ViribusUnits

Does anyone know what the curves look like on our stock cars?

The reason I ask is because when I got to looking around at US Detroit, most of it's torque was measured at a lower RPM.

It would seem to my simple mind that the most torque would come when the engine's got the least drag on it. Since the forces friction caused by rubbing metal, and compreasing springs should remain the same at all rpm, the single biggest effect on torque would be restrictions on the air flow of the engine. Keep the flow to the minimul, you keep the restriction to a minimul, and the resulting subtractions on torque to a minimum. Thus it would seem to me that at much lower rpms, the engines should have more tork, untill you reached the point of trying to lug the engine. Certainly before we made it to 4,000 rpm which is where our cars torque was measured for the manuals.

Mean while, hp is at it's best when the largest amount of air/fuel mixture is "processed" or near the rev. limit of the engine.

As an aside, what do DIM, and SAE mean?

Yes ? No? Am I compleatly off the wall?

Thanks

Rich9928p

If you do a search on the topic, you'll see that HP is just a calculated number based on torque measured and engine RPM. The formula is

HP = torque * RPM / 5252

The equation tells us that at 5252 RPM HP is the same number as torque.

Keep in mind that we "small minded" Americans are the last hold outs for our antiquated systems of measurement ... why, are we lazy? I just don't know. If we were in tune with the rest of the world (and with the scientific community) we would be using the metric system and put Fahrenheit, inches, feet, yards, lb-ft, slugs, quarts, gallons, acres, fathoms, lbs., ounces, bushels, pints, cups, and other obsolete measurement in the museum where they belong.

We would express the power of our cars in kW (kilowatts) and torque in Nm (Newton-meters). We would have zero to 100 km/hr times. We would have a simple and logical system for measuring distance, volume, mass (not weight), and temperature.


You will find detail about HP computation this URL

<a href="http://www.revsearch.com/dynamometer/torque_vs_horsepower.html" target="_blank">http://www.revsearch.com/dynamometer/torque_vs_horsepower.html</a>

Rich9928p

As an aside, what do DIM, and SAE mean?

&gt;&gt;&gt; I think you mean DIN, or Deutsches Institut fur Normung

SAE stands for Society of Automotive Engineers

There are many "Standards" and standards bodies for different things,

DIN - JIS - ISO - SAE - NIST

You're probably wondering what is SAE HP or DIN HP is. Each body defines "standards" of how power should be tested. They are slightly different so specs often don't match even for the same engine. Europe has higher octane fuel, so newer cars even generate more HP in Germany than the US because their computer control can use more spark advance than in the US.

How can you compare an HP measured in Alaska at sea level at - 30 degrees F, 3% RH and 30 inches of mercury barometric pressure to an HP measured at 5000 ft., 98 F, 88% RH and 27.6 inches of mercury barometric pressure? Well, the SAE has correction factors that are supposed to enable an "apples to apples" comparison.

In the past car manufacturers measured HP at the flywheel with no accessories (not even a water pump). That is why older cars have such high HP ratings ... a lot of hot air was used. Naturally disconnecting all of these extras inflated the HP, since no car would drive without accessories. In an effort to better communicate, there was a term NET SAE HP that required accessories to be included in HP measurements.

jpitman

What happened to the definition I learned of

HP = Ft/lbs * RPM / 33,000 in minutes

or

HP = ft/lbs * RPS / 550 in seconds.

I cant quite grasp why there should be such a constant as 5252 as the crossover point of torque and HP curves. Is it the same in metric units - CV, nm ? If not why not?

jp

ViribusUnits

Intersting.

I long ago concluded that the US is way to stubern to ever adopt a logical measurement system. If we were lazy, we would notice how much easer SI is to use, and go with it. If we were smart, we would notice how much more logical it is. But we're not, we're way to stubern to adopt ANY thing pionered by the French. (Kinda like the French have trouble adopting anything pionered by the US. :-))

Intersting artical. From my experence in college so far and the sgi system, it's basicly correct. I was thinking it was something like that, but wasn't able to nail it down.

