DP Torque/HP

[JpalmerCass]

Originally Posted by DirtRider

You got a big chuckle out of me on that, well done.

I can't take credit, it comes from an old SNL skit of a game show called "Common Knowledge".

[paddler1954]

We had an 08 Expedition, 36 footer, with the Cummins 6.7L and we pulled a 4,000 pound toad. Our travelling buddy had a 37 foot, V10 gasser. Both rigs had about the same performance envelope. I could usually climb the hills a little faster but not by enough to brag about. Of course the big difference was the ride quality of the DP was superior to the gasser. Quieter and smoother with the air suspension. We now have a 41 footer with the Cummins ISL motor, very sweet, plenty of hill climbing power and still has reasonable fuel economy.

[DirtRider]

I tried to figure out the best way to explain the math on this. Here's what I came up with, I hope it helps explain the situation.
So, say you have two similar motorhomes. We'll ignore weight here, but assume they're similar. One has a Cummins L9 (450hp @2100rpm, 1250lb-ft of torque @1400rpm), one has the new Ford 7.3 Gas (430hp @5500rpm, 427lb-ft @4000rpm). Note I cheated right there--I wanted a close power comparison, so I used the power specs for the Ford Pickup version of the engine, NOT the motorhome engine.

Both our RVs have an ideal transmission/rear end ratio combination. As in, at whatever speed we decide to calculate, we have a total gear reduction that achieves our target engine rpm. For my example, I'll calculate at 45mph, so the gas engine has the proper gear reduction to be at 5500rpm at this 45mph, and the Cummins will be at 2100rpm at 45mph. We're assuming no frictional losses in the drivetrain. Again, that's not really possible, but hopefully it demonstrates the point.

The gear ratios we'll talk about, again, are TOTAL. As in, the ratio of how fast the engine is turning to how fast the tires are turning. This total gear ratio is multiplied by the engine torque at that rpm to get torque at the wheel, then divided by the radius of the tire to convert to thrust applied to the ground. This thrust is the actual force we have available for acceleration, holding speed, whatever. All of this is of course calculated at 100% throttle, since that's what we're trying to compare here. For each engine, you'll see two gear ratios and two resulting thrusts. One is calculated at Peak HP RPM, and one at Peak Torque RPM (again, our perfect transmission allows our Cummins L9 to be at either 2100rpm for this 45mph climb, or 1400rpm, whichever we want).


As you can see, in each case, more thrust is created at the peak HP rpm than the peak torque RPM. That's because HP is the measure of how quickly work can be done. Also, the gas engine is very close to the Cummins L9, because 430HP is pretty close to 450HP, even though their crankshaft torque is world's apart. Also, the Cummins X12 (425HP) that makes very high torque at very low speeds--it's slower than both of the other engines, because it makes less horsepower.
Our best guess for how quickly an engine can do work is represented by Horsepower. Torque absolutely matters--as has been said, you can't have one without the other. But the torque that is most representative of how well something will climb a hill is the torque applied to the wheels, not the torque at the crank. Horsepower effectively describes how much torque can be applied to the wheels at a given ground speed. Crankshaft torque does NOT describe that, since it ignores the rpm aspect of HP.

I'm sure almost no one has been willing to read this whole diatribe. But, in case you have, thank you.

[Jzack]

DirtRider- I actually did read it. Your Welcome. Not sure I understand it correctly. The way those numbers make sense to me is that the MPH is left out of the equation. The gas motor has a much higher "total gear ratio" so it has to be traveling MUCH slower speeds in order to match the "ground applied thrust" of the bigger diesels.

Back in the real world, I drive a 12.7 liter Mack with very similar HP and torque numbers as the Cummins X12. I know from experience my truck will hold a hill much better at 1250rpms than 1750rpms in the same gear because 1250 is where the torque peak is. Almost all OTR diesels pull like this. For those that claim there rig climbs better by downshifting early to a higher RPM, that's because your in a lower gear giving the engine better leverage.

[DirtRider]

Originally Posted by Jzack

DirtRider- I actually did read it. Your Welcome. Not sure I understand it correctly. The way those numbers make sense to me is that the MPH is left out of the equation. The gas motor has a much higher "total gear ratio" so it has to be traveling MUCH slower speeds in order to match the "ground applied thrust" of the bigger diesels.

Back in the real world, I drive a 12.7 liter Mack with very similar HP and torque numbers as the Cummins X12. I know from experience my truck will hold a hill much better at 1250rpms than 1750rpms in the same gear because 1250 is where the torque peak is. Almost all OTR diesels pull like this. For those that claim there rig climbs better by downshifting early to a higher RPM, that's because your in a lower gear giving the engine better leverage.

Thanks for reading! And I agree that if you cannot adjust gearing, then the engine works best at its peak torque. If you can shift--like you said, you can take advantage of the additional leverage the transmission provides, and can typically put more torque to the ground with the higher rpm in a lower gear.

