Playing with trucks is an affliction that never ends. For most of us, we don’t just build our trucks and then sit back and admire them. They’re a constant work in progress with endless changes taking place throughout the course of ownership. No matter how good the truck looks or how well it runs, there is always a “next step.”

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John Thompson— a high-energy Duramax addict—illustrates this point to a tee. Over the past seven years his ’08 Silverado 2500 HD has been lifted, lowered, sported large single-turbo setups as well as a compound arrangement, and has had parts combinations ranging from 600 to 1,100 rwhp thrown at it. Along the way, he’s cracked a piston after pushing the stock LMM bottom end too far, snapped a crank at the 950hp mark, and been kicked off the dragstrip for going too fast. When it comes to pushing the limits, Thompson has definitely been there, done that.

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Built to endure plenty of abuse, the LMM-based Duramax in John Thompson’s ’08 Chevy Silverado 2500 HD was pieced together by Illini Outlaw Diesel. An Industrial Injection girdle, R&R connecting rods, 15.5:1 billet-aluminum Diamond pistons, and alternate firing order camshaft from Wagler Competition Products all make for a stout bottom end. A set of Stage 2 Wagler Competition Products heads (fitted with REV dual valve springs) reside up top and anchor to the block via ARP Custom Age 625+ studs. Worthwhile insurance items include a keyed crank, cam, and water pump.

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Sitting atop a Wagler T4 Twisted Pedestal is a 2.5 Class turbo spec’d from Diesel Technology Source. The Garrett-based charger features a billet compressor wheel with a 2.5-inch inducer, an 87mm turbine wheel, and makes use of a ball-bearing center cartridge. With the engine under load in the midst of a pull, the turbo produces 65 psi of boost, which it sends through a Banks intercooler. The intercooler piping and coolant tank were provided by Wehrli Custom Fabrication and finished in Candy Copper at Wehrli Custom Powder Coating.

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To maintain rail pressure for a thirsty set of Exergy Performance 250% over injectors, a twin CP3 system from Wehrli Custom Fabrication is employed. A 12mm stroker pump from Exergy sits in the factory location, while a stock displacement, LBZ-intended CP3 gets belt driven up top.

After Thompson took past experiences with big horsepower (and the inevitable breakage that went along with it) and his desire to hook to the sled into consideration, he recently put the truck under the knife once more in order to find a happy medium. Reliability with respectable horsepower was the game plan. Thanks to the finished product sporting an overbuilt engine and driveline, this late-model Bowtie was successfully transformed into a competitive, 850hp 2.5 Class puller.

Billet, Forged, and Cast Parts

Built by Illini Outlaw Diesel, some of the industry’s best aftermarket parts are put to use inside the LMM Duramax under the hood. A balanced rotating assembly consists of a brand-new crankshaft that’s been keyed; R&R connecting rods; Diamond billet-aluminum, 15.5:1 compression pistons; and a Wagler Competition Products alternate firing order (keyed) camshaft, while an Industrial Injection girdle holds down the fort. A set of Wagler Competition Stage 2 heads, equipped with REV dual valve springs and incorporating Merchant Automotive’s chromoly pushrods, clamp to the block via ARP Custom Age 625+ studs.

No Frills Fuel System

No competitive puller would be complete without copious amounts of fuel available, and a set of massive, 250% over injectors from Exergy Performance ensure that’s always the case. The big sticks are supported by a Wehrli Custom Fabrication twin CP3 system that employs an Exergy 12mm stroker pump in the factory location and a stock LBZ pump up top. A Deviant Race Parts tank sump and a 220gph FASS system provide plenty of low-pressure fuel supply for the CP3s.

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The job of feeding the CP3s is left to a Titanium series fuel system from FASS, which is capable of flowing 220 gph. The FASS system is gravity fed fuel from the tank thanks to a Deviant Race Parts sump.

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With tread patterns vaguely resembling the BFGoodrich All-Terrain that’s had so much success in truck pulling over the years, a set of General Grabber AT2s help propel Thompson through the dirt. The allterrain tires ride on 16×10-inch ION Alloy 174 wheels.

