posted 11-04-2009 11:32 PM ET (US)            

Best fuel economy in a 4-stroke-cycle piston engine occurs at wide open throttle, but at maximum torque. In automobiles with manual transmissions, you can possibly force this scenario, and keep it that way with careful gear selection. But it is not easy on the equipment, particularly if the motor is powerful and torquey. It is a critical apex in fuel control, spark timing and load, and is dangerously close to lugging. In automatic transmission cars with computer control and warranties, the PCM would never allow this. I'm not sure how you could run at WOT at precisely to peak torque rpm in an outboard unless you chose precisely the right prop for the weight and conditions. And again, you would be dangerously close to lugging.

Best acceleration and top speed are always attained WOT and at the rpm where maximum power is produced.* This is not the most fuel efficient situation, but you are also not a max load, and as long as the reciprocating parts and valve train are up to the task of sustained operations at that rpm, it's easier on the motor than the first scenario, which I consider to be mostly hypothetical because of it's impracticality to employ. It is also common practice in the EFI world for the PCM calibration to command slightly rich of peak fueling to keep piston tops cooler and what not, thereby "wasting" some extra fuel as well.

*Neato, little known fact: Horsepower and torque in ft*lbs are always equal to each other at 5252rpm all the time, every engine whether outboard or F1 auto racing engine.

posted 11-09-2009 10:32 AM ET (US)            

"*Neato, little known fact: Horsepower and torque in ft*lbs are always equal to each other at 5252rpm all the time, every engine whether outboard or F1 auto racing engine."

I sure don't understand that "little known fact" at all. I just bought a diesel VW Sportwagen, whose turbocharged motor produces an amazing 236 ft-lbs of torque at around 2000 rpm, and 140 horsepower at 4000 turns. What's happening here, Peter?

Tony

posted 11-09-2009 10:48 AM ET (US)            

The relationship between HP and Torque is as follows:

HP = (Torque x RPM) / 5252

posted 11-09-2009 11:49 AM ET (US)            

Tony

posted 11-09-2009 06:28 PM ET (US)            

Who can put this into layman's terms that even a mechanically-challenged person such as me can understand?

posted 11-09-2009 07:52 PM ET (US)            

--1foot-- 10 lbs

----------------------10 feet------------------------ 1 lb

The torque measured at the left end of the lever would be the same.

Even as mechanically challenged as you may claim to be I'm sure you've used a screwdriver before and have experienced torque. In order for you to make the screw rotate with the screwdriver, you have to apply a twisting force known as torque with your hand to the screwdriver which is turning the screw. I'm sure you've found that some screws are easy to turn with the screwdriver, some are harder to turn. The easier screw requires less torque to overcome its resistance to twisting than the harder screw. There may have been some screws that you cannot cause to turn at all because its resistance to twisting exceeds your capability to twist (the available torque you can provide).

posted 11-09-2009 08:13 PM ET (US)            

quote:

---

A BMW M series, 240 hp at 6000, 236 torque at 3800 rpm ...snip...[first assertion]:, and can tell you the torque and hp do not occur at the same point. ...snip...[second assertion]:Max torque on the torque curve is where the power is the greatest[third assertion]....snip.... high sierra

---

Almost there, high:

Second assertion:
I'm pretty sure nobody in this thread says that hp and torque occur at the same point... EXCEPT at 5252 rpm, where horsepower ALWAYS equals torque. This is not somebody's old saw, this is a mathematical certainty borne of the fact that:

[1] hp = torque x rpm/5252

There is no room for interpretation on this matter. This formula can be derived from the definition of horsepower, where:

[2] 1hp = 550 lbs.ft/second

This again is not open to interpretation. This IS the definition of horsepower. The mathematical proof of the first formula uses the definition of horsepower, which is the second formula.

Third assertion:
Because of the first formula, which shows horsepower as a mathematical function of BOTH torque AND rpm, your second assertion, that max torque occurs where max power occurs is incorrect. further, this statement runs contrary to your first statement, which was that hp and torque do not occur at the same point. Unless I read it wrong or you misspoke.

First Assertion:
Ah yes, the late, great BMW s52 derivative of their great I6. This was the very same motor in my old ///M3 race car. I would posit that the s/m 50/52/54 series of BMW straight six has simply got to be one of the top 10 greatest auto engines ever. And that is without knowing every other engine ever produced. Too bad they discontinued the s52 after 1999, with the end of the e36m3's run. It's hard to believe it's been gone [in that particular iteration], for 10 years now, though it still lives on in derivative form in the twin turbo 3.0L series of the 135i, 335i and 535i.

