GM doesnt currently offer the 6.6 gas in medium duty trucks so there is no dyno certification numbers yet.

 

I am correct in what I state. I am not dismissive of HP, but HP is NOT what moves a load. FORCE is what moves a load. And force is a component of torque; accurately describing torque as this: T = F x D, where F is force and D is distance of the lever arm.

HP is only a measure of how FAST work is done. But to understand how much load any engine can pull, you have to understand what the torque value is.

Also, I stated that we're comparing engines; all other inputs are to be said as equal (trans gears, diff, etc). I do fully understand the value of gearing. I have a complex Excel tool I wrote that allows one to evaluate the effect of HP and torque, relative to gearing inputs, etc. If you want to PM me, I'd be willing to discuss it with you. The point we were discussing was engines, not the entire drivetrain effect. So my comments are completely accurate and valid.

And we're not talking about the diesel engine here; this is the 7.3L forum. Gas engines do not have the same characteristics for torque curves that diesels do. Gas engine have a peak torque well below peak HP, when means the torque rises as you fall away from peak HP.

 

"Gas engine have a peak torque well below peak HP, when means the torque rises as you fall away from peak HP. "

Read that out loud a few times and go back and edit your post before someone notices.

Serving suggestion.

 

Too late....

 

Sorry - I was in a hurry prior to leaving for church services.
Gas engines have a peak torque well below peak HP, which means the torque rises as you fall away from peak HP.
What I'm describing in the rise in magnitude of torque, relative to the drop in rpm. As rpm's drop away from peak HP, the torque will rise until it reaches it's peak, and then it will fall away as the rpms continue to fall away.

People often think that revving up an engine will make it move more load; if they rev her up to the peak HP, they'll be able to "pull" more weight. This is patently untrue. The most load can be moved when at peak torque in rpm. Staying as close to that as possible is the assurance to moving the load. (again, ignoring the discussion of gearing because for any gear selected, the same concept still applies.)

Most simply stated, any object is moved by force. To create that force, we use the engine torque and gear multiplication to produce a thrust force. That thrust force will be maximized at "peak engine torque", for any gear selected.

 

" Gas engines have a peak torque well below peak HP, which means the torque rises as you fall away from peak HP."

Sorry, I can't find a single dyno graph to support that. While we usually find peak torque below the 5252 RPM where torque equals HP, it is possible to find engines where both torque and HP continue to climb beyond that speed.

I disagree about running the motor above peak torque towards peak HP. Even my old 300 would accelerate faster and faster as rpms climbed towards the HP peak.

What have you ever driven that would hold speed better at the torque peak than at the HP peak? What have you ever driven that was quicker if you kept the engine at the torque peak vs. HP?

 

Torque is simply a force being applied. Horsepower introduces a time component which in effect introduces an acceleration component. The acceleration component is what pushes you back in the seat. Force = Mass x Acceleration.

 

This conversation is so great I think I'll unsubscribe from the thread. You guys have fun.

 

No kiddin! Like a bunch of NASA rocket scientists in here...

It's not hard to figure out 935ft-lbs > 475ft-lbs

 

Where is that in question?

 

You guys can argue this until your blue in the face but until I see a what it's doing at the rear wheels were the power meet the pavement it's just numbers.

When I went from my 2000 V10 2 valve to a 6.2 with the same 4.30 gears the HP numbers were higher and the torque was lower and the 6.2 is a better pulling setup with the 6 speed over the 4 speed and I'm sure the 7.3 will be the same way with the 10 speed, it's all about the power at the pavement not at the crankshaft.

Denny

 

" the torque rises as you fall away from peak HP..."

Perhaps I am misunderstanding here. This makes sense if you are reading the HP/TQ graph from RIGHT to LEFT as RPMs drop, instead of left to right as RPMs rise.

 

You say a truck pulls hardest (max torque to the ground) at peak engine torque.
I say a truck pulls hardest at peak horsepower.

