Duramax 6600 Performance Pull-Off - Reprint from October 1999

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Duramax 6600 Performance Pull-Off
By Jim Bigley

What follows is a reprint from an article that appeared here shortly after October of 1999.
By now, those of you watching the Duramax story unfold here in TheDieselPage.com have probably guessed that we ran the Duramax trucks right along with the 6.5TD Project truck and Ford & Dodge during the recent Pull-Off competition.

The Diesel Page was given an opportunity to test-drive both a new Duramax Chevy and GMC here in Montana a full year before any Duramax powered pickup trucks would become available at dealerships. Not only were we able to get a feel for how the trucks handled on the highway in a variety of empty and loaded configurations, but we also ran these trucks against a 1999 Ford F250 Super-Duty 7.3L Powerstroke and a 1999 Dodge 24-valve 5.9L Cummins in an ultimate test of power we call the "Pull-Off".



The Duramax performance is impressive. After you read what these new Duramax diesel prototype trucks can do, I'm sure you'll want to share with as many people as possible just what Ford and Dodge are up against. I don't blame you, I do too. But, please honor our copyright, and don't re-post the images and text. Simply post the link to this page.

I was skeptical of the early Duramax performance rumors, but after having an opportunity to drive these trucks, I can say that if anything, GM is underplaying the performance of their new diesel. In my opinion, the Duramax/Allison powertrain was designed not to compete with Ford and Dodge, but to dominate the light-truck diesel market.

During two days of driving, we ran 0-60 after 0-60, and loaded hill climb after grueling hill-climb, comparing the Duramax performance to Ford & Dodge. GM's new 6600 is a real killer. The power and performance is so far ahead of the new Ford and Dodge trucks that it'll be a long time before they become a threat to the Duramax superiority.



To make this strictly a test of diesel engine power, all the trucks were "weight-matched" with sand bags before any runs were made. I initially disagreed with this philosophy. I felt that if a truck manufacturer produced a heavy truck, they should pay for it in reduced performance and payload capacity. The majority out-voted me though, and all the trucks weighed exactly the same for each of the performance runs. The Ford Super-Duty (at 6820 pounds) turned out to be the porker of the bunch, weighing fully 500 pounds more than the quad-cab Duramax Chevy at 6320. Even the Dodge with its heavy I-6 Cummins was 180 pounds lighter than Ford.

Ford and Chevy were both equipped with 3.73 gearing, while the Dodge carried 4.10 gears. Chevy and Dodge were both equipped with an automatic transmission, but the only Ford we had access to came with a six-speed manual. And finally, all three trucks were quad-cab shortbox ¾-ton 4x4's. This was as close a match as we could have hoped for.

First in the series of tests was a direct comparison in 0-60 mph acceleration. We ran all the trucks on the same track, under the same conditions and with the same driver. For this portion of the performance test, Willie Worthy (Four Wheeler magazine tech editor) performed the driving duties. Willie did his best to treat each truck fairly in launching method and driving style, and I rode shotgun, operating two G-Tech performance meters in tandem.



Here we are at the track (an undeveloped industrial park) used for the 0-60 acceleration runs. Aside from the gas Suburban, this is one impressive lineup of diesel trucks. The Duramax Chevy is just behind the Suburban and the Duramax GMC is bringing up the rear. The 6.5TD Project truck and the new Dodge are just behind the enclosed trailer hooked to the Ford.



Empty acceleration performance is a test of horsepower. Even with 500 lbs of sand bags added to the bed, the Duramax Chevy easily outran both Ford & Dodge. The 300 horsepower Duramax clearly had the advantage.

Keep in mind that these numbers were generated under less than ideal conditions, due to a rain-slicked track. Willy just mashed the throttle at the beginning of each run, to keep the engines from developing too much power from a standing start. A turbo diesel needs a couple seconds for the turbo to spool up before real power can be generated, and to produce the quickest elapsed times.

We also planned to perform a series of 0-60 runs with the 9,700 lb trailer, but had to give it up. The Powerstroke wouldn't accelerate fast enough through a loaded 0-60 to trigger the G-Tech performance meters. The Duramax, on the other hand would trigger the G-Tech meters every time, and produced a 26.08 second average. Since the Powerstroke couldn't do it, we didn't try it with the Dodge.




With a flat torque curve that peaks at an advertised 520 lb-ft, the Duramax Chevy stunned us with a run of just 83 seconds, reaching a top speed of 55 mph from a dead stop on a 6% grade while pulling a 9,700 lb trailer. The Ford Powerstroke (with an advertised 500+ lb-ft of torque) finished a distant 2^nd in the mile-long hill-climb, producing a run of 113 seconds and a top speed of 50 mph. The Dodge 24V Cummins came in last at 122 seconds, with a low top speed of just 40 mph.

Nine seconds separated Ford and Dodge, but the Duramax finished the mile-long loaded hill-climb a full 30 seconds ahead of Ford, and 39 seconds ahead of Dodge. The Duramax performance is in a class by itself.

