Which V-8 diesel is best? Duramax/PSD comparison
#16
09-01-2004, 01:13 PM
I like both motors. Performance can't really be a factor cause 'king-of the-montain' changes every year. But I beleive the PSD is finailly getting the bugs worked out. I'll be interested to see M.I's numbers. I beleive the chart from edge products to be more accurate.
#17
09-01-2004, 01:23 PM
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I took a look at both those links. They use measurements the wheels so the values are too low. I need the engine.
This is what i will do. I need someone to post to me four things:
Idle rpm/torque
Max torque (@ rpm)
Max Horsepower (@ rpm)
and drop-off redline rpm
I'll extrapolate from there. Can anyone get this?
This is what i will do. I need someone to post to me four things:
Idle rpm/torque
Max torque (@ rpm)
Max Horsepower (@ rpm)
and drop-off redline rpm
I'll extrapolate from there. Can anyone get this?
#18
09-01-2004, 01:35 PM
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Before my father bought his 04 F350, he and I test drove all three diesels, Cummins, Duramax, and PSD, all with automatic trannys(also all three in crew cab dually pickups). We agree that the PSD was the best performer from the test drive with Duramax at a close second, both pulling far from the Cummins. And although I personally would never purchase one of those new chevy's, I would never contest any decision made about building a diesel engine when it came from Isuzu. They make great engines! Not quite like a Volvo diesel though...but that's a different story! Any how, I have towed with my father's 04 6.0 PSD and my uncles Duramax, both are very strong towing trucks with smooth enough shifts to keep the cattle in the trailer!
Scott
Scott
#19
09-01-2004, 04:30 PM
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PSD/TS vs DURAMAX/ALLISON Results
PSD/TS vs DURAMAX/ALLISON
A Ford PSD/Auto-TS and a GM Duramax/Allison are driving across the salt-flats. They each have an empty trailer into which we are going to add weights until someone must downshift to maintain speed. They both must also maintain the same speed. The first forced to slow down loses the race. We will start with the most ideal speed for the GM, which is 1600 engine rpm (max torque of 590 ft-lbs). Each has a 4.10 rear axle, the best for towing and 31inch diameters tires on a 16-inch wheel. Therefore the speed they are running at is: (1600 rpm x 3.14 x 31 inches) / (0.71 x 4.1 x 1056) = 50.7 mph, which is a nice comfortable highway speed.
My assumptions for this analysis include the following:
FORD:
I’m using the published PSD torque/HP curve from the PSD brochure, adjusted upward 10 ft-lbs for their new 570 ft-lb 2005 PSD, with the exception that I smooth that back into no extra torque gain at the 325HP point. For the Duramax, I am using their published max Torque/HP as shown below, and I assume a drop-off distance of 450 rpm after peak HP, which is identical to the curve shape for the PSD. In review:
PSD:
2100 rpm = 570 ft-lb
3300 rpm = 325 HP (518 ft-lb)
3750 rpm = drop off to zero
1st: 3.09
2nd: 2.20
3rd: 1.538
4th: 1.00
5th: 0.712
Duramax 6.6L
1600 rpm = 590 ft-lb
3100 rpm = 310 HP (525 ft-lb)
3550 rpm = drop off to zero (assumed based on peak horsepower at 200 rpm less than PSD)
slope of the torque line is y = 655 – 0.040625
Allison
1st: 3.10
2nd: 1.81
3rd: 1.41
4th: 1.00
5th: 0.71
(1) DROP IN THE FIRST LOAD (874 ft-lb-rev/min): Ford must downshift to 4th
GM/Chevy work potential is 590 ft-lb x 1600rpm = 944K ft-lb-rev/min, transmission axle rpm = 1600 / .71 = 2253 rpm.
Ford must go transmission axle of 2253 rpm, so engine turns 2253 * .712 (5th gear) = 1604 rpm, Torque @ 1604 rpm = 545 ft-lbs, work potential of the Ford = 545 ft-lb x 1604rpm = 874K ft-lbs-rev/min).
The GM/Chevy has more torque reserve. We drop a load in each trailer equal to 874K ft-lbs-rev/min) of load.
The Ford must downshift. Ford downshifts, must maintain transmission axle rpm = 2253rpm, 4th gear is 1.0 so engine is going 2253rpm Torque @ 2253 rpm = 560 ft-lbs. Work = 2253 rpm x 560 ft-lb = 1,262K ft-lbs-rev/min.