Thanks

It still doesn't answer the exact problem with the formula. Hp is greatest when rpm, and torque are the greatest, BUT torque isn't constance. Where in the engine curve is the torque typicaly highest?

Rich9928p

JP, read the link. You'll see that it all works out.

Power

Power is a measure of how much work a machine can do in a unit of time.


The formula for power is:

Power = (Torque x Speed) / Constant

The value of the constant changes depending upon the units that are used for torque.

For Metric the equation is:

Power (kW) = (Torque (Nm) x Speed (RPM)) / 9551

SteveG

Rich, as usual, you have provided an excellent explanation, thanks.

John..

Horsepower is a rate of doing work. In an engine it is the combination of speed and torque. For elecric devices, it is the combination of current and voltage.

1HP=550ft-lb/sec

Lets say a motor makes 300 ft-lbs of torque at 4000 rpm and 200 ft-lbs at 6000 rpm:

HP @ 4000 RPM: 300 ft-lb*4000 rev/min*2pi Rad/Rev*min/60 sec=125,663.70 ft-lbf/sec

125,663.70 ft-lbf/sec*1HP/550 ft-lbf/sec=229 HP

At 6000 RPM, HP=229 HP

As can be seen, the power is the same, even though the torque has dropped off. Half the torque and twice the speed gives the same HP. This is why an engine can make a lot more power, even after the torque has dropped of significantly. Can you say Formula 1???

The 33,000 and 5252 numbers are unit conversions only and do not descriibe any interaction between the two variables.

2Pi/(60*550)=5252

ian928s4

I am looking at the dyno chart for my 87 s4 5 speed. I have headers, exhaust, and a rising rate fuel pressure regulator. My peak rear wheel torque is 307 lb/ft at 3300 rpm. At 2600 rpm, my engine is producing 272 lb/ft. The torque continues to rise up to 3300 rpm where it reaches its peak. Also, on my chart Hp and torque are indeed equal at 275 hp and 275 lb/ft at 5252 rpm.

Ian
'87 s4 5 speed

jpitman

So in metric units, the torque/kw curves cross at 9551, but in Hp/ft-lbs, they cross at 5252?

Get outa here! They are the same curves !

What has Pi got to do with the calculation of power? time rate of doing work has no circles in it.
If a gang of guys (as in a tug-of war) pull a rope over a pulley, lifting 550 lbs up 1 foot in 1 second to give one horsepower, is the diameter of the pulley relevant? Crap it is!

John..

Are you questioning my calculations? The 5252 is a unit correction factor. Pi is in the calculation because there are 2 Pi (6.28) radians per revoultion. This is what gets rid of the revolutions in the calculation. As I said before the 5252 is a unit correction factor, that is it. It has absolutely nothing to do with the torque or horsepower, or how their respective curves relate to each other. HP from an engine is: Engine Torque* Engine speed, pure and simple.

Rich9928p

JP, you are correct, the equation that I posted was wrong. It is correctly stated as:

For Metric the equation is:

Power (HP) = (Torque (Nm) x Speed (RPM)) / 9551

This equation takes into account the difference between Nm and lb-ft.

Rich

ViribusUnits

OK, now to ask about something thats bothering me.

The 80 to 82 US 4.5l cars are suppost to have 265 ft/lbs of torque at 4,000 rpm.

The 83 to 84 US 4.7l cars are suppost to have 263 ft/lb of torque at 4,000 rpm.

This REALY bugs me. How does my larger displacement, higher compression engine have less torque at 4,000 rpm than the smaller earler engine?????

Why???

Thanks

ian928s4

Just to clarify: Horsepower and torque are equal at 5252 rpm. This is because horsepower is a function of torque and rpm.

Ian
928s4 5 speed