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Somewhere between working seven days a week on the family farm, Thompson finds time to do what he does best: hook to the sled. And now that he’s transformed his Bowtie into a full-blown 2.5 truck, he told us that his trips to the track will only grow more frequent.

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In order to hold up to truck pulling, the front end has been fitted with some of the best parts in the industry. Kryptonite series tie rods and a solid center link rule out flexing under load, and a Cognito Motorsports pitman and idler arm support kit eliminates fore and aft movement in the center link. For maximum traction, an Eaton ELocker is employed, while Fleece Performance Engineering’s TufShaft axle shafts add strength and eliminate the failure-prone central axle disconnect.

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Torque transfer begins with a competition-ready Allison 1000 that was built by Illini Outlaw Diesel. Internal modifications include a GMax-6 clutch kit from SunCoast and billet input, intermediate, and output shafts. An ML series triple-disc torque converter from Precision Industries features a 2,800rpm stall and a billet stator. A transmission blanket from DJ Safety is also utilized, which is a requirement per Thompson’s local 2.5 Class rulebook.

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Keeping the rear end digging as opposed to hopping is a set of traction bars from Big Chevy Hitch. Made from heavy-duty 2-inch-diameter tubing, they incorporate chromoly heim joints and polyurethane bushings. They secure to the frame and bottom of the AAM 1150 axle tube via bolt-on brackets.

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A Dirty Hooker Diesel spool, 4.56 gears, and Yukon 38-spline axle shafts reinforce the AAM 1150 in the rear. Additional cooling of the Allison comes by way of a BD Xtruded transmission cooler mounted to the rear cross member.

Innovative Turbo

As is the case in any air-limited pulling class, Thompson’s Duramax is on the cutting edge as far as turbo technology is concerned. A competition charger from Diesel Technology Source incorporates a billet compressor wheel, ball bearing center cartridge, and an 87mm turbine wheel into a Garrett-based package. The turbo mounts to a Twisted T4 Pedestal from Wagler Competition Products and Wehrli Custom Fabrication intercooler piping routes boosted air in and out of a Banks Techni-Cooler intercooler.

Battle-Ready Allison

Just like the Duramax in front of it, the Allison 1000 was prepped for the rigors of sled pulling abuse by the folks at Illini Outlaw Diesel. A GMax-6 clutch kit from SunCoast along with billet input, intermediate, and output shafts reside inside the six-speed automatic, while a 2,800rpm stall Precision Industries tripledisc torque converter handles power transfer and brings the 2.5 charger to life at the right time. Thompson utilizes an AlliLocker from Fleece Performance Engineering to control converter lockup, and all ECM and TCM tuning was performed by DuramaxTuner, with the truck laying down 854 hp on the company’s in-house chassis dyno.

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Dyno-tuned by the EFI Live experts at Duramax- Tuner, the truck sends a respectable 854 hp and 1,375 lb-ft of torque to the ground. Thompson navigates the custom tuning files via a DSP5 switch mounted next to the steering column.

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When it comes to truck pulling with an automatic transmission, the timing of when you lock the torque converter is crucial (as well as when you unlock it at the end of the track). For full control over lockup, Thompson uses an AlliLocker from Fleece Performance Engineering, which overrides the Allison TCM.

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A-pillar-mounted Cobalt series gauges from Auto Meter allow the driver to keep tabs on EGT, boost, and rail pressure. Virtually every other parameter can be monitored via the Edge Insight CTS2 mounted to the windshield.

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Dedicated

Despite owning a truck that’s worn many hats over the years, Thompson tells us he’s content leaving it set up as a dedicated 2.5 Class contender for the long-term. However, that doesn’t mean he’s done making changes. Before the 2017 pulling season gets underway, he plans to rework the rear suspension and install the latest and greatest 2.5 charger on the market. Like we said, no matter how good the truck looks or how well it runs, there is always a “next step.” DW