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2004 GMC Sierra - Discreet-Max

An Unassuming, 900HP GMC

Mike McGlothlin
Oct 9, 2013
Photographers: Mike McGlothlin
Photo 2/12   |   2004 Gmc Sierra Front Three Quarter
In the age of high-powered street trucks, Steven O’Brien’s ’04 Sierra is one of the gnarliest GMs you’ll come across. There’s a reason it sports a lengthy set of traction bars…so it can plant its 900 rwhp and put the hurt on just about anything it lines up against. In the highly populated diesel region that is the Midwest (namely Ohio in Steven’s case), it’s kill or be killed when it comes to your daily driver and—whether he meant to or not—Steven made it a point to ensure this fire red GMC will never be anyone’s lunch.
Thanks to a blown head gasket, Steven was able to get his hands on the LB7 Duramax fairly cheap. After the head gasket was taken care of, he knew the gateway to making more horsepower involved bulletproofing the five-speed Allison 1000. For that, a Gmax 5-pac rebuild kit, billet flexplate, and 1058 torque converter from Sun Coast got the call. With the transmission capable of surviving virtually anything Steven could throw at it, an aftermarket turbo, bigger injectors, LBZ CP3, and aggressive EFILive tuning proved too much for the LB7’s stock bottom end. A rod decided to exit the block, leaving him stranded.
Seeking a ground up, indestructible rebuild, Steven contacted Fleece Performance in Brownsburg, Indiana. And, after dropping the truck off, he had only one thing to say: “I want anything and everything to keep me from breaking it again.” The guys at Fleece immediately got to work on the short-block, and the balanced and blueprinted rotating assembly includes forged steel Carrillo rods and cast-aluminum, OE-replacement Mahle Motorsports pistons. Billet main bearing caps and ARP main studs were thrown in to eliminate crank walk, and a SoCal Diesel 3388 cam got the nod for its ability to improve spool up and low-rpm performance in large, single-turbo applications. The crank, cam, and water pump are all keyed.
A set of SoCal Diesel’s Stage 2 heads, complete with stiffer, ovate-style valvesprings, titanium retainers, and a CNC port job, are fastened to the block via A1 Technologies’ H-11 head studs. The rocker arms are secured thanks to ARP rocker arm studs, and SoCal chromoly pushrods link them to the lifters. Billet injector hold-downs, coupled with ARP hold-down studs, keep the injectors from lifting out of the heads (a notorious problem on LB7 engines).
The power-making puzzle begins with a set of 100-percent-over injectors from Exergy Engineering, and an ATS Twin CP3 arrangement. The two LBZ pumps share the task of pressurizing fuel sent to the rail and have no problem maintaining rail pressure. Low-pressure diesel is delivered to the CP3s courtesy of an AirDog II fuel system. Forcing some serious air into the engine is an S468 Billet Cheetah turbo built by Fleece Performance. A Banks intercooler, and high-flow exhaust manifolds and up-pipes from PPE team up to keep EGT at bay. From there, the 5-inch turbo-back exhaust system handles the rest. Fine-tuning of the ECM and TCM was performed by Fleece, using EFILive software.
Photo 3/12   |   2004 Gmc Sierra Rear Three Quarter
Of all the wild street trucks we climb aboard here at Diesel Power, this was one we won’t soon forget. When the turbo lit, it was time to hold onto something. And thanks to the unique turbine wheel being employed in the S468 turbo, it’s also the nastiest sounding LB7 we’ve ever heard. Its high-rpm cackle is like nothing our ears have ever picked up on before. It’s got its own distinct sound…and it’s pretty darn cool.
Fast Facts
Year/Make/Model: 2004 GMC Sierra 2500HD
Owner: Steven O’Brien
Hometown: Reily, Ohio
Odometer: 85,000 miles
Engine: 6.6L LB7 Duramax V-8
Transmission: Allison 1000
Tires: 305/50R20 Cooper Zeon LTZ
Wheels: 20x10 Fuel Throttle
Fuel: Exergy Engineering 100-percent-over injectors, ATS Twin CP3s, AirDog II
Aspiration: Fleece Performance S468 Billet Cheetah and Banks intercooler
Horsepower: 918 hp
Torque: 1,607 lb-ft
Fun Fact: In a deal too sweet to pass up, Steven recently sold the truck to a neighbor. But, the best part about the sale is that he’s still driving a Duramax—and he still gets to see his old truck in action.



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