this 500 dragster, looks rather Nortonish and only shifts once.

[video]http://www.youtube.com/watch?v=h1x8CMqtzj0[/video]

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I reckon it's a 500 Triumph

comnoz said:

And a Norton is a screamer if you are thinking about piston speeds.

A Hyabusa at 10,000 RPM has a piston speed of 68 FPS [very high for a modern engine].

A Norton at 7200 RPM is moving it's pistons at 70 FPS. Jim

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What a comfort you are - have you watched Jim Scmidt's video where he describes his long rods and short pistons in terms of piston weights ? My 850 motor usually revs easily to 7200 RPM with aluminium rods and the heavy pistons, almost regardless of what gearing I use.

chasbmw said:

hobot said:

What make of engine is this 500 dragster, looks rather Nortonish and only shifts once.

[video]http://www.youtube.com/watch?v=h1x8CMqtzj0[/video]

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I reckon it's a 500 Triumph

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It sounds like my old 500cc short stroke Triumph however I don't know how it could pull so hard with such high gearing unless it has a bullet proof clutch which will cop a lot of revs off the start - it is probably a long stroked 650 with really nasty cams and small inlet ports.

hobot said:

This is math intense simpleton graphing of torque vs hp in two similar cars with dramatically different engine character and gear ratio testing to show it don't matter much.

[video]http://www.youtube.com/watch?v=CxK0x7AE3s8[/video]

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A very interesting video, however it doesn't tell you what happens to the acceleration if you fit the 7 speed F1 gearbox to the Nascar engine and appropriately raise the overall gearing to maximize the use of the higher torque output. Someone once said to me that you don't need a CR gearbox if you have a torquey engine - I say that statement is rubbish. The common 4 speed American CR gearbox is really great everywhere with my torquey commando motor, except off a clutch start where you need the very low first gear to avoid cooking the clutch or bending mainshafts. It should not be a problem having a really big gap between first gear and second. Once the bike is rolling on almost any race circuit, you rarely go below second gear and the up-change from first gear often occurs at around the first corner where a lot of other guys back off. As I've said before, I am a cheapskate - it's the difference between $700 and $5000 for a satisfactory result.

[video]http://www.youtube.com/watch?v=CxK0x7AE3s8&feature=player_embedded[/video]

I have a huge problem with this video. All of his acceleration numbers are calculated on what that much horsepower would do with that much weight. No time calculated for shifts and no consideration of the engines ability to accelerate. In other words, his numbers would have come out all the same if you added a 1000 lb flywheel to the engine (as long as that did not increase the weight of the car ) One of my theories is that some engines won’t get out of their own way so in the motorcycle acceleration model they do not have useful horsepower. Said another way, they would maintain speed fully loaded going up a steep hill but not have that great an ability to quickly get up to a fast speed.

Jim hit the nail on the head when he said that a drive system that allows an engine to just operate at its full rated horsepower and just pulls off as much power as possible by a torque converter or clutch would level the playing field between all engines of the same horsepower.

Hobot, feel free to use the example any way you wish. Glad you liked it.