Next up, was a loaded hill-climb. Many people buy a diesel to pull trailers; sometimes really big trailers. The mighty Powerstroke has always excelled at this game because of its torque advantage. Torque is what pulls heavy trailers up steep grades, and the heavier the trailer, the more torque plays in deciding the winner. Our 9,700 lb trailer was heavy enough to clearly show which truck was developing the most torque.

When the Pull-Off was still in the planning phase, I wanted to use a 7500 lb travel-trailer for the loaded performance runs. The GM team "suggested" we use a trailer that weighed as close as possible to 9,800 lbs, which is the limit in some states, above which the driver must have a commercial license. So, we loaded our trailer to 9,700 lbs. They knew the Duramax would have the advantage with a heavier trailer.

Evaro hill, a few miles Northwest of Missoula Montana on US93 provided the site for the hill-climb. This hill is infamous among diesel truckers for its 25-35 mph grind in the slow lane. To make the hill-climb even more demanding, each truck was required to pull the same 9,700 lb enclosed trailer up this 6% grade from a dead stop. Each run was then timed for exactly one mile using the conveniently located mile markers. A more brutal hill-climbing power test would be hard to find.




Besides spending a lot of time driving the GM trucks, we also spent a nearly equal amount of time in the new Ford and Dodge. Having driven several other Dodge diesels over the years, I couldn't help but notice the Cummins has come a long way since the early days. Aside from a few minor issues, I actually enjoyed driving the Dodge, even if it didn't do well in our performance tests. The Cummins was quieter than the Powerstroke, and I liked the open engine bay. The ISB 24V engine revved willingly to 3000 rpm, and once under load, the diesel clatter became almost tolerable. Dodge has a long way to go to stay competitive, but I'm sure the "industrial" nature of the Cummins will continue to appeal to lots of folks, regardless.

Even while trying to remain objective, I found the new Ford Super-Duty Powerstroke to be the most objectionable truck to drive. The PSD was noisier than even the Dodge Cummins, and road noise transmitted through the chassis quickly became a nuisance. After spending an hour in this truck I was asking myself why it was so popular.

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The Duramax Chevy and GMC were in a class by themselves where driving comfort, handling, and quietness were concerned. The lower truck height helped considerably while towing the nearly 10,000 lb trailer. No sway, no wandering, only precise control. The independent front suspension also gave us the smoothest ride. These trucks are the best riding among heavy-duty diesel pickups available today.

Since the first Duramax truck article appeared here in the Diesel Page, I've been asked to compare the diesel rattle to the 6.5. In my opinion, the Duramax produces less noise than any 6.5 at idle and during slow speed driving. While accelerating, or cruising, the noise level is comparable to a typical 6.5. I've driven a few 6.5 trucks that were quieter, and some that were noisier than the Duramax.

It was an interesting experience meeting the GM Marketing and Engineering team who brought the trucks to Montana. What I noticed most about them was their confidence and pride in the Duramax/Allison powertrain. The demonstrated performance is something to believe in, and now, GM owners can take satisfaction in knowing their favorite trucks will have the most powerful diesel engine.

More Power!!! Jim Bigley

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Acknowledgements:
First, my thanks goes to GM Chevrolet Division for making the Montana Duramax event possible. Their willingness to include the Diesel Page in the Duramax launch says a lot about GM's commitment to the Internet community and their faith in the Duramax. I also want to thank the staff at Four Wheeler magazine for their help to me. Karl Tyler, Vaughn LaTour and Jason Nordberg at Karl Tyler Chevrolet were extremely helpful. The Pull-Off would have been difficult to do without them.


Karl Tyler Chevrolet
3219 N. Reserve St
Missoula, MT 59801
406-721-2438
Largest GM dealership in Western Montana, Truck specialists, and diesel enthusiasts

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A Behind The Scenes Look At Duramax 6600 Performance
By Jim Bigley

What follows is a reprint from an article that appeared here shortly after our GM Prototype Duramax Chevy/GMC testing in October of 1999.

The Pull-Off performance numbers you've read about for the Duramax are certainly impressive. After analyzing the data and having ridden in the Duramax through three hill-climb runs, I can say that the 83 seconds it took run the mile and the 55 mph top speed doesn't tell the whole story. I wish we would have had a ZF 6-speed Duramax as well as an Allison. I believe a 6-speed Duramax would have reduced that time even further, and would have made 60-65 mph possible in the standing start mile-long hill-climb while towing 10,000 lbs.

Let me clarify....... The Duramax/Allison was able to reach 50 mph at about the half-mile point in the run. That's right! At the half-mile point! This was a tremendous feat when you consider the Ford PSD just scratched 50 at the one mile mark. At a little past the half-mile point, the Allison upshifted into 4th (direct). The TCM (Transmission Control Module) saw that the rate of acceleration had slowed once in 4th, so to ease the burden on the automatic transmission, it shifted back to 3rd and held it there for the duration of the run. (GM calls it "Shift Stabilization") This meant the Duramax was effectively running against the governor for the final 1/3 mile. A manual transmission could have been upshifted into direct, which would have produced another 5-10 mph along with a corresponding reduction in elapsed time.