(2) 2nd DROP OF MORE LOAD (944 ft-lb-rev/min): GM/Chevy must downshift to 4th
Now Ford has more torque reserve (1,262 to 944 ft-lbs-rev/min). We drop more load in the trailer and the load is upped to 944K ft-lbs-rev/min) and GM/Chevy must downshift.
GM/Chevy downshifts, must maintain transmission axle rpm = 2253 rpm. 4th gear is 1.0, so engine is going 2253 rpm. Torque @ 2253 = 563 ft-lbs. Work capacity = 2253rpm x 563 ft-lb = 1,268K ft-lb-rev/min
(3) 3rd DROP OF MORE LOAD (1,262 ft-lb-rev/min): Ford must downshift to 3rd
Now GM/Chevy has more torque reserve (1,268 to 1,262 ft-lbs-rev/min) but the margin is almost zero, and from an engineering view, within error margin. However, we will press on. We once again add more weight and the load is upped to 1,262K ft-lb and Ford must downshift.
Ford downshifts, must maintain transmission axle rpm = 2253 rpm. 3rd gear is 1.538, so engine is going 2253 rpm x 1.538 = 3465 rpm. Torque @ 3465 = 500 ft-lbs. Work capacity = 3465 rpm x 500 ft-lb = 1,733K ft-lb-rev/min
(4) 4th DROP OF MORE LOAD (1,268 ft-lb-rev/min): GM/Chevy must downshift to 3rd
Now Ford has more torque reserve (1,733 to 1,268 ft-lbs-rev//min. We add more weight and the load upped to 1,268K ft-lb and GM/Chevy must downshift.
GM/Chevy downshifts, must maintain transmission axle rpm = 2253 rpm. 3rd gear is 1.41, so engine is going 2253rpm x 1.41 = 3177 rpm. Torque @ 3177 is approx 520 ft-lb. Work capacity = 3177 rpm x 520 = 1,652K ft-lbs-rev/min.
(5) 5th DROP OF MORE LOAD (1,652 ft-lb-rev/min): GM/Chevy must downshift to 2nd
Ford STILL retains more torque reserve (1,733 to 1,652 ft-lbs-rev/min). The GM/Chevy is now in a position of never being able to catch up. The race is soon over. However, note at this point that the GM/Chevy and Ford were VERY CLOSE when Ford was in 4th Gear (1,262 ft-lb-rev/min) and when GM/Chevy was in 4th gear (1,268 ft-lb-rev/min). It’s technically and virtually a tie. But what has happened is that the ramp down of torque for Ford is flatter, going out to 518 ft-lbs at 3300 rpm, while the GM/Chevy’s ramp down is a bit steeper, going down to 520 … but 200 rpms earlier at 3100. This is THE WEAK point of the Duramax. It’s peak HP comes 200 rpm before the PSD, and the PSD will outlast.
Yet more weight is added again, and the load is upped to 1,652K and GM/Chevy must downshift again.
GM/Chevy downshifts, must maintain transmission axle rpm = 2253rpm. 2nd gear is 1.81, so engine is going 2253 x 1.81 = 4077 rpm. REDLINE. Cannot maintain that engine rpm… the truck is forced to slow down to a slower transaxle speed, and go slower with the new load of 1,652 ft-lbs-rev/min in the trailer. So what should the GM/Chevy slow down too? Well, if the max RPM for the dodge is about 3550 rpm, then 1,652 ft-lb-rev/min divided by 3550 rpm means it would need a torque of 465 ft-lbs. Can a GM/Chevy produce that amount there? We’ll assume so. The new speed of the GM/Chevy will now be:
Speed (mph) = (3550 RPM x 3.14 x 31 inch tires) / (1.81 x 4.1 x 1056) = 44 mph
Notice also, that the final load for the Dodge/Cummins in this race (other thread) was 1,513 ft-lb-rev/min, forcing it to slow down to 32 mph. However, in this case the GM/Chevy-Duramax is taking a higher load (1,652 ft-lb-rev/min) and only slowing down to a much higher speed of 44 mph.
What that tells me is the outcome of Dodge/Cummins vs. GM/Chevy-Duramax: The Duramax will win.
Well ... that's the results of my analysis. You can shift the starting speed up/down a bit, and this one is close enough where you may find a tough spot for the PSD to get out of (one gear is too low, and the next to high) but generally PSD should will most of these scenarios against Duramax.
Robust Horsepower further up the RPM chain. That's the key for the increasing load test.
