Will my road speed increase at same RPM with higher torque?

AgLaw · Dec 1, 2008

Markvfl said: ↑

If a tree falls in the forest and no one hears it, does it make a sound??? JEEEEEEZZZZ...........
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Don't really know, but I have HEARD that question asked thousands of times! I've got some great ideas though. Would you like another long winded post of me explaining? I've got the logical, scientific, legal, and philosophical background to come up with a convincing answer!

Not tonight though...I've got a load to take early in the morning.

C16KIWI · Dec 1, 2008

AgLaw said: ↑

Don't really know, but I have HEARD that question asked thousands of times! I've got some great ideas though. Would you like another long winded post of me explaining? I've got the logical, scientific, legal, and philosophical background to come up with a convincing answer!

NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO.
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AgLaw · Dec 1, 2008

[chuckles...] I knew that'd elicit a response!

smd5231 · Dec 1, 2008

AgLaw said:

Anyway, what popped out to me is the 3.73 rears with LP 22.5's. That seems to be an awfully low gearing for the Acert Cat, and it could be the cause of your fuel mileage complaint--that's how I interpret your postings. By my calculations, at 58 mph, you would be turning around 1348 rpm, not 1300; which is fine if you were cruising at 60 mph all day long.
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Believe it or not, even though the revs per mile of my drive tires is "512", the actual number is nowhere near that! I had a mechanic play with the ECM's "pulse per minute" settings and he manually measured the tire - to make the speedometer more accurate. I think the RPM numbers tire makers put on the their web sites are for cases when the thread is practically gone. Until then, I'm doing 1300 rpm at 58 mph, even though the formula states different.

Aglaw said:

Cat recommends gearing the Acerts to run 1325 to 1400 rpm at 65 mph...
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They recommend 1325 rpm at cruise for 65 mph only if the engine's torque is 1750 lb/ft or higher. For my torque of 1650, the recommended gearing is 1400 rpm at 65 mph.

oldcornbinder · Dec 1, 2008

smd5231 said: ↑

The way my truck is set up, I do 58 MPH at 1,300 RPM. This is with 1,650 torque (CAT C15). According to the formula (HP = Torque X RPM / 5252) my CAT develops 408 HP at that speed. If I increase the engine torque from 1650 to 1850, the engine will produce 457 HP at 1,300 RPM instead of the previous 408 HP. Where does this extra horse power go? Will my road speed increase to 65 MPH at the same 1,300 RPM?
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Your kidding right?

If not:
max torque and max hp are measurements of potential power and work not the production of work. The rpm is the measurement of rotations of engine which equates directly to the rotation of the wheels through what ever gear your in of the transmission.

Therefore an increase in either torque or hp will do nothing to engine rotation just the potential to maintain that rotation

oldcornbinder · Dec 1, 2008

AgLaw said: ↑

Let's give the guy a break, there are those times that each of us has gotten into a rut of misunderstanding. That's why he asked the question, and to his (and everyone elses) benefit, I hope he steps away with a better understanding. For starters, vehicle speed is a direct function of engine speed and gearing, not horsepower or torque. Just because and engine can develop 1650 lb-ft of torque at 1200 rpm, does not mean it will always be producing that torque at that rpm. Same with horsepower...it varies with the fueling, and thus is the maximum power produced at a particular rpm with maximum fueling.

On to your power/economy question. Let me first say that I don't run any Acerts in my fleet. I have an "E", a "C", a "B", and an "A" model; but in no way does it qualify me as to the 'real world' of Cat's latest and greatest (well, however you classify it!). Anyway, what popped out to me is the 3.73 rears with LP 22.5's. That seems to be an awfully low gearing for the Acert Cat, and it could be the cause of your fuel mileage complaint--that's how I interpret your postings. By my calculations, at 58 mph, you would be turning around 1348 rpm, not 1300; which is fine if you were cruising at 60 mph all day long. However, at 1500 rpm, you would be going 64.6 mph--Cat's recommended maximum cruise rpm and thus, your maximum recommended cruise speed. Anything over 64 mph, and your gulping fuel!

Cat recommends gearing the Acerts to run 1325 to 1400 rpm at 65 mph, with a max cruise speed of 1500 rpm. You don't mention your normal cruise speeds nor your current average fuel economy, so I'll let you do the math. Here are some helpful gearing formulas:

Engine RPM = (Veh Speed in mph * Trans Ratio * Axle Ratio * Tire Rev per mile) / 60

Veh Speed in mph = (Engine RPM * 60) / (Trans Ratio * Axle Ratio * Tire Rev. per mile)

[FYI, the constant of '60' in each formula is for the conversion from minutes (as in revolutions per minute) to hours (as in miles per hour) or vice versa.]

Startability = (engine's clutch engagement torque * trans low gear * axle ratio * tire rev per mile) / (10.7 * gross vehicle weight)

[the '10.7' is constant, and make sure the gross vehicle wieght is your total weight, not just the GVWR of the tractor. It is usually 80,000 lbs, 85,000 lbs, or whatever your normal max weight will be. Make sure your low gear is the low hole ratio...for a super 13 (RTLO1x913A) it would be 12.31 (low forward), or 13.22 (low,low reverse). I don't know what the Acert clutch engagement torque is. The "E" model was 940, so I speculate somewhere between 940 and 1000. Perhaps one of the CAT guys can help you out with that number.]

In general, regarding startability, you want:
>14 -- turnpike applications
>16 -- general highway use
>25 -- on/off highway use
>30 -- severe on/off highway applications

I personally don't think going from 1650 to 1850 torque will help your fuel economy. As goes, it will take more fuel to develope the 1850 than 1650, and thus the power increase becomes a wash. I do acknowledge that there is that magical line with any engine where, when underpowered, it will use more fuel; however, I don't think you are anywhere near underpowered for 45,000 lbs.

Play with the numbers...going from a 3.73 ratio to something along the lines of a 3.58 and see how that affects your engine rpm at your desired cruise speed. Unfortunately, gearing changes don't come cheap. With that in mind, you could simply change tire size from a LP22.5 to conventional 11R22.5's or LP24.5's (around 493 rev/mile), or even conventional 11R24.5's (around 471 rev/mile). You just might find reason to switch to a larger tire when it comes time to buy new rubber.

Good luck.
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I'm in awe!

Wish I could delete my post now

kent0242 · Apr 29, 2012

AgLaw said: ↑

Let's give the guy a break, there are those times that each of us has gotten into a rut of misunderstanding. That's why he asked the question, and to his (and everyone elses) benefit, I hope he steps away with a better understanding. ....

Good luck.
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A good posting to answer the question and clear up a lot of mistaken beliefs by many truckers out there.

highside · Apr 29, 2012

AgLaw put it best...your engine has the POTENTIAL of putting out it's rated torque/horsepower at any given RPM. Increasing the torque/hp rating of the engine will only increase it's maximum potential - not what it is putting out at part throttle.

SuperLiner · Apr 29, 2012

smd5231 said: ↑

The way my truck is set up, I do 58 MPH at 1,300 RPM. This is with 1,650 torque (CAT C15). According to the formula (HP = Torque X RPM / 5252) my CAT develops 408 HP at that speed. If I increase the engine torque from 1650 to 1850, the engine will produce 457 HP at 1,300 RPM instead of the previous 408 HP. Where does this extra horse power go? Will my road speed increase to 65 MPH at the same 1,300 RPM?
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WhaT!!

MNdriver · Apr 29, 2012

um...

The OP is over 4 years old.

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