To get the GVW rating up to 26,000 lbs for the Allison, the TCM programming manages the powertrain in ways that protect it from excessive heat and stress. This will help ensure a near bullet-proof auto transmission, but it reduced the performance in our "damn the torpedoes" run up the hill.

A lot has also been said here in The Diesel Page about a "built-in competitive power growth potential" for the Duramax. One indication of that was apparent in the engine operating rpm I witnessed. The tachometer was redlined at 3400 rpm, but our test trucks governed at 3100. Also, the current horsepower rating for the Duramax is at 3100 rpm. Curious....... I suspect, all GM has to do to get 310-330 horsepower, is let it rev to the indicated redline. Since boost pressure is regulated at 18 psi independent of the computer, torque will also remain high at higher rpms. I know the GM EFI 6.5TD produces maximum horsepower at 3400 rpm. I believe the Duramax would as well.

During our "after event" bull session, I asked one of the Powertrain engineers what sort of exhaust gas temperatures the new Duramax was running. These new trucks were equipped with a transmission temperature gauge, but not an EGT gauge. The engineer told me that he didn't know what the EGT was, and that it didn't matter. At first, I thought to myself "of course it matters", but on reflection I think I understood what he meant.

When we increase power in the 6.5, we have to install an EGT gauge to help protect the engine. This is because we're now operating near the thermal limits. If the Duramax were operating well below the thermal limits, there would be no need to monitor the EGT. This could mean that there is considerable potential to increase power in the future.



As far as engine bays go, all of them are pretty crowded. The Duramax trucks are no exception. However, ease of service was designed into the Duramax under-hood layout. The center mounted turbo frees the technician from several laborious routines should the engine require more than minor service, and the heads can be removed without removing the turbocharger or the injection system.



The Ford PSD under-hood layout strikes me as being somewhat more complicated, with the biggest detractor coming from the recessed firewall and cab-forward design. This places the engine well into the firewall. The entire rear section of the engine is enclosed by the firewall, with the turbocharger completely under the cowl.

While I've never been fond of the Ford trucks, I've always been a fan of the Powerstroke engine. Ford did good, partnering with Navistar. Our 1999 PSD seemed to develop considerable torque at 1800 or so RPM, and pulled strong up to about 2700 rpm, then the engine really wanted another gear. It would continue to pull out to the redline, but the power had fallen off quite a bit by then.

In analyzing the performance, I feel the 7.3L PSD didn't perform as well as the Duramax because of the narrower powerband. It may have had as much torque as the Duramax, but couldn't use it over the same rpm range.



Dodge has the most open engine bay of all, but this truck also uses a "cab forward" design. The rear 1/3 of the engine is under the cowl. This might make some kinds of engine work a little tedious.

While driving the Dodge, I could tell the computer was protecting the automatic transmission. During a wide-open 0-60 run, the engine didn't make much power till the tach reached 2000 rpm, then the 5.9L Cummins came alive and ran strong to 3000 rpm. Between 2000 and 3000 rpm, the power felt very linear, without the typical peakiness I've experienced in earlier Dodge Cummins equipped trucks. As with Ford, I found the Cummins a little too noisy for my tastes. I also thought the A-pillar windshield frame obstructed the view more than that of either Ford or GM.



The second day the trucks were here, the Chevy dealership loaned us a lift so we could get under the new trucks. Aside from oogling over the Allison transmission, one item that jumped out at me was the remote fuel cooler mounted just in front of the fuel tank. This is a plate type cooler with a perforated aluminum shield protecting the front surface. I wasn't surprised to see a fuel cooler after learning of so many other innovative diesel concepts incorporated in GM's new truck.

The hard working Electronics Distribution Unit is what controls the electric fuel solenoids in all eight injectors. The electronic drivers in the EDU generate heat while is use, and the engineers felt additional cooling would provide more durability and reliability. This thermal problem was solved by fuel flow. Fuel from the tank flows through the cooler, then to the EDU, then to the injection system. We weren't told what kind of heat load the electronics dump into the fuel, but I couldn't help thinking the electronic 6.5 trucks might also benefit from a fuel cooler, for the very same reasons GM is using one with the Duramax.

Also notice the frame construction in the above photo. Not only is the frame considerably deeper than the C-frame construction used in earlier model trucks, but the rolled inner edges of the C-channel add considerable strength. GM did this to help provide industry leading GVW ratings.



Both the ECM and TCM are under the hood. These two computers are separate, but operate together to provide the best powertrain management. This "separateness" allows more modularity in packaging, meaning the engine and transmission will be suited for a variety of applications, and can be used independently when the configuration calls for it.

The Duramax is the wave of the future. I'm sure the implementation will vary with Ford and Dodge as they introduce their own new diesel engines, but you can bet they'll offer similar operating characteristics. I couldn't help but notice how crisp, smooth and quiet the new Duramax is. All other diesel engines appear dated by comparison. It's like comparing a modern multi-port EFI gas engine to a carbureted engine of the 60's. Quiet, powerful, and very easy to drive. - Jim Bigley