Diesel Power Curve Comparison! 2011 Power Stroke V-8 vs 2011 Duramax V-8

[SVTOffroad News Bot]

For the first time, we present the horsepower and torque curves for both the 2011 Power Stroke and 2011 Duramax diesel engines. There's a story to be told reading between the lines.

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[frogslinger]

Well one of those graphs is lying...

 

[FSM06]

I'm curious frog. Which one do you think is lying?

 

[frogslinger]

Dunno... however being as how HP, for all practical purposes (note I said practical purposes) is not a real thing, and TQ is, something is screwy.

The reason I know that one of the graphs is lying is purely mathematical

To wit;

HP= tq*rpm/5252 for any given rpm.

so if the ford TQ is higher at a given rpm, the hp MUST be higher at that rpm and vice versa.

By this RULE if we assume that the TQ curves are accurate then the GM must make more hp than the ford between ~900 RPM and ~2250 RPM. They must make the same hp from there up to ~2750 and thenthe GM should take over again.

If we assume the HP curves are accurate, then the GM TQ curves are too high at those same ranges.

Again this is based on math and how HP is CALCULATED. There is no way to measure HP directly... dyno's measure TQ and calculate HP using the method I formula above.

This is not an "I think ford or chevy is better" issue. I have no clue which is wrong, I just know that at least one line (and quite possibly all four) lines are wrong.

If anyone mentions different engine designs or flow characteristics etc to try to refute this, I simply say, you do not know what you are talking about. There is no way to make more HP at a given RPM and TQ... HP is for our purposes simply a number which is directly proportional to rpm x tq.

 

[MarkT]

More than you probably want to know about HP/torque

Again this is based on math and how HP is CALCULATED. There is no way to measure HP directly... dyno's measure TQ and calculate HP using the method I formula above.

Yes, there is a mathematical relationship between torque and horsepower. (I can derive the formula for you if you want to see where "(torque x rpm)/5252" comes from) And yes, those graphs look goofy.

But the mathematical relationship goes both ways. You can measure horsepower and calculate the torque.

Horsepower is simply a measurement of the *rate* of doing work. Someone (James Watt?) decided a long time ago that a "standard horse" (yes, the animal) attached to a harness and pulley system could lift 550 lbs of weight one foot off the ground in one second. That rate of doing work became the standard we call "one horsepower".

If the horse can lift the 550 lb weight at the rate of one foot per second, it has just achieved a one horsepower rating. If a second horse can lift the 550 lb weight at a rate of two feet per second, it is a two horsepower horse!

The second horse would have to be pulling twice as hard as the first horse to lift the same weight twice at twice the velocity... But did we need to measure that force to know the horsepower? No.

(Engines rotate and the measure of force is called torque. I don't want to get all mathematical here... but suffice to say that each 550 lb-ft of work an engine can do per second equals one horsepower.)

You can measure torque (force) and calculate HP or you can measure HP and calculate torque (force).

Brake dynos measure torque. Other types of dynos measure the rate of doing work (horsepower) and calculate the torque.

P.S. If BigJ lived in the 1800's, can't you just see him testing all the horses in town to see how fast each one could lift a 550 lb weight?

 

[frogslinger]

Believe it or not I knew all that... I said "for practical purposes" because HP in reality does not mean much in the real world other than for bragging rights purposes.

It is also impossible to accurately measure a cars HP output directly as a practical matter... there is no standardised available equipment to do it... unless you can find a way to drive a car straight up a vertical cliff at 1 engine speed... then do it again at another... and this still would not work because you would need maximum throttle at that speed so you would have to change the cars weight... as I said not practical.

On a side note humans can put out as much as 2.5 HP... for about a second... if they are olympic athletes...

The point of all this is that one (or more) of the graphs is wrong.

 

[MarkT]

Believe it or not I knew all that... I said "for practical purposes" because HP in reality does not mean much in the real world other than for bragging rights purposes.

It is also impossible to accurately measure a cars HP output directly as a practical matter... there is no standardised available equipment to do it... unless you can find a way to drive a car straight up a vertical cliff at 1 engine speed... then do it again at another... and this still would not work because you would need maximum throttle at that speed so you would have to change the cars weight... as I said not practical.

On a side note humans can put out as much as 2.5 HP... for about a second... if they are olympic athletes...

The point of all this is that one (or more) of the graphs is wrong.

Sorry, not true. There is definitely specialized equipment that measures horsepower.

If a horse was actually able apply 1100 lbs of force to lift that 550 lb weight, the weight would not be moving at a constant velocity but accelerating at a constant rate. The average velocity would be 2 feet per second in my example... but the horse would be accelerating. The horse would not be able to keep applying the same force as its speed increased. The force would drop off at some point. By taking multiple measurements of the position of the weight versus time, you could calculate the force and horsepower versus the velocity of the horse... Then you would would have a "force (torque) and horsepower" graph for that horse.

Some engine dynos apply an "inertial load" such as a heavy flywheel. All you need to measure is the change in rpm of that flywheel. By measuring the change in velocity (acceleration) of that known inertial load you can determine the horsepower.

Some chassis dynos also use this principle. This is why you'll see people claiming a change to heavier wheels and tires reduced their horsepower. Not true. What has changed is the new wheels have increased the overall inertial loading by an unknown amount. Same horsepower is now working against more load.

In contrast, a true "brake dyno" directly measures torque by applying a "braking force" load until the engine is held steady at the desired rpm and is not accelerating. A simple lever arm and scale can be used to measure the torque the brake is applying and if you know the rpm, the HP can be calculated.

Changing to heavier wheels and tires on a chassis brake dyno would not change the torque readings... but a different tire rubber compounds could. The problem with chassis brake dynos is getting enough traction on the rollers. And the tires can get really hot which adds rolling resistance and reduces the measured torque reading.

Horsepower is very real and relevant. It is the measurement of how much work can be done in a period of time.

Torque is simply a measurement of a static force.