Marine Ironman
A Ford PSD/Auto-TS and a GM Duramax/Allison are driving across the salt-flats. They each have an empty trailer into which we are going to add weights until someone must downshift to maintain speed. They both must also maintain the same speed. The first forced to slow down loses the race. We will start with the most ideal speed for the GM, which is 1600 engine rpm (max torque of 590 ft-lbs). Each has a 4.10 rear axle, the best for towing and 31inch diameters tires on a 16-inch wheel. Therefore the speed they are running at is: (1600 rpm x 3.14 x 31 inches) / (0.71 x 4.1 x 1056) = 50.7 mph, which is a nice comfortable highway speed.
My assumptions for this analysis include the following:
FORD:
I’m using the published PSD torque/HP curve from the PSD brochure, adjusted upward 10 ft-lbs for their new 570 ft-lb 2005 PSD, with the exception that I smooth that back into no extra torque gain at the 325HP point. For the Duramax, I am using their published max Torque/HP as shown below, and I assume a drop-off distance of 450 rpm after peak HP, which is identical to the curve shape for the PSD. In review:
PSD:
2100 rpm = 570 ft-lb
3300 rpm = 325 HP (518 ft-lb)
3750 rpm = drop off to zero
1st: 3.09
2nd: 2.20
3rd: 1.538
4th: 1.00
5th: 0.712
Duramax 6.6L
1600 rpm = 590 ft-lb
3100 rpm = 310 HP (525 ft-lb)
3550 rpm = drop off to zero (assumed based on peak horsepower at 200 rpm less than PSD)
slope of the torque line is y = 655 – 0.040625
Allison
1st: 3.10
2nd: 1.81
3rd: 1.41
4th: 1.00
5th: 0.71
(1) DROP IN THE FIRST LOAD (874 ft-lb-rev/min): Ford must downshift to 4th
GM/Chevy work potential is 590 ft-lb x 1600rpm = 944K ft-lb-rev/min, transmission axle rpm = 1600 / .71 = 2253 rpm.
Ford must go transmission axle of 2253 rpm, so engine turns 2253 * .712 (5th gear) = 1604 rpm, Torque @ 1604 rpm = 545 ft-lbs, work potential of the Ford = 545 ft-lb x 1604rpm = 874K ft-lbs-rev/min).
The GM/Chevy has more torque reserve. We drop a load in each trailer equal to 874K ft-lbs-rev/min) of load.
The Ford must downshift. Ford downshifts, must maintain transmission axle rpm = 2253rpm, 4th gear is 1.0 so engine is going 2253rpm Torque @ 2253 rpm = 560 ft-lbs. Work = 2253 rpm x 560 ft-lb = 1,262K ft-lbs-rev/min.
(2) 2nd DROP OF MORE LOAD (944 ft-lb-rev/min): GM/Chevy must downshift to 4th
Now Ford has more torque reserve (1,262 to 944 ft-lbs-rev/min). We drop more load in the trailer and the load is upped to 944K ft-lbs-rev/min) and GM/Chevy must downshift.
GM/Chevy downshifts, must maintain transmission axle rpm = 2253 rpm. 4th gear is 1.0, so engine is going 2253 rpm. Torque @ 2253 = 563 ft-lbs. Work capacity = 2253rpm x 563 ft-lb = 1,268K ft-lb-rev/min
(3) 3rd DROP OF MORE LOAD (1,262 ft-lb-rev/min): Ford must downshift to 3rd
Now GM/Chevy has more torque reserve (1,268 to 1,262 ft-lbs-rev/min) but the margin is almost zero, and from an engineering view, within error margin. However, we will press on. We once again add more weight and the load is upped to 1,262K ft-lb and Ford must downshift.
Ford downshifts, must maintain transmission axle rpm = 2253 rpm. 3rd gear is 1.538, so engine is going 2253 rpm x 1.538 = 3465 rpm. Torque @ 3465 = 500 ft-lbs. Work capacity = 3465 rpm x 500 ft-lb = 1,733K ft-lb-rev/min
(4) 4th DROP OF MORE LOAD (1,268 ft-lb-rev/min): GM/Chevy must downshift to 3rd
Now Ford has more torque reserve (1,733 to 1,268 ft-lbs-rev//min. We add more weight and the load upped to 1,268K ft-lb and GM/Chevy must downshift.
GM/Chevy downshifts, must maintain transmission axle rpm = 2253 rpm. 3rd gear is 1.41, so engine is going 2253rpm x 1.41 = 3177 rpm. Torque @ 3177 is approx 520 ft-lb. Work capacity = 3177 rpm x 520 = 1,652K ft-lbs-rev/min.