I agree with you that if you hold a truck in a particular gear, it will pull hardest at peak engine torque. So at max torque, its pulling as hard as in can in THAT gear.
But, in practice, when you are pulling a load, the transmission allows you to downshift and bring the engine near max horsepower. In that case, engine torque will be lower than max torque, but the gear reduction will be higher, delivering a higher overall torque to the rear wheels. Of course there are limits to how well this works, particularly on older transmissions with only 3 or 4 widely spaced gears

In your example you are holding the gear and varying the speed as RPM climbs.
In my example I'm allowing the trans to vary the gear. This allows the transmission to do its job by holding low gears longer, or downshifting to a lower gear to take advantage of the additional torque multiplication of the lower gear.

As an example of this I'm repeating my calculations from post #92 to show how more torque is produced at the rear wheels at Max horsepower rather than max torque:

"When you consider the effects of a transmission, trucks pull the hardest (most torque delivered to the rear wheels) at peak horsepower, not at peak torque (unless you are in first gear because you cant downshift any further.) Lets take the 6.7 PSD as an example. Assumptions: 450hp at 2800rpm, 935lb-ft at 1800rpm, no torque management, 844 lb-ft at max horsepower from T=(450X5252)/2800

Torque at the rear wheels at max engine torque = 935lb-ft x 1.516(3rd gear) x 3.55 = 5032 lb-ft @34 MPH
Torque at the rear wheels at max engine horsepower = 844lb-ft x 2.318 (2nd gear) x 3.55 = 6945 lb-ft @34 MPH"

The more gears the better this works. if you had a perfect CVT and you compared pulling a load at max torque vs max horsepower the truck would always pull harder at max horsepower.

My above example is a diesel and this is a gas thread. We don't have gear ratios yet for the new 10 speed or I would have gone with the 7.3/10 speed. The concept is still the same.

 

Denny you hit the nail on the head. It's torque at the rear wheels that matters, not max engine torque. If we are talking about a truck pulling a load we have to look at the complete system: Engine, transmission, differential, horsepower etc.

 

This is about two different topics, Shamrock, that you're confusing. I'll elaborate ...

There is a difference between what is needed to move a load, and how fast it can be moved.

If you are pulling a load uphill (lets say 10k pounds towing), and there is some grade to that hill (say 5% grade), there is a load requirement that must be overcome to move that load uphill. It takes force to overcome force. To move the load uphill, you must overcome the resistance of the load wanting to move downhill. I think that's simple enough for anyone to understand. To generate that uphill force, we use engine torque multiplied by the gearing of tranny and diffs, and then divided by the wheel radius. That results as thrust force at the road surface.

Anytime you are at a condition LESS than a max force requirement, then HP will move a load faster. The more rpm you can generate towards peak HP (but not past that rpm), the "quicker" it will move uphill. However, as the loads increase (either the weights get larger and/or the hill gets steeper), the more you need to be at peak torque to produce the max amount of available thrust for any given gear selected. With lighter loads or slighter slopes, you can sustain a higher velocity because you can sustain a higher gear selected; that's all.

Typical gas engines produce their peak torque at a lower rpm than their peak HP rpm. Nearly always, that relationship results in an effect called "torque rise" as you fall away from peak HP towards peak TQ. Think of the rpm band between peak HP and peak TQ as the "power band". Your power-train will ALWAYS be able to produce the most thrust force when the engine is a peak TQ; period. So starting at peak HP, as the load increases (either more weight or more slope), the engine will come down in the rpm band towards peak TQ; the torque will "rise" in magnitude as the rpms drop. As long as there is sufficient TQ present for the force requirement, then the vehicle will stay in that gear at that road speed. If the force it too great, the trans will drop down a gear and the process starts all over again.

TQ is about how much work can be done. HP is about how fast that work can be done. All engines (gas and diesel) can do the most work at peak TQ; they can move the largest load when the engine is at peak TQ, for any given gear selected as long as the gear selected can provide a force element larger than the resistant force load.

This is the true, physics explanation of how things move.

Please read my post (#6) in this thread for more detailed explanation:
https://www.ford-trucks.com/forums/1...que-value.html