GOT THE BUG
Aiming for 1,000 hp in a Duramax street truck
How many of us have bought a truck thinking, “I’ll just do exhaust and a chip?” How many of us have actually stopped there? Collin Casstevens from Middletown, Indiana, had the same idea when he bought his 2007 GMC Sierra 2500—until he caught the diesel performance bug and saw the possibilities. Working at one of diesel motorsport’s hubs at Fleece Performance Engineering at the time of the build, and now as a machinist for Neturen USA, the 21-year-old Casstevens well knew the potential inside his Duramax, and the idea of a 1,000hp street truck was too much temptation to pass up. With the help of several of the industry’s top shops and their best components, he set out to overhaul his daily driver into a road warrior.
After Casstevens initially did his exhaust work and a tune, he felt compelled to give the truck a little more. Once he had bolted on a stockappearing Fleece Cheetah turbo, upgraded CP3 and new injectors, he realized he was going to have to bolster his stock Allison transmission to handle the power he was making with the upgraded Duramax. Jacob Richards at Fleece helped Casstevens tear into the transmission to install a Suncoast Diesel 6Pac kit with Raybestos clutches, Transgo Jr. shift kit, a Suncoast Diesel 1058 billet triple-disc torque converter, Fleece Performance Allilocker, and Merchant Automotive’s Pump Rub upgrade and transfer case brace. With his newly rebuilt transmission, the truck seemed like a complete build—until Richards said, “You should be good to go now, unless you want to build the motor.” All it took was that seed of an idea to get Casstevens’ mind churning, envisioning a 1,000-horsepower street beast.
Soon Casstevens was talking to Chase Fleece and LJ Benshoof and Greg Lossi from Freedom Racing Engines to get the process started building his block. Using a core LBZ block as a starting point, the team at FRE was set to machine and build the short block. With a 0.020-over engine bore, cylinder heads milled with a competition valve job, bronze valve guides, FPE max life PSI springs, chromoly retainers and keepers, Trend Performance Stage 3 push rods, Carrillo connecting rods, Mahle Motorsport pistons, keyed and polished crank, Socal 3388 cam, ARP main bolts and head studs with Grade C head gaskets, and all-new Clevite internal bearings, this LBZ was ready to breathe fire.
Back at Fleece Performance, Casstevens and some of the guys in the shop helped pull some long nights assembling the engine and installing the new heart in the ’07’s engine bay. Casstevens and the Fleece team further tightened up the Duramax with an ATI Superdamper, new oil pump and oil cooler, PPE exhaust manifolds and uppipes, ATS Diesel dual pump kit with twin Fleece Powerflo 750 10mm CP3S, Pureflow Airdog 200 4G fuel system, Banks Power Intercooler and a Suncoast billet flexplate. Then they bolted the engine in place with a set of Merchant Automotive engine mounts. Expelling the fumes from the beast of an engine is a 5-inch JAMO exhaust system with a 6-inch tip. But even the built engine and single S366 turbocharger wasn’t enough to quench Casstevens’ thirst for more power.
The next stop on the road to glory was Seth Wallace and Jake Bells of BOFA (Build Overhaul Fabricate Anything) Performance in Fairmount, Indiana, to custom build a compound turbo setup for his GMC. This would be BOFA’S first kit for the Duramax powerplant, but they didn’t back down from the challenge. After tooling and retooling their fabrication, the crew was able to craft a sleek and powerful compound setup, sporting the S366 turbo as well as an S480. Casstevens’ attention to detail shined as he made sure to dress up the upgraded engine components in an electric Illusion Blueberry powdercoat finish by Shane Marshell at PCT Coatings tied in throughout the engine bay. With an expert tune by Brayden Fleece, this LBZ was set to top some charts.
Later, Casstevens upgraded his transmission further to keep up with the power with Suncoast
billet input, output and intermediate shafts, billet C2 and P2 hubs, modified P1 sun gear, and swapped to a Suncoast 1053 torque converter for a higher rpm stall speed. With this configuration, Casstevens was able to put down an impressive 950 horsepower and 1,765 lb-ft of torque in his street-driving Duramax. And with some even more aggressive tunes loaded now, he plans to break that coveted 1,000-horsepower mark next time he straps the truck down to a dyno.
With all that power coursing through the veins of the Sierra, Casstevens had to focus on getting it to the street and keeping it there. To counter a weak point often found in powerful GMS, Casstevens installed Fleece Performance tie-rod sleeves. Tightening up the performance truck’s ride are the two-inch Belltech drop shackles, Bilstein 5100 front shocks and steering stabilizer, Caltrac traction bars, and the removal of the rear overload springs. Hostile Havoc 20-inch wheels where selected and paired with 305/50 Nitto 420s for grip down the road.
The focus of the build had been to cram immense power into a streetable machine, so when it came to aesthetics, Casstevens opted for a subtle, sleeper exterior and comfortable interior. Spencer Baledge in Alexandria, Indiana, helped color-match the Silver Birch factory paint into the mirrors, handles, and bumper plastics for a sleek custom look. A billet aluminum grille with the GMC logo removed, aftermarket headlights, and tinting on all three brake lights added some subtle touches to catch your eye. Inside, the
factory gray leather remains with the only visible modifications being the Autometer A-pillar gauge pod with Ultra-lite II gauges and an Edge tuner overhead. Audible customization comes from a JL Audio 6W3 sub replacing the factory Bose model inside the center console and a JL Audio 200-watt amp tied into the sound system.
All it took was that first upgrade and a little encouragement and Casstevens caught the performance bug. Even with a lofty goal, he was able to pull together his dream truck with help from friends at Fleece Performance Engineering, BOFA Performance, Freedom Racing Engines, and his brother Jarren Casstevens. Living up to the goal, Casstevens 2007 GMC Sierra 2500 rides like a daily driver—that is, until you mash the throttle and let the beast come to life. Continuing to prove it can take the street or the track, on a recent 200 mile trip with the nearly 1,000-horsepower machine, it averaged 19 mpg. Casstevens truly built an awesome diesel truck. UDBG
Text : Kyle Tobin Photography: Chris Tobin
Collin Casstevens’ sleek, subtle 2007 GMC Sierra hides a monstrous engine caged inside ready to devour any road in front of it.
The S480 atmospheric turbo feeds air directly into the secondary S366 to boost the intake charge further before it cycles through the Banks intercooler and finally into the LBZ’S intake manifolds.
The Duramax configuration can make it challenging to keep a tidy engine bay, but Casstevens did it right with powdercoated Illusion Blueberry accents and outstanding pipe and wiring work.
With the pulley powdercoated to match the engine accents, the second Fleece Powerflo 750 10mm CP3 stands out as it does its job of providing a high volume of highpressure fuel to the oversized Fleece injectors.
With Fleece’s tie-rod sleeves and Bilstein 5100 shocks and steering stabilizer, the front end is equipped to hold steady when the potent LBZ is unleashed.
Vital components to the high-powered engine are the smoothly fabricated JAMO 5-inch exhaust (part of which is seen here) and the Pureflow Airdog 200 4G fuel pump and filter system to get plenty of clean #2 up to the dual Fleece CP3S.
BOFA Performance expertly fabricated the compound setup, which not only looks fantastic in the stellar blue powdercoat, but also helps this engine get close to 1,000 horsepower and still manage to be street-friendly.
After removing the rear overload springs, Casstevens installed Caltrac traction bars to ensure the rear end would hook up under power. Grippy Nitto 305/50R20 NT420S tires wrap around 20-inch Hostile Havoc wheels on all four corners to add some style as...