Torque v. HP.

[papa al]

My friend has a thumper bike which he likes because of the low end torque…less down shifting.

I ask him, does it require less downshifting at low rpm due to plenty HP rather than torque?

'No, no. Torque and HP not same.'

Yes but…

Consider various engines that produce say 15 HP @ 4000rpm,

which would produce the highest torque at that rpm?

A.) a 4-stroke single, vs. a 2-stroke single;

B.) a single cylinder, vs. a multiple; and

C.) a long stroke, vs. a short stroke.

My friend thinks 4-stroke, long stroke, single.

What y’all think?

[Engineers, kindly forgo answering this for a couple of days.]

Edited January 19, 2015 by papa al

[ll2]

it depends on how the engine is tuned.

a twin can give more torque than a single if tuned properly.

Both torque and hp is needed for every bike, hard to choose one.

[seedy]

HP and Torque lines for all engines cross at 5252 rpm - because HP is just torque over time

That being said - generally in motorcycle engines, long stroke tends to feel as if it has more torque. Feels like ...

The old slower revving bikes, like my Norton 750, and my HD's, were not very high revving compared to today's engines. But they felt quicker on the bottom end - the lower rpm range. that low-end lunge forward compared to the small nudge given by higher-revving engines.

That being said - the real cause is in camshaft and cylinder head design. Small valves induce fast velocity in the intake charge, which helps at low rpm. Along with valves which do not stay open for long - thus inducing more vacuum in the cylinder which will encourage cylinder filling at low rpm.

But there is always a price to pay - the above example does not work well at high rpm, where you want the valves to stay open for a long time, with more overlap, and larger valves to take advantage of the greater time the valves are open.

Put simply - Torque is acceleration. HP is top speed. That's how they build drag race engines - Torque is King.

[dave_boo]

You want me to stay out of it for now?

[papa al]

Okay.

[Briggsy]

You want me to stay out of it for now?

Nah, you might as well come in. This will end in equations, one way or the other. Perhaps you could simplify for those of us without physics or engineering degrees.

[papa al]

Patience grasshopper.

[garryjohns]

In the most basic simple terms ,the way i see it it is that torque is the force that gets you off the start and up to speed quickly.

HP is what maintains that speed.

In reality, engineering wise there are a huge amount of variables to alter the HP/torque end result.

My XJR has huge torque .It is easy to start off in 5th/top without any problem at all.

Makes for effortless touring type riding ,but i generally prefer to ride it hard up and down the gears like a sportsbike.

As for a small engine as in the OP question i think that the 4 stroke/long stroke would be the answer.

Edited January 19, 2015 by garryjohns

[ktm jeff]

papa al , your friend is generally correct , long stroke single. But as ll2 says , some twins ( most engines infact ) can be engineered for torque. At 4,000 RPM , some 400cc 2-stroke twins would only produce your mentioned 15 BHP , but would have a good amount of torque . They wouldnt be "tuned for torque" as these type of engines often gey reved . Now throw in diesels , or supercharged 2-stroke diesels , and a big discussion will go on forever !.

[papa al]

"Do you want your engine to make HORSEPOWER or TORQUE?"

And the question is usually asked in a tone which strongly suggests that these experts believe power and torque are somehow mutually exclusive.

In fact, the opposite is true, and you should be clear on these facts:

1. POWER (the rate of doing WORK) is dependent on TORQUE and RPM.
2. TORQUE and RPM are the MEASURED quantities of engine output.
3. POWER is CALCULATED from torque and RPM, by the following equation:

HP = Torque x RPM ÷ 5252, per ref:

_http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

_______________________

or, re-arranged:

Torque = HP x 5252 ÷ RPM

As in OP,

15 x 5252 ÷ 4000 => 19.7 lb-ft. of torque.

Single/ multi, 4-stroke/ 2-stroke, diesel, carb/ fi, long-/ short-stroke crank, valve train, ignition parameters, tuning, electro-, hydro-, hydraulic, wind turbine, gerbils on a wheel…the motive force is irrelavent to the question.

Sirs, 15 ponies @ 4000rpm is 19.7 lb-ft of torque. Always!

(And that equation, oh Seedy one, is also why the torque & HP curves for all motors intersect at 5252 rpm.)

Right Dave?

Edited January 19, 2015 by papa al

[dave_boo]

"Do you want your engine to make HORSEPOWER or TORQUE?"

And the question is usually asked in a tone which strongly suggests that these experts believe power and torque are somehow mutually exclusive.

In fact, the opposite is true, and you should be clear on these facts:

1. POWER (the rate of doing WORK) is dependent on TORQUE and RPM.
2. TORQUE and RPM are the MEASURED quantities of engine output.
3. POWER is CALCULATED from torque and RPM, by the following equation:

HP = Torque x RPM ÷ 5252, per ref:

_http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

_______________________

or, re-arranged:

Torque = HP x 5252 ÷ RPM

As in OP,

15 x 5252 ÷ 4000 => 19.7 lb-ft. of torque.

Single/ multi, 4-stroke/ 2-stroke, diesel, carb/ fi, long-/ short-stroke crank, valve train, ignition parameters, tuning, electro-, hydro-, hydraulic, wind turbine, gerbils on a wheel…the motive force is irrelavent to the question.

Sirs, 15 ponies @ 4000rpm is 19.7 lb-ft of torque. Always!

(And that equation, oh Seedy one, is also why the torque & HP curves for all motors intersect at 5252 rpm.)

Right Dave?

If you look at the problem from the HP line of thought...you are correct.

[dave_boo]

No math, but the answer is below. Just highlight my post and see.

The OP set some specific guidelines in his post. The biggest 'gotcha' was the 15hp @ 4,000 RPM. The torque across all the engines will be the same.

