With the 2.5 charger on the truck and the added weight out front, it was noticeable that the truck was making more power and the front end was biting harder. This time, Karker’s GMC would go 302 feet before spinning out. We should note that as the second to last truck to hook in the next class (Open), Karker definitely lucked out in having ample time to perform another turbo swap in the pits.

A Remote Mount Turbo Makes The Duramax Infinitely Adaptable

In the age of purpose-built pulling trucks, sled pulling is a far cry from what it used to be. Unlike the early days of the sport, it’s rare to see one truck churn out competitive efforts in multiple classes at a single event. Whether it’s due to the rules not permitting it, the truck itself not producing enough power to run at the top, or the fact that hooking several times a day is extremely hard on equipment, it’s near impossible to see the latter scenario play out. But that’s precisely why we were intrigued with what Andrew Karker—owner of Illini Outlaw Diesel in San Jose, Illinois—had planned for his ’04 GMC Sierra 2500 HD.

At a local pull held at the beginning of the summer, Karker entered his Duramax in three different classes, swapped turbos in between to remain legal, and finished near the front of the pack each time. The classes he ran were: 8,500-lb. Work Stock (which allows T4 S300-based turbos with an inducer limit of 66mm), 8,000-lb.  2.5 (63.5mm inducer turbo limit,) and Open (anything goes). While he didn’t take home a First Place, Karker proved that his Sierra could hang with the best-running trucks in each class. So how was he able to pull it off? Read on for the full scoop on this multi-class contender. DW