For the speed portion, I meant that I kept that constant across all 3 examples (45mph). I was using the same tire height for all 3 (37.3"), which gives a tire RPM of ABOUT 417 RPM. The "total gear ratio" is simply engine rpm divided by tire rpm. So the engine rpm divided by that 417 rpm gives the total effective leverage the engine has on the tire. So, crankshaft torque times gearing leverage= wheel torque. Wheel torque divided by tire radius = ground-applied thrust.

[n2zon]

The whole torque/HP thing confuses everyone.

HP is a measure of work and how fast it can be done. One HP will lift 550 pounds over 1 foot, in 1 second. More HP means you can climb at a higher speed, assuming the gearing is reasonable and lets you use it (which we ought to be able to rely on).

Torque is a measure of force. It's more useful for acceleration than getting up hills. F=MA, and F (force) can be calculated from the combination of engine torque, gearing, and tire size. M (mass) is what the rig "weighs" and A (acceleration) is what's provided by the relationship between F and M, and gearing, which provides torque multiplication (which can increase or decrease what's at the driving wheels). The area under the part of the curve you can use (given the gearing) is more important than the peak.

Just to confuse things further, HP = (Torque * RPM) / 5252 (grin).

[Jzack]

Originally Posted by DirtRider

Thanks for reading! And I agree that if you cannot adjust gearing, then the engine works best at its peak torque. If you can shift--like you said, you can take advantage of the additional leverage the transmission provides, and can typically put more torque to the ground with the higher rpm in a lower gear.

For the speed portion, I meant that I kept that constant across all 3 examples (45mph). I was using the same tire height for all 3 (37.3"), which gives a tire RPM of ABOUT 417 RPM. The "total gear ratio" is simply engine rpm divided by tire rpm. So the engine rpm divided by that 417 rpm gives the total effective leverage the engine has on the tire. So, crankshaft torque times gearing leverage= wheel torque. Wheel torque divided by tire radius = ground-applied thrust.

Ok I think I see what your math is saying now. A high revving, low torque engine could do the same amount of work at the same ground speed as a low revving, high torque engine because it can take advantage of a much lower gearing ratio (torque multiplication). The key variable is the magic transmission (basically a CVT) that puts each motor in the perfect gear. I wonder if the numbers would change much if you changed the speed to say 70 MPH?

Still seems crazy to me that your numbers say a 7.3L gas engine can put more thrust to the ground at 45 MPH than a 12L diesel could. Is that right?!!

[momcat]

You are correct...The hp vs torque is confusing, and has gotten VERY confusing. My bus is exactly like yours in size and weight, I have the 400 ISL. My question to you is a simple one...I think. If you enter a 3/4 mile, 7 degree incline...and a bus identical to yours was beside you...what production GAS engine could he have in his bus that would out perform your bus up this grade. This Hillbilly might be able to understand this. As stated, my bus is the Allegro Bus 40 QDP, 2007. Just like yours. Thank you very much in advance, for hopefully a simple answer. MOMCAT

[KanzKran]

DirtRider is exactly correct. It's all about horsepower. Climbing a hill at a given speed, weight, rolling resistance, wind resistance, etc. takes a certain amount of power, and on the macro level, that's all that matters.

Where it gets confusing is that published performance figures are peak values, which happen at two different engine speeds. Some engines have much larger max torque values than the torque at rated power ("Torque Rise"; a term that doesn't seem to be used much any more), like most 4-stroke highway diesels these days, and some have very little, like all the old 2-stroke Detroit Diesel offerings.

The higher the torque rise figure, as a general rule, the flatter the hp curve over the engine's operating range, and the flatter the hp curve, the fewer gears you need to keep it in the 'power band'. The most extreme example I know is the Mack Maxidyne engine of the 1970's and 80's. It had essentially a flat hp curve over more than the top half of it's range, which results in a very straight, steep, and long torque curve over the whole range.*

That engine could haul an 80,000 lb rig on 237 hp with only a 5-speed transmission. You'd have to drop down to 4th for most hills, and grind along at 35 mph in the right lane with the 4-ways flashing, but it only needed five speeds for the full speed range.

A huge torque rise/flat hp curve isn't really necessary if you have more gears to select from. The opposite extreme was the Detroit Diesel 2-strokes, with their narrow power band. 13- or 18-speed Road Ranger trannies were required, as you only had real power (for the engine size) in the last few hundred rpm; you had to keep the revs up or you'd slow to a stall on anything but a down hill. Worse than most gas engines. Horrid engines in trucks, but great in stationary machines where you're working at or below the available power at a constant engine speed. Like pumps, generators, compressors, etc.

Someone mentioned it way back in this thread, but big published torque numbers go with big published hp numbers, as a general rule. So the perception may be that big torque drives easier, but for the most part that's because the rated hp figure is big, too. Have enough gears available to keep it high in the power band, and it'll climb hills like nobody's business. But whether it's a 25,000 rpm turbine, or a 1200 rpm diesel, geared to the same drive axle rotational speed, if they're outputting the same power, the're climbing the same hill in the same weight RV at the same speed.