Peter
Santa Barbara, CA

posted 11-09-2009 08:15 PM ET (US)            

In fact, the latter question underlines my confusion about this concept. I get it that a longer lever (with the fulcrum and object to be moved remaining in the same location) increases power to turn or move something, and I can grasp how lower gearing can change the ability of a motor to overcome the inertia of an object that higher gearing on the same motor couldn't move, but I still don't see how the concept of 'torque' fits into all of this, unless it's just another word for the same physical effects.

Tony

posted 11-09-2009 10:02 PM ET (US)            

It sounds like you now have an intellectual grasp of torque. The whole concept of torque is related to turning or twisting things. There is an exact equivalent analogy to moving things linearly as well, [more on that later]. Well, power is the rate of output of torque. And HORSEpower is simply a specific amount of power.

A good analogy to understanding power that I like to use, [and have used, while tutoring junior college students], is the following, and it uses the linear analogy, instead of the angular, [which torque applies to]:

Suppose you can bench press 275 lbs. If you press it up 1 foot, you have just exerted 275 ft.lbs. If you press it up 2 feet, you have just exerted 550 ft.lbs.

Now, it is a fact that 1 horsepower = 550 ft.lbs in 1 second. Or, 550 ft.lbs/s

So if you can press your 275lbs up 2 feet in ONE SECOND, you are putting out 1 horsepower. You are one strong hombre. Now suppose you have a nerdy scientist friend who couldn't possibly lift 275 lbs, 2 feet in one second. But, he builds a contraption with pulleys and gears and a crank handle, like we might have on our trailer winches, and he lifts the 275 lb barbell the same two feet you just did... But it takes him 10 seconds. Well then, he just put out 1/10th of a horsepower.

Sooo, then. Back to engines and such. When you put an engine on a dyno, whether it's a wheel dyno or an eddy current dyno, it's actually only measuring torque. You then go back and take the torque at each rpm and apply the formula:

hp = TQ x rpm/5252

to get a horsepower for each of point of torque that the dyno measured. Well, YOU don't do the calulations, the software built into the dyno does it... unless you're a masochist. :)

Ok, dinner bell's ringing. I'll be back to check on this in a bit.

posted 11-09-2009 11:34 PM ET (US)            

By definition, Horsepower = Torque x RPM/5252

posted 11-10-2009 10:15 AM ET (US)            

Thanks for the time and mental torque employed sharing information about this, Whalers. As must seem obvious to many here, high school physics was more than a little challenge for me; and those guys with foot-long slapsticks* in scabbards hooked to their belts wandering the campus while I attended UC Berkeley in the '60s were living in another intellectual universe ;-)

Tony

posted 11-10-2009 02:54 PM ET (US)            

If you are the typical (all right, we know the typical part may not apply but it sounds good) adult male weighing 180 lbs and the typical length of the crank arm of a bicycle is 1 foot from crank center to pedal, then when you stand on the pedal with the crank arm parallel to the ground with the bicycle brake applied, you are providing 180 lb-ft of torque to the sprocket. Until you release the brakes, the bike won't move because it needs to have more than 180 lb-ft of torque applied to the sprocket to overcome the resistance of the brakes.

posted 11-10-2009 04:23 PM ET (US)            

Did you also know that a Watt is a unit of power? Almost everybody I talk to, (except fellow science and engineering grads), will say, "yebbut Watts have to do with electricity." Well, no, they do not. It's just that in this country, the electric power industry uses Watts as the unit for selling power. They could just as well put on your bill, "horsepower-hours" consumed as they do "kilowatt-hours" consumed. So, in our country, the auto and marine (and aviation) industry use horsepower to describe power, but the electric power industry uses Watts.

But there are other parts of the world where the 500hp BMW ///M5 is said to produce, 373kW of power, because 1,000 Watts or 1kW = 1.341 horsepower.

So next time you see your electric bill, just for fun, multiply the number of kilowatt-hours you used last month by 1.341, and you will see the number of horsepower-hours you used.

Fun!

posted 11-11-2009 01:20 PM ET (US)            

quote:

---

Horsepower has a stronger marketing appeal when it comes to power equipment. Watts is better left to the marketing of hair dryers and light bulbs.

