GM Cummins Engine Swap - Diesel Power

Arrington cleaned up the new hybrid Dodge/GMC NV4500 and bolted it back up to the GMC's original Borg-Warner transfer case. With the Dodge input shaft and bearing retainer in place, the original Dodge/Cummins flywheel, clutch, clutch fork, and throw-out bearing could all be reused. Even the Dodge hydraulic slave cylinder bolted back up and worked with the GMC's clutch master cylinder and hydraulic line. The Dodge bellhousing added close to 3 inches to the overall length of the drivetrain, though.

With the 6.5L engine out of the way, the original engine mounts were torched off the frame and ground smooth on both sides. This was the perfect time to hose the whole engine compartment down with some Simple Green and rinse off the gunk with a hose.

The donor engine was a second-generation Cummins with the simple yet powerful P7100 injection pump. Rated at 175 hp at 2,500 rpm and 420 lb-ft at 1,600 rpm, this engine's a few modifications away from producing twice as much power as any 6.5L ever could.

When lowered into place and bolted up to the transmission, it became clear the Cummins engine would have to be offset slightly to the passenger side of the truck. This position put the Dodge A/C pump into the passenger side of the GMC's frame and the exhaust downpipe into the firewall. It also became clear just how tall the Cummins engine was compared to the 6.5L. McCully was able to sit the engine lower in the frame than most people could because his truck has been converted to a solid-axle front suspension. He didn't want to put one on his truck, but McCully concedes that a 3-inch body lift would make this engine fit a lot better in an '88-'98 GM truck.

With the Cummins engine hanging from the engine hoist, McCully went to work positioning the powerplant. The engine has to sit low enough for the hood to clear, high enough not to interfere with the steering, and tilt back far enough to maintain a reasonable rear driveshaft angle. When he was sure he had his Cummins in the right spot, he built these 3/8-inch steel plate frame mounts to tie into the factory Dodge truck engine mounts.

With the Cummins back in the truck, you can see how the fabricated mount captures the Dodge/Cummins rubber mount securely. From this angle, you can also get an idea of how little clearance there would be between the stock IFS differential and the Cummins engine block. Installing a 4-inch lift kit that lowers the factory IFS differential would free up some critical real estate.

Here's where the new exhaust downpipe interfered with the GMC's passenger-side firewall. McCully removed the factory sound insulation from the firewall and sheathed it with a piece of 1/4-inch high-temperature heat insulation. It's not clear in this photo, but the GMC's factory heater core fitting and heater hose also ended up very close to the exhaust. McCully wrapped them with insulation as well to prevent damage.

Cooling a diesel is critical. Ironically, the indirect injection 6.5L GM diesel will inherently need more cooling than the direct injection Cummins engine. McCully thought about using the original GMC radiator, but in order to package the Dodge's intercooler in front of the radiator, he decided to stick with the Dodge unit.

With the '96 GMC radiator support out of the truck, McCully began trimming and clearancing it for the taller Dodge radiator. He decided to reuse the GMC's radiator mounting system (rubber isolators on the top and bottom), but in hindsight thinks he would have been better off modifying the GMC support to use the Dodge's side mount rubber grommets instead.

Originally, the plan was to saw out the radiator support in the areas traced out by the white paint lines. This would need to be done to make way for the Dodge intercooler tubes. By the time he was through, McCully removed almost twice as much metal as he planned in order to fit the tubes around the GMC's frame. Custom mounts were then made to hang the intercooler in front of the radiator and still leave room for the factory GMC A/C condenser.

With the radiator, intercooler, and A/C condenser test-fitted, McCully had to find a way to mount the Dodge A/C compressor and alternator. Originally, he thought this water outlet/mount off a Kenworth truck application from Reliable Goods would do the trick--but no such luck. He used it anyway and built his own adapters to hang the accessories high on the passenger side of the engine.

McCully wanted to use a mechanical fan, but his second-generation Cummins pulley was too tall and would have pushed the fan blades into the radiator core. McCully swapped it for an '89-'93 (first generation) pulley (right) and gained almost 2 inches of clearance. This swap required McCully to remount the factory Cummins serpentine belt tensioner in order to reuse the stock length fan belt. Dual electric fans would have been easier but more expensive.