Square Body Cummins

July 22, 2023

An ’85 GMC Body, A Second-Gen Chassis, and A Compound Turbo’d 4BT

How many of us have seen your dream truck parked along the highway? When it comes to old iron—especially the classics we grew up with—this method of shopping for a vehicle is more common than you think. Some time ago, Austin Magruder spotted this ’85 GMC, then in two-wheel drive form, while passing through Rolla, Missouri. He immediately inquired about it, bought it and spent years touring the Show-Me states in it. Then a friend put his 4BT Cummins-powered step van up for sale and the wheels started turning…

Perfect Timing

Luckily for Austin, around the same time he acquired his delivery van, the folks at nearby MCS Midwest Performance Diesel were parting out a second-gen. Not only did Austin buy the ¾-ton chassis, suspension, NV5600, and NP241 transfer case, but he also enlisted their help in making his 4BT square body dream a reality. With a trip up to Brown’s Auto Body, the GMC’s body was flawlessly swapped over to the Dodge chassis and the project was returned to MCS, where months of work would be poured into Austin’s classic creation.

Performance Overhaul

Austin was not willing to risk the engine’s 400,000-mile-take-out, so they had MCS disassemble the 4BT for an overhaul. They also made a few upgrades. The forged steel crank and connecting rods remained, but the rod beams had been micro-polished. The pistons with overbore would be OEM, but the skirts of these pistons were coated in dry film to reduce friction. On top, a ceramic thermal shield was applied. The stock camshaft was swapped in favor a 188/220 unit from Hamilton Cams to drive the turbo(s) harder, with Hamilton’s bolt-on cam retainer, tappets and heavy-duty pushrods making the cut as well. The original cylinder was found to be cracked. An OEM replacement was then ordered and fitted with Hamilton Valve Springs before being machinized to accept O Rings by LinCo Diesel Performance. ARP 425 head screws secure the head to the block.

Plucked from an Interstate Battery step van, the 4BT Cummins in Austin Magruder’s ’85 GMC was rebuilt by Keith Summers of MCS Midwest Performance Diesel. With the 4BT’s stock hardware more than capable of handling in excess of 400-hp, nothing crazy was required to ensure the 3.9L remained reliable. The short-block upgrade consisted of a factory crankshaft and original forged-steel connecting rods (micro-polished), overbore OE Pistons with ceramic-coated tops and skirts, as well as a 188/220 Hamilton camshaft.

DW-2106-4BTGM-03 — MCS supplied a new cylinder head to Austin due to the crack found in the original. It was also fitted with Hamilton Cams’ stiffer valve springs. LinCo Diesel Performance O-ringed the cylinder head to ensure a good seal during high boost. A composite OEM head gasket was used and the head was anchored to block with ARP 425 Head Studs.

DW-2106-4BTGM-05 — Relocating the oil filters makes it easier to change oil when compound turbos come into play. Pacbrake’s remote oil filter relocation kit makes greater accessibility possible on Austin’s square body, and LinCo mounted the oil filter on the passenger side of the engine bay, right next to the air filter.

The Spooling Solution

To get around the 4BT Cummins’ inherent spooling issues, Austin enlisted the help of LinCo Diesel Performance to build a compound turbo arrangement. The guys at LDP responded by spec’ing out a combination that placed the factory Holset HX25 over a BorgWarner S300-based charger from Stainless Diesel, then fabricated the piping to make it work. The Stainless Diesel S300 uses a 5 blade compressor wheel with a 63mm inducer, and a 68mm turbo wheel within a T4 divided.83A/R exhaust housing. The HX25 bolts on to a Steed Speed T3 exhaust manifold. This combination delivers 70-psi through a factory intercooler.

DW-2106-4BTGM-04 — A compound turbo system forces 70 psi boost into the 4BT. Spec’d and fabricated at LinCo Diesel Performance, the combination includes a stock-based Holset HX25 over an S363 atmosphere charger from Stainless Diesel. The S300 is equipped with a five-bladed, 63mm, billet compressor wheel and 68mm turbine wheels, as well as a T4 divided exhaust system. The second-gen factory intercooler limits EGT to 1,100 degrees F, which is a very manageable temperature.

DW-2106-4BTGM-06 — Northeast Diesel Service finessed a significant amount of additional fuel out of the engine’s Bosch P7100. During the overhaul, the P pump was equipped with new 12mm barrels and plungers, as well as higher governor springs. The fueling capacity of this engine’s Bosch P7100 was also increased using standard parts. Incredibly, the little P7100 is capable of flowing 425cc’s.

Rebuilt And Benched P7100

Northeast Diesel Service was tasked with unleashing the potential of the Bosch P7100 12mm pump. The renowned pump shop created a street-friendly, smoke-free version of the Bosch P7100 that delivers solid power. Using genuine factory parts, including brand new plungers and barrels, the P7100 is capable of flowing 425cc’s worth of fuel but timing is set conservatively in order to aid spool up and cold-weather starting. AFC Live controllers from Power Driven Diesel are used to fine-tune the engine in the cab. A single filter Platinum series of FASS systems ensures that the P-pump is always supplied with enough fuel.

California Tested Different Minimum Speed Limits For Freeway Lanes. It Didn’t Work

DW-2106-4BTGM-07 — A single Platinum series FASS filter is mounted on the driver-side frame rail to deliver low-pressure fuel to the P-pump. The lift pump pulls fuel from a FASS sump installed in the factory second-gen tank, the tank having also been part of the deal when the GMC body was swapped on top of the ’00 Dodge chassis.

Second-Gen Drivetrain

The ’00 Ram 2500 donor at MCS Midwest Performance Diesel proved invaluable in transforming the square body into a true heavy-duty truck. The transmission, an NV5600 six-speed, was matched to a South Bend one-disc clutch along with a brand new flywheel. MCS added a free-spin Hub Kit to the equation. The front solid Dodge Dana 60 was retained with the second generation chassis. The tough-as-nails Dana 80 was retained, too—complete with all the factory leaf springs above it. Both axles were re-geared with 4.10’s (from 3.55’s) while in the care of Northeast Diesel Service.

DW-2106-4BTGM-13 — Northeast Diesel Service treated the Dana 80 and 60 to a new gear while in its care. In switching from 3.55’s to 4.10’s, the 4BT is kept perfectly within its happy place in the power curve. Rancho RS5000 steer stabilizers help reduce bump steer and vibration.

DW-2106-4BTGM-08 — Along with the frame, axles, and most of the suspension coming from MCS Midwest Performance Diesel’s second-gen donor, Austin received the truck’s NV5600 manual and NP241 transfer case as well. The combination of a single disc clutch from South Bend and the six-speed transmission has zero issues coping with the square body’s estimated 400 to 450 hp.

Work & Play

Even though it wasn’t necessarily built for work, with quick spool up, low EGT, a solid foundation, and a manual transmission, Austin’s GMC will have zero issues when it’s attached to the occasional bumper-tow trailer. Austin will enjoy his truck for the moment, and take it to all local car shows. If he has the urge, Austin may even hook up the square body a few times this summer. Wherever it goes, we expect Austin’s old-school, tractor engine’d creation to spark numerous conversations—as well as inspire other swap projects.