(5) 5th DROP OF MORE LOAD (1,652 ft-lb-rev/min): GM/Chevy must downshift to 2nd
Ford STILL retains more torque reserve (1,733 to 1,652 ft-lbs-rev/min). The GM/Chevy is now in a position of never being able to catch up. The race is soon over. However, note at this point that the GM/Chevy and Ford were VERY CLOSE when Ford was in 4th Gear (1,262 ft-lb-rev/min) and when GM/Chevy was in 4th gear (1,268 ft-lb-rev/min). It’s technically and virtually a tie. But what has happened is that the ramp down of torque for Ford is flatter, going out to 518 ft-lbs at 3300 rpm, while the GM/Chevy’s ramp down is a bit steeper, going down to 520 … but 200 rpms earlier at 3100. This is THE WEAK point of the Duramax. It’s peak HP comes 200 rpm before the PSD, and the PSD will outlast.
Yet more weight is added again, and the load is upped to 1,652K and GM/Chevy must downshift again.
GM/Chevy downshifts, must maintain transmission axle rpm = 2253rpm. 2nd gear is 1.81, so engine is going 2253 x 1.81 = 4077 rpm. REDLINE. Cannot maintain that engine rpm… the truck is forced to slow down to a slower transaxle speed, and go slower with the new load of 1,652 ft-lbs-rev/min in the trailer. So what should the GM/Chevy slow down too? Well, if the max RPM for the dodge is about 3550 rpm, then 1,652 ft-lb-rev/min divided by 3550 rpm means it would need a torque of 465 ft-lbs. Can a GM/Chevy produce that amount there? We’ll assume so. The new speed of the GM/Chevy will now be:
Speed (mph) = (3550 RPM x 3.14 x 31 inch tires) / (1.81 x 4.1 x 1056) = 44 mph
Notice also, that the final load for the Dodge/Cummins in this race (other thread) was 1,513 ft-lb-rev/min, forcing it to slow down to 32 mph. However, in this case the GM/Chevy-Duramax is taking a higher load (1,652 ft-lb-rev/min) and only slowing down to a much higher speed of 44 mph.
What that tells me is the outcome of Dodge/Cummins vs. GM/Chevy-Duramax: The Duramax will win.
Well ... that's the results of my analysis. You can shift the starting speed up/down a bit, and this one is close enough where you may find a tough spot for the PSD to get out of (one gear is too low, and the next to high) but generally PSD should will most of these scenarios against Duramax.
Robust Horsepower further up the RPM chain. That's the key for the increasing load test.
Marine Ironman
#20
09-01-2004, 05:16 PM
I don't know if theoretical numbers games really are what this discussion is about as I don't think anyone is saying that the 6.0 wouldn't be a bit faster but the newest dmax is 605lb ft of torque, not 590. The only rear end available in the 3/4 ton series with the dmax is a 3.73.
#21
09-01-2004, 05:29 PM
Nice numbers there. Too bad an increasing load test has no real world implications what-so-ever. The only thing it proves is that all three trucks have different redlines and power bands, and that has absolutley nothing to do with which motor is better.
Last edited by BVister; 09-01-2004 at 06:28 PM.
#22
09-01-2004, 06:28 PM
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My apologies, DieselDonor, and I won't mention that whole thing again, at least not in this thread.
Anyway, like Logical Heretic said, it hauls butt empty, and I'm sure it's a good engine, I've not heard of any problems with them. I definitely think a V design is the way to go, too, and also, like was mentioned, I'm sure if Isuzu built it, then it's going to be right, but you could have said the same about Ford before the 03 SD came out and they had troubles with the first batches of them. I think that's it's an engine used specifially for the application, but not for heavy duty work. Yeah, heavier duty than their 1/2 ton, but it's geared more towards retirees who wants to haul campers or whoever wants a smoother ride with the IFS, I don't think it's built to be an all-out decades-lasting work machine. I think the Ford is more of a piece of equipment, while the Chevy is more of an automobile.
Anyway, like Logical Heretic said, it hauls butt empty, and I'm sure it's a good engine, I've not heard of any problems with them. I definitely think a V design is the way to go, too, and also, like was mentioned, I'm sure if Isuzu built it, then it's going to be right, but you could have said the same about Ford before the 03 SD came out and they had troubles with the first batches of them. I think that's it's an engine used specifially for the application, but not for heavy duty work. Yeah, heavier duty than their 1/2 ton, but it's geared more towards retirees who wants to haul campers or whoever wants a smoother ride with the IFS, I don't think it's built to be an all-out decades-lasting work machine. I think the Ford is more of a piece of equipment, while the Chevy is more of an automobile.