Like most truck pullers these days, the engine parked in Andrew Karker’s ’04 GMC is built. The LB7 Duramax is fitted with an Industrial Injection girdle, R&R forged connecting rods, Mahle Motorsports valve-relieved pistons, a Wagler Competition Stage 1 alternate fire camshaft, Brodix/Wagler ported heads with beehive valve springs, Harland Sharp roller rockers, and ARP head studs. Relocating the turbo to the passenger side fender greatly improved its accessibility. The remote mount turbo kit came from Wehrli Custom Fabrication, incorporates a T4 mounting flange, and new hot and cold side intercooler pipes, which reverse the route in which air flows through the Banks intercooler.
The remote mount turbo system was built around using a set of PPE high-flow exhaust manifolds, and both the up-pipes and downpipe were covered in heat wrap. As for the up-pipes, the driver side unit wraps around the rear of the engine in order to feed into the T4 mount/pedestal. The passenger side up-pipe loops forward (back toward the front of the engine) in order to feed the other port on the T4 collector.
With much more available space in the valley, Karker did away with the traditional style Y-bridge and opted for one of Wagler Competition Product’s Street Intake manifolds. Made from cast-aluminum, it connects the cold side intercooler pipe via V-band and greatly simplifies the busy nature of the 6.6L’s factory lifter valley.
Putting the fuel system on steroids is a twin CP3 kit from Wehrli Custom Fabrication. The dual pump system puts two Exergy Performance (Sportsman model) pumps to good use in support a set of Exergy 150 percent over injectors injection pumps..
Twin Titanium series FASS systems keep the dual CP3’s happy, and Karker found a nice home for them where the spare tire used to reside. Both lift pumps flow 150-gph and pull fuel from a dual outlet tank sump from T/Rex Technology, so there will never be any shortage of fuel making it to the pumps.
Further reinforcement of the rear AAM 1150 lies in the LPW axle tube brace kit. It’s designed to stop any forward movement (i.e. twist) of the axle tube ends during the stress of a pull. For insurance beyond that, a set of Moser Engineering axleshafts is employed, along with a Detroit Truetrac for maximum traction.
This is a driveshaft that’s been built for a 900hp truck. Made by Driveshaft Specialist of Texas, the custom shaft is made of 1/8-inch wall aluminum and incorporates battle tested 1480 series U-joints.
For ultimate strength, and because the IFS system’s steering would have to indirectly endure the stresses of having a 900hp engine in the mix, Dmax Store’s Kryptonite tie rods were selected to replace the failure-prone factory units. They utilize 1-1/8-inch hex steel bodies and are more than five times stronger than the stock tie rods.
The five-speed Allison automatic benefits from Sun Coast internals, Raybestos clutches, and an ML-R triple disc torque converter from Goerend Transmission. The converter features a fairly loose stall in order to light a large single turbo, and a billet stator to hold up to sled pulling, boosted launches, and drag racing.
Torque converter lockup can be controlled manually thanks to this BT DieselWorks controller. On a typical pull, Karker locks the converter about 60 feet out. With a larger turbo on the truck (for the 2.5 and Open classes), the transmission was left in Fourth gear, whereas Third gear was used with the smaller S300 based turbo feeding the engine (in the Work Stock class).
Because the first hook of the day would be in the 8,500-pound Work Stock Class, Karker showed up with a BorgWarner S300 already bolted in place. Built by Engineered Diesel, it features a billet 66mm compressor wheel and a 71mm turbine wheel (exducer). This particular Work Stock class permitted the use of remote mount turbo kits, along with T4 foot S300-based turbos with a compressor inducer of 66mm or smaller. Believe it or not, the T4 flange S300 rule came about to help out the Cummins competitors, as the previous “stock-appearing turbo only” rule greatly hampered the Dodge trucks in the class.
After taking a look at the track, Karker aired the front tires down to 20 psi, and left 34 psi in the rear. His tread of choice is a 35×12.50R20 General Grabber AT2 all terrain at each corner.
As the third hook in the Work Stock Class, Karker’s Sierra put in a 290-foot effort, which would end up being good enough for Fourth Place (out of 14 trucks). An early hook in Work Stock and drawing the last hook in 2.5 meant he would have plenty of time to perform the turbo swap in the pits. For this class, only ballasted weight is allowed (no hanging weights), so tractor weights were added in the bed to get the truck as close to the 8,500-pound mark as possible.
During a few trial runs at his shop, Karker was able to swap turbos in 20 minutes. At the track, and with several helping hands, the task was accomplished in just 15 minutes. To ease the removal process (and because the turbine housing was still sizzling hot), the compressor housing was removed, followed by the compressor wheel, center cartridge, and turbine wheel getting pulled out as one assembly. The last order of business was removing the turbine housing. We have to give Karker and his crew credit for swapping turbos so quickly between hooks. Trust us, after seeing EGT in excess of 1,600 degrees, the welding gloves were well warranted!
While we couldn’t get Karker to spill the beans on his 2.5-inch charger, we know it’s Garrett-based, has one heck of a map width enhancement groove, and utilizes a ball bearing center cartridge. During street testing, Karker told us it was just as drivable as his billet S475. All secretive specs aside, we have to admit it’s a nice looking piece of hardware.
Reversing the removal process, the 2.5-inch charger was semi-disassembled, with the turbine housing being bolted to the T4 collector first. After that, the turbine wheel, center cartridge, and compressor wheel assembly was installed (shown).
While Karker buttoned up the install of the 2.5 charger, Ryan Sieh added six 100-pound tractor weights to the front weight bracket (and removed five weights from the bed). This would be the weight configuration the truck would compete with for the remainder of the day. Both the 2.5 and Open classes call for an 8,000-pound maximum.
With the 2.5 charger on the truck and the added weight out front, it was noticeable that the truck was making more power and the front end was biting harder. This time, Karker’s GMC would go 302 feet before spinning out. We should note that as the second to last truck to hook in the next class (Open), Karker definitely lucked out in having ample time to perform another turbo swap in the pits.
Just as they’d done during the removal of the S300 (i.e. removing it in sections), Karker and crew made quick work of pulling the 2.5 charger. It would be replaced by a 75mm S400 for an all-out effort in the Open Class.
Installed back to street status (air filter, turbo wrap), Karker was ready to give another BorgWarner based, Engineered Diesel-built unit a try. The S475 features a billet 75mm compressor wheel, the street-friendly 83mm turbine wheel, and a 0.90 A/R turbine housing.
In the Open Class the truck clawed its way to a respectable Fifth Place finish. For its third hook of the day, and having competed against several trucks making north of 1,000-rwhp, we’d say Karker and his team put in an impressive effort. It was a lot of work, but showed a lot of enthusiasts that you can just about pull in as many classes as you want, so long as you’re willing to turn a wrench in order to meet the rules and you’ve got a truck built to survive all the abuse. For us, it was enlightening to see a truck capable of keeping pace with a wide array of class-specific, purpose-built trucks.


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