---

Sure, in this country it does. Possibly in England, too. And possibly also amongst pockets of Europeans whose hobbies comprise vintage motor things. I have lived in Australia and New Zealand as a teen [and a car nut], in the 80's and, and even then, power on the new car brochures was given in kilowatts. What I don't remember is, if there were parenthetical notations of horsepower.

As I write this, I have a browser tab open to General Motors' Australian division, Holden, and am looking at their online brochures. Today, right now, horsepower is not even listed parenthetically. It's kilowatts and kilowatts only.

In my recent travels to France, [averaging 6 times per year], I see that newer OMC's represent well over there. I must say, it never crossed my mind to see if the convention is to list power in kW or horsepower. I'll check the next time I'm on the Cote d'Azur strolling one of the _many_ harbors.

posted 11-11-2009 10:41 PM ET (US)            

If I remember correctly it went something like this:

"Horsepower sells cars, torque wins races."

posted 11-12-2009 10:29 AM ET (US)            

Since, as JimH points out, the Outboard engine manufacturers don't publish the torque curves of their products, how do we know the rpm band for maximum torque?

I wonder if what I did with my boat might have accidentally arrived at something like figuring this out.

After putting 40 hours on my 2007 200 Dauntless/175 HP Verado I did several two way runs in flat water, making notes of GPS speed and gallons per hour according to the Smart Craft fuel flow meter.

I found that the nautical miles per gallon vs. rpm curve was virtually flat between 3500 and 4000 rpm, so that's where I run most of the time.

Could one of the engineers comment?

posted 11-22-2009 03:21 PM ET (US)            

1. Torque is rotational force that can accelerate a mass in rotation, just like a linear force that can accelerate a mass in translation. When applied to propellers, torque is linearly related to thrust. The constant of proportionality is defined by the propeller's characteristics. Thrust is the force that moves the boat. Thrust times speed is applied (horse)power.

2. Torque produced by an IC engine (optimally timed, with optimal fuel mixture, etc.) is linearly proportional to manifold pressure at any given RPM. The torque curves given in the above reference are for (I assume) full throttle operation (29-30" mercury for normally aspirated engines).

3. Peak full throttle torque coincides, more or less with the RPM at which the engine experiences peak volumetric efficiency, which is where it is the most efficient air pump. However, peak VE (the "sweet spot" referred to above) has nothing to do with fuel economy or specific fuel consumpton. Assuming a constant fuel mixture the engine uses the most, not the least, fuel (and air) per RPM at peak VE.

4. The "fact" that torque in foot pounds (some purists use pound-feet only) equals horsepower at about 5200 RPM is definitional and true only when the units of measurement are as above.

5. RPM is related to horsepower by roughly the cube. For instance if an engine produces 100 hp at 6000 RPM, at 4000 RPM it produces only 30 (not 66) horsepower. This result applies to static mounted engines, fixed-pitch airplane propellers at higher speeds, water pumps with impellers in pipes, and boats traveling fast enough that their total drag is proportional to the square of the velocity. (For boats going slower, where wave making resistance is significant, the relationship of power and RPM is usually an exponent between 2 and 3.)

posted 11-22-2009 05:39 PM ET (US)            

http://continuouswave.com/whaler/reference/graphics/150-HP_Engines.png

In a boat driven by a propeller, for a given propeller shaft speed there will be a required horsepower needed to turn the shaft. The relationship is known as the propeller power curve. In a REFERENCE section article I showed how this relationship is developed. Cf.:

http://continuouswave.com/whaler/reference/propellerPowerCurve.html

If an engine has more power available at a particular engine speed than is required to turn the propeller shaft, then it is possible for the engine speed to accelerate. The amount of reserve power determines how fast the propeller shaft speed can be accelerated. I showed this relationship graphically in that same article linked above.

posted 11-27-2009 10:43 AM ET (US)            

--operate the boat at maximum throttle, and note the engine speed obtained;

--assuming the engine speed is in the range of recommended maximum throttle speed, make the assumption that the engine is developing its rated horsepower;

--plot a curve of horsepower as a function of engine speed according to the relationship:

HP = C x RPM^2.7

where HP is the engine power produced at a particular RPM, and C is chosen from

C = Rated-HP / RPM^2.7

For example, if an engine is rated at 225-HP and reaches 5,500-RPM, we can deduce C from

C = 225 / 5500^2.7
C = 225 / 12,559,699,873
C = 0.000000017914

We then use C to deduce horsepower at other engines speeds according to

HP = 0.000000017914 x RPM^2.7

For example, if we are cruising along at 3,500-RPM, the horsepower being used to turn the propeller would be approximately