#23
09-01-2004, 07:15 PM
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BVister and Jeb:
What brand of drugs do you fellow use, Columbian Gold or Morrocan Red? First, these are real numbers, gents. If you were a fly-on-the-wall at GM or Ford, you'd see this stuff scribbled all over the white-boards and comprising hundreds of power-point briefings. This data fills loads of modeling and simluation software. Brief but reasonalby accurate estimations like these are performed all over the world for a host of engineering applications.
Secondly, independent tests show what my analysis has predicted: Ford 1st, Chevy 2nd, Dodge 3rd. So, my results corroborate real-world tests.
Finally, this analysis represents real world applications. For example, hook up a 10,000 lb trailer to your truck and head for the rockies. The first time your Ford or Chevy downshifts .... you have entered my scenario below.
Ironman's reply: Puff, puff. [deep inhale] The redlines and power bands have EVERYTHING to do with which motor is better for towing a 10,000 lb trailer in real conditions. IF what you said is TRUE .... then ... we could install a little 100 horsepower VW beetle diesle engine and be just fine.
What brand of drugs do you fellow use, Columbian Gold or Morrocan Red? First, these are real numbers, gents. If you were a fly-on-the-wall at GM or Ford, you'd see this stuff scribbled all over the white-boards and comprising hundreds of power-point briefings. This data fills loads of modeling and simluation software. Brief but reasonalby accurate estimations like these are performed all over the world for a host of engineering applications.
Secondly, independent tests show what my analysis has predicted: Ford 1st, Chevy 2nd, Dodge 3rd. So, my results corroborate real-world tests.
Finally, this analysis represents real world applications. For example, hook up a 10,000 lb trailer to your truck and head for the rockies. The first time your Ford or Chevy downshifts .... you have entered my scenario below.
The only thing it proves is that all three trucks have different redlines and power bands, and that has absolutley nothing to do with which motor is better.
#25
09-01-2004, 07:26 PM
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P.S. - 605 ft-lb for the Duramax will not save it. My analysis put the 600 ft-lb Cummins in dead last, matching the tow tests coming out in Truck Trend.
Higher Low-end torque simply allows you to wait a bit longer for the first two downshifts. After that ... when you're bogging down to 25 mph over the mountain pass ... your neighbor in his Ford/PSD will zip by you at 40 or 50 mph. It's the total area under the power/torque curve that counts. Flatter but substantially broader give you more workability all together.
Herr Von-Daimler and Tokyo-GM are resorting to snake-oil-salesmanship and counting on selling trucks to the bottom-half of the bell curve education wise. 1/2 of all drivers have an IQ of less than 100, and this is their targeted sales audience.
Ford: made in the USA, better performance. (Hum John Philip Sousa's Stars-and-Stripes-Forever to yourself and salute the American flag).
The Marine Ironman
Higher Low-end torque simply allows you to wait a bit longer for the first two downshifts. After that ... when you're bogging down to 25 mph over the mountain pass ... your neighbor in his Ford/PSD will zip by you at 40 or 50 mph. It's the total area under the power/torque curve that counts. Flatter but substantially broader give you more workability all together.
Herr Von-Daimler and Tokyo-GM are resorting to snake-oil-salesmanship and counting on selling trucks to the bottom-half of the bell curve education wise. 1/2 of all drivers have an IQ of less than 100, and this is their targeted sales audience.
Ford: made in the USA, better performance. (Hum John Philip Sousa's Stars-and-Stripes-Forever to yourself and salute the American flag).
The Marine Ironman
#26
09-01-2004, 07:27 PM
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P.S.S. - you can substitute a 3.73, makes no difference. Just means the whole scenario is at about 47 mph vice 50 mph.
#27
09-01-2004, 07:35 PM
Originally Posted by Marine Ironman
BVister and Jeb:
Ironman's reply: Puff, puff. [deep inhale] The redlines and power bands have EVERYTHING to do with which motor is better for towing a 10,000 lb trailer in real conditions. IF what you said is TRUE .... then ... we could install a little 100 horsepower VW beetle diesle engine and be just fine.
