jmzvet

Went to a dyno day yesterday and several of the graphs showed HP & TQ crossing at an rpm other than 5250rpm (5800, 4500, etc.)
Isn't it mathmatically impossible for this to occur? Was the dyno screwed up? My numbers seemed about right but others were a bit strange. 2 new Z06s were there. Both bone stock. One did about 450hp and the other 400hp?

Sully C5

All 6 of mine with two different engines intersected at 5250.

Patches

It can happen if the dyno operator shifts the axes relative to each other as you can see in the dynograph below. Notice the values of the left and right scales - HP and TQ, respectively - do not match up. If the scales are properly aligned, the plots should cross at 5252.

Btw, curious as to why one of those Z's dynoed so low - any insight into that?

Y2Kvert4me

The relationship between horsepower and torque is mathematical:

Horsepower = torque x rpm / 5252

Chassis dynos measure torque and rpm, and calculate horsepower using the above equation. Because of the 5252 constant, this is always the point where the tq and hp curves cross.

RockyL32

If that formula is true why do I see some engines making more horsepower than torque and some that make more torque than horsepower? Im not questioning you, Ive just been told that some mods give more horsepower at the price of toque.

Shane_S

My LT1 camaro made 380rwtq and 370rwhp. I think it has to do where each one peaks. *shrugs*

Horsepower is how hard you hit the wall. Torque is how far you take the wall with you

Shane

indy300

That explains a lot Without a disertation, does anyone know where the constant 5252 comes from or is related to? Or is that one of those goofy numbers like pi....

Y2Kvert4me

Actually, it does involve PI.

If you think back to all those wonderful terms you learned in physics class...and the fun times you had learning equations in algebra class...

Torque is a force.
Hp is power.

Power = force X distance/time.

James Watt invented the term and value for horsepower...1 hp is the equivalent of moving 150 lbs a distance of 220 feet in 1 minute.

150 pounds of force is the torque.

Since the "distance" part of the formula is in revolutions, (the engine produces rotary motion), and not linear as in moving something 220 feet...the force of 150 pounds is "applied" tangentially to a one foot radius circle. This would be 150 foot pounds torque.

The time part is minutes.

So we need to express 220 feet in one minute as RPM.

The circumference of a one foot radius circle is 6.283186 feet. ft. (Pi x diameter 3.141593 x 2 feet)
The distance of 220 feet, divided by 6.283185 feet, gives us a RPM of 35.014.

We then figure 150 pounds of force (150 foot pounds torque) that turns 35 RPM, is one horsepower.

Constant (X) = 150 ft.lbs. x 35.014 RPM / 1hp

35.014 x 150 / 1 = 5252.1

5252 is the constant.

So then hp = torque x RPM / 5252


You probably regret asking now.

NEED FOR SPEED

You da man!!!!

indy300

That explains it perfectly. That was the disertation I was talking about! At least you didn't say "area under the curve" or "at the rear wheels" that gets thrown around a lot here

Hardcorvette

ALWAYS 5252!

edensknight

Well said, Tom (Y2kvert4me)!

slwhite

Without a dissertation, it is simply a scale factor which matches the units that we are interested in: RPM, HP, and foot-pounds. If we were somewhere else, they may be interested in RPS (revolutions per second), watts (power), and newton-meters (torque). In this case there would be a different scale factor.

MY1STPEWTER

No questions around here go unanswered.

slwhite

Whoever told you that is mightily confused. Torque and horsepower are often confused. When someone says that an engine has a lot of torque, what they really mean is that it has a lot of low-RPM power. When an engine is modified for more peak power, usually this is at the expense of low-RPM power. This is where the "area under the curve" statements come from. A tradeoff is made with the mods that shifts low-RPM power to a higher RPM resulting in a higher peak power. This results in an engine with a narrower power band and requires a lot more shifting.

Consider this simple example. Let us replace the LS7 engine in a C6Z with a different engine. The new engine will consist of me and a 4 foot wrench. If I pull on the wrench with 125 pounds of force, I am generating 500 foot-pounds of torque at the input to the transmission. This is more than the LS7, but how much power am I producing? Let us assume that it takes me 1 minute to turn the wrench 1 time. Then, using the HP-torque equation, I am producing about 1/10 HP which is not much.

You can see from this simple example that merely saying that an engine has a lot of torque is meaningless unless you specify an RPM with it. It also shows that you can have a very large peak torque value and still only have a tiny amount of power. You can make the wrench as long as you want and generate as much torque as you want, but the power will depend on how fast you can turn that wrench.

jdmvette