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