The idea was simple: build a Duramax-powered ’06 GMC that could dust a Corvette, rotate the earth with its torque, and still tow and drive like a normal, everyday truck. This project was also undertaken to show the performance gains at each of the three most commonly utilized performance combinations: 1) Single turbo/single CP3, 2) Compound turbo/single CP3, 3) Compound turbo/dual CP3. We’d dyno and drag race the truck after each of these modifications in order to get some good performance data.
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Parts One and Two
In the first part of “The Sleepermax” (Nov ’10), we dyno’d the truck to the tune of 576 hp with a single Garrett GT37 turbocharger. On the hottest tune, the truck’s power actually dropped (due to low rail pressure) and the EGT was in the danger zone at 1,600 degrees with the PPE Hot+2 E.T. Race set on Level 5, which was the highest level that still held rail pressure. At the dragstrip, the truck ran well, clicking off an 8.31-second eighth-mile elapsed time at 83.4 mph.
| With twin turbos and twin CP3 pumps, it’s obvious the engine in the Sleepermax is far from stock. With 744 rwhp and 1,360 lb-ft of torque, it had all the power we would ever need for a daily-driven street vehicle.
Since our EGT was smokin’ hot, our next installment of the project, “The Sleepermax: Episode Two” (Jan ’11), saw the installation of two Garrett turbochargers (a GT4094 and a GT45) set up in a compound arrangement (one blowing into the other). This made the engine produce 52 psi of boost and increased midrange power, although the peak numbers didn’t improve all that much. Still, the truck was quicker and faster on the dragstrip, running 8.02 seconds at 86.6 mph in the eighth, while the new dyno numbers were 587 hp and 1,102 lb-ft, respectively.
Part Three
After the installation of PPE’s compound turbos, the truck’s EGT was an ultra-manageable 1,200 degrees. This meant we could now add more fuel to increase the truck’s horsepower. However, there was an issue—rail pressure was dropping like a rock on the higher tunes. Luckily for us, PPE also markets a Dual Fueler system. This kit runs a second CP3 injection pump that ties in with the factory unit and keeps that all-important rail pressure where it should be, providing maximum atomization of the fuel and max power.
| The key to unleashing another 150 hp lied in PPE’s Dual Fueler kit, which retails at $2,400 and includes a second Bosch CP3 injection pump.
On the Dyno
With twin CP3 pumps and twin turbos, we were expecting huge dyno numbers, and the truck didn’t disappoint. After a few 700hp dyno pulls, we left the dyno cell with a smoking 744 rwhp reading, along with 1,360 lb-ft of torque. The EGT was up to 1,500 degrees because of the extra fuel, but it still wasn’t hitting 1,600 like it was in the first test with the single turbocharger.
On the Street
On the street, the truck was so fast that one could easily exceed any speed limit in the country without ever having to glance at the EGT gauge. At this power level, we wish all-wheel drive was an option on the diesel, as the truck was virtually tractionless at speeds less than 70 mph. On the lower power level tunes, the truck was still completely manageable and smoke-free.
Final Thoughts
A couple of years ago, we overheard a diesel enthusiast explaining his rationale for modifying his truck. “I was going to spend $50,000 on a new Shelby GT500 Mustang,” he said, “and then I realized what I really wanted was to just make my diesel faster. Now it’s as quick as the Shelby for a lot cheaper, and I still drive it every day.” This mentality was the driving force behind this project. It was our hope to illustrate that for the $50,000 to $60,000 one could spend on a new diesel (or a hot rod toy), that same amount of money could be spent making a low-mileage Duramax into a beast that would run circles around most modern performance cars, while holding onto the utility and comforts of today’s brand-new trucks. Considering Sleepermax’s excellent horsepower and torque numbers and killer dragstrip times, we’d say the project was a success.
Another SleeperMax
While we were at PPE watching the Sleepermax get dynoed, we noticed a pretty innocent Duramax-powered GMC pull up with its stereo thumpin’. When Dan Grant, one of PPE’s head honchos, got out of the truck, we realized he was driving the very same rig that ran low 7s at 100 mph at Irwindale’s eighth-mile just a week before. From the outside, the exhaust and traction bars were the only giveaway that anything might be up, but it was still cool to see Dan daily-driving his 80mm-turboed, 800hp truck.
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The $5,000 Question
It’s a lot to ask of an engine to make almost triple the horsepower it did in stock form without any type of internal modifications. While diesels are built to be durable and strong, there are limits. With twin turbos and twin injection pumps, we learned it’s possible to exceed those limits. When busting 700 rhwp, it is usually only a matter of time before the engine bends a connecting rod, or (in the case of the LBZ) cracks a piston. Another matter of reliability is the injectors, which are overtaxed at this power level and will fail given a long enough time span with a huge tune. Since Sleepermax is destined to live on as PPE’s shop truck, the injector nozzles were replaced with units that flowed 40 percent more fuel than stock, and the engine was also rebuilt with new pistons and connecting rods to live at the 700-plus-rwhp power level. If you’re a Duramax owner who is considering this final step, it is often easier to buy a core engine (such as the roasted long-block pictured) rather than take your truck out of commission for an extended period of time. Although a set of pistons and rods comes in at a whopping $5,000 (and that’s not counting any shop labor), the short-block is now safe to make upward of 1,000 rhwp, which in reality is probably only a big nitrous system away!
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Driving Impressions
We not only took Sleepermax to the dragstrip, we also hit a local company-sponsored car show and cruised it around town long enough to get an impression of how it drove. We can honestly say the truck is still pretty mild mannered. It may be a little smoky at times, but we were driving it around with the engine on the hottest setting. The ride was good, the steering was nice and smooth, and if passing was needed, 50 to 70 mph was accomplished in a little more than a second, once the boost hit. If we were to make any changes, we’d probably try to get rid of some of the low-end smoke, but that would be about it.
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