Ironman's reply: Puff, puff. [deep inhale] The redlines and power bands have EVERYTHING to do with which motor is better for towing a 10,000 lb trailer in real conditions. IF what you said is TRUE .... then ... we could install a little 100 horsepower VW beetle diesle engine and be just fine.
Better read a little closer. I said nothing about peak power. The analysis you did is entirely dependent upon gear ratios and where the power band is. They also show no data as to how a vehicle would react from a dead stop, which is far more important in real world applications that how a vehicle react to extreme amounts of weight when already in motion. That, coupled with the fact that you use a 4.10 ratio in the Ford (which it isn't even available with) makes your data inconclusive at best. Since when is it important how fast a vehicle can pull huge amounts of weight when in first gear anyway?
Also, if your analysis really did hold any water, when you apply the same analysis to the F-150 with the 5.4 and the Ram 1500 with the Hemi, the Hemi is clearly the winner. This is contrary to anything anyone on this site has ever tried to prove.
#28
09-01-2004, 07:50 PM
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Originally Posted by BVister
Marine Iron-on: Better read a little closer. I said nothing about peak power. The analysis you did is entirely dependent upon gear ratios and where the power band is.
Originally Posted by BVister
They also show no data as to how a vehicle would react from a dead stop, which is far more important in real world applications that how a vehicle react to extreme amounts of weight when already in motion.
Originally Posted by BVister
That, coupled with the fact that you use a 4.10 ratio in the Ford (which it isn't even available with) makes your data inconclusive at best.
Originally Posted by BVister
Since when is it important how fast a vehicle can pull huge amounts of weight when in first gear anyway?
Originally Posted by BVister
Also, if your analysis really did hold any water, when you apply the same analysis to the F-150 with the 5.4 and the Ram 1500 with the Hemi, the Hemi is clearly the winner. This is contrary to anything anyone on this site has ever tried to prove.
Don't forget your bag of Columbian Gold.
The Marine Ironman
#29
09-01-2004, 07:53 PM
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Somebody opened a bag of nuts, and a few rolled in here. Also, the 3.73 makes it a few miles faster, my typo.
#30
09-01-2004, 08:32 PM
"25 at the top of the hill while the PSD is doing 40 or 50."
We are still talking about Duramax Vs. Powerstroke, right? Most of the testing comparos show differences in top speed between the diesels in the single digits at the end of long grades. I believe you stated the difference in torque peaks was 200 rpm or so between the Duramax and PSD. Does that justify a statement of 60 to 100 percent more speed at the top of the grade? MI, you're going to have to show me that math to justify that statement. Use realistic, within GVWR loads.
I wouldn't spit twice for a difference in rpm torque peaks for diesels of 200 rpm, especially if they are both V8's. I think the discussion needs to turn to technology in these motors to define the real differences. How about durability of construction? Main bearing surfaces? Rods, wrist pins? Which one can take more power upgrades? I've seen 800 horse Cummins motors. How high has anyone taken the Duramax/Powerstrokes, using reliable dyno numbers? Which motor held up better? Extreme applications can separate the contenders from the dogs.
Even though V's have more speed potential, I am an inline fan. A Ford straight six gasser won me over after a youth spent building up V8's. Therefore, contrary to the prevailing Ford (diesel) wisdom, I like torque lower in the rpm band, but I make distinctions between the capabilities of motors based on torque peak differences much greater than 200 rpm, like 302 vs. 300.
We are still talking about Duramax Vs. Powerstroke, right? Most of the testing comparos show differences in top speed between the diesels in the single digits at the end of long grades. I believe you stated the difference in torque peaks was 200 rpm or so between the Duramax and PSD. Does that justify a statement of 60 to 100 percent more speed at the top of the grade? MI, you're going to have to show me that math to justify that statement. Use realistic, within GVWR loads.
I wouldn't spit twice for a difference in rpm torque peaks for diesels of 200 rpm, especially if they are both V8's. I think the discussion needs to turn to technology in these motors to define the real differences. How about durability of construction? Main bearing surfaces? Rods, wrist pins? Which one can take more power upgrades? I've seen 800 horse Cummins motors. How high has anyone taken the Duramax/Powerstrokes, using reliable dyno numbers? Which motor held up better? Extreme applications can separate the contenders from the dogs.
Even though V's have more speed potential, I am an inline fan. A Ford straight six gasser won me over after a youth spent building up V8's. Therefore, contrary to the prevailing Ford (diesel) wisdom, I like torque lower in the rpm band, but I make distinctions between the capabilities of motors based on torque peak differences much greater than 200 rpm, like 302 vs. 300.