HOWEVER, there will be differences. Let's go with the extremes. A super short stroke 2T versus a multi-cylinder 4T. We all know that the 2T will have less mass; there is but one piston/conrod/crankpin and 2 counterweights. The 4T will have all of the previous multiplied by the number of cylinders.

Why does that matter? Inertia. While the lightweight engine is in fact putting out the same torque; when a load is applied to the engine in question it will not have the mass to overcome the applied force the same the other engine will. Much the same way if you accelerated a tennis ball and a billiard ball to the same speed and forced someone at the point of a gun to stand in front of one of them...most sane people would choose to stand in front of the tennis ball's path!

So, in answer to the question, being constrained as I am to design an engine that has 15 hp @ 4000 RPM and would require the least amount of shifting...it would be a multi-cylinder (preferably in a vee configuration) extremely long stroke 2T. The inertia reasoning has already been discussed, but the 2T may be a surprise. Yes, it would technically produce less torque per power stroke; but as the force would be applied in time intervals that were 1/2 that of the 4T the reaction to load would be that much quicker.

Hope this answered the question.

[papa al]

OP:

"Consider various engines that produce say 15 HP @ 4000rpm,

which would produce the highest torque at that rpm?"

Simple enough question. Don't complify.

You'd have to have a lot of gerbils for 15HP I guess.

But if you could you could get them to spin a dynamometer @ 4000, it would be

15hp <==>16.7 pound feet.

Inertia, power strokes per rev, &c, &c, IRRELEVANT TO THE QUESTION.!

The horsepower to weight ratio would make gerbil power a poor choice for a motorcycle.

Edited January 19, 2015 by papa al

[seedy]

Oh Seedy one !!!

[BSJ]

There was a time when I would have said you cannot have to much of either but I now think a 1800cc to 2000cc V twin with a supercharger is probably the best choice for me. I guess ones opinion changes as one gets older!

[JonnyF]

There was a time when I would have said you cannot have to much of either but I now think a 1800cc to 2000cc V twin with a supercharger is probably the best choice for me. I guess ones opinion changes as one gets older!

Yeah that's probably enough

I think there can be too much now for the average rider. Which is why the emphasis is increasingly on electronics to make some of these modern sports bikes rideable. Whether your sweet spot is 80 hp, 120hp, 180hp (at various RPM) is a personal preference. For the street I like the middleweights as (for me) it's more fun to ride a bike nearer to it's limits rather than cautiously feather the throttle until you reach your destination intact. I say 'nearer' because I still can't ride my 600 to it's limit unless on a very straight road with no traffic.

The torque of the litre bikes is fun for overtaking at 5k rpm without changing gear, but for me not as much fun as screaming a middleweight through the gears.

[papa al]

There is a guy in North Carolina, USA who builds bikes with 427 cu. in. V-8s.

No need to shift, as no transmission.

You can get a 327 built for the wifey too.

[andrewrooney]

There is a guy in North Carolina, USA who builds bikes with 427 cu. in. V-8s.

No need to shift, as no transmission.

You can get a 327 built for the wifey too.

Any pictures?

[RubberSideDown]

^

It's a 'Boss Hoss'- you can get one with a 502.

[papa al]

I do not find big bikes ostentatious.

[andrewrooney]

^

It's a 'Boss Hoss'- you can get one with a 502.

What do you figure the import tax on that would be?

[dave_boo]

What do you figure the import tax on that would be?

Let's see...

More than 3 litres?

Check.

More than 220 HP?

Check.

Import Duty...80%.

Excise Tax...111%.

Interior Tax....11%.

VAT..................7%.

Total.............209%.

But(!) you may get lucky and only have the 3.1% excise tax applied to motorcycles...I don't know.

[seedy]

HD Road Glide special Bangkok - 1,749,000 baht

Same bike Canada - 851,370

Throw in 15000 baht shipping - 866,370 at 30 baht to CDN dollar which is high right now

Tax, duty - the rest

[RubberSideDown]

The price varies, but you can find decent used ones for ~USD30K (this is dependent on displacement and age, but that seems to be the average)- if you could find an importer to bring one in, figurè about 2m baht with shipping, taxes, bribes, and BS. I don't think you'd be hit with the 200% car rate.

[seedy]

The price varies, but you can find decent used ones for ~USD30K (this is dependent on displacement and age, but that seems to be the average)- if you could find an importer to bring one in, figurè about 2m baht with shipping, taxes, bribes, and BS. I don't think you'd be hit with the 200% car rate.

Hot Rod Pattaya brought in four 2015 models - sold 'em all in one day. 1,450,000 baht. With legal green books.

[RubberSideDown]

^

Doubt it was the 502 at that price- more likely the 293ci (4807cc) version, which runs ~USD45K new:

As monstous as these bikes are, they'll get beaten in the quarter mile by literbikes and up- they don't hook up well at all, and their ~500kg wet-weight makes them tough to handle.

Edited January 23, 2015 by RubberSideDown

[Rhys]

IS that an old BSA 441 Victor?

[papa al]

IS that an old BSA 441 Victor?

The bike I yearned for as a lad.!

Reality was a 100cc Yamaha.

[andrewrooney]

Not as expensive as I thought. Not actually considering buying one or anything, but they're cool to look at!

[dave_boo]

The price varies, but you can find decent used ones for ~USD30K (this is dependent on displacement and age, but that seems to be the average)- if you could find an importer to bring one in, figurè about 2m baht with shipping, taxes, bribes, and BS. I don't think you'd be hit with the 200% car rate.

Hot Rod Pattaya brought in four 2015 models - sold 'em all in one day. 1,450,000 baht. With legal green books.

Amazing they were able to sell them at the sticker price they are in the States.