*hp x 5252 / rpm is torque at rated power. Max torque minus rated power torque divided by rated power torque (times 100%) is the 'torque rise' value, and is usually around 20-25% for modern 4-stroke highway diesels. Or just divide max torque by rated power torque and subtract 1 - close enough. The DD 8V71NA has a torque rise value of only about 7.6%, which is why they were coupled to 13 and 18 speed Road Ranger trannies.

Curves are of Mack Maxidyne at 237 hp, and Detroit Diesel 8V71NA (truck version) usually referred to as the 318 (hp).

[Rod Fosback]

Originally Posted by Onthehunt49

We are shopping for first DP motorhome. We presently have gas southwind.
I keep reading about varied torgue ratings and that some people dont like the Cummings with 660 @ 1600 RPM.

Looking at 35-38 ft DP pulling 4000-6000 lb vehicle.
What would be min HP and Torgue I should be looking for?

Appreciate all info and education. I know I have alot to learn and process.

Cummins or Cat ? Is there major differences I should be concerned with ?

Thanks in Advance.

This past Sunday we were Coming back from the coast on HWY 20 from Newport Oregon we hit a passing lane and I ended up "shifting" down to 3rd and at 1800 RPM i was pulling 1157 FLBS of Torque, and was pulling the hill at 55 MPH.
I have a 2005 Alpine 36' FDDS with a 400HP 8.9L Cummins.
We were not pulling our boat or or pickup so we were just the coach (28,000 LBS) but I am so pleased with the performance of this coach!!

if you have not found a coach you might put ALPINE on your list.

Rod and Jackie Fosback
2005 Alpine 36' FDDS(Front Door Double Slide), 400 HP 8.9L Cummins, second owner.

[n2zon]

Originally Posted by momcat

You are correct...The hp vs torque is confusing, and has gotten VERY confusing. My bus is exactly like yours in size and weight, I have the 400 ISL. My question to you is a simple one...I think. If you enter a 3/4 mile, 7 degree incline...and a bus identical to yours was beside you...what production GAS engine could he have in his bus that would out perform your bus up this grade. This Hillbilly might be able to understand this. As stated, my bus is the Allegro Bus 40 QDP, 2007. Just like yours. Thank you very much in advance, for hopefully a simple answer. MOMCAT

Getting a coach up an incline is a "work" question, not a "force" question. So my answer is, "any gas engine that makes 400HP." (Someone else said something very like this earlier but I'm too lazy tonight to go back and see who it was.)

The caveat is that the gasser will run at much higher RPM to make that HP, and the gearing will be a lot different to make it climb the same as a 400HP diesel. What's best for a 7% grade between the two will depend on that engine/transmission/differential combination. 6.5% and 7.5% grades would yield different answers. So it's quite possible that on one hill the gasser combo would win while on another the diesel combo would win, just because the gearing was better for one or the other for that particular grade.

[Tha_Rooster]

Originally Posted by Ken Thompson

Stay away from CAT. When ever a coach has engine troubles, it’s a CAT. They’re extremely expensive to fix. That is if you are able to find someone to work on them.

I don’t know where you got your info but they all have the same number of problems just saying.

[n2zon]

Originally Posted by n2zon

Getting a coach up an incline is a "work" question, not a "force" question. So my answer is, "any gas engine that makes 400HP." (Someone else said something very like this earlier but I'm too lazy tonight to go back and see who it was.)

The caveat is that the gasser will run at much higher RPM to make that HP, and the gearing will be a lot different to make it climb the same as a 400HP diesel. What's best for a 7% grade between the two will depend on that engine/transmission/differential combination. 6.5% and 7.5% grades would yield different answers. So it's quite possible that on one hill the gasser combo would win while on another the diesel combo would win, just because the gearing was better for one or the other for that particular grade.

I missed "outperform" in your post. So "any gas engine that makes 401HP" is a better answer!

[stuccosteve]

So maybe 25 years ago we are chugging along in my 40’ Southwind tag with a 454 under the hood. Just so happens we are climbing the East side of the Eisenhower tunnel. No toad just a fully loaded Spartan chassis with 6 family members onboard. Having passed this point before in underpowered rigs I was impressed that I only dropped down to 2nd gear At around 3600 rpm, we maintained A speed of maybe 20-25 with everything pretty much cranking at max V, by the summit my wife was pushing her feet through the dash trying to coax more velocity, wasn’t gonna happen, we just screamed through the crest thinking “holly crap, the motor is really pulling!!!

So fast forward 20 years and I happen to be looking at the same grade around the same time of the year but with the Navigator packing a 500 Cummins. Question being what will another 25,000 lbs on the GVW and a big block diesel dialed in for torque do on this pull?

All I can say is what grade? What pull? What problem? I left the big motor in cruise and rolled with the traffic, so why feel bad when I am in the left lane moving quite briskly passing comfortably at 60-65 without touching the pedal. The difference between the two couldn’t have been more dramatic all those years apart. And the best part is once we crested the pass my wife didn’t have to push her feet through the dashboard worrying about stopping, this rigs jake brake took care of all my worries.