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New Hampshire's Original Diesel Performance and Repair Specialists

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Scott's Cummins M998

As if an Ex-Millitary M998 Humvee isn't already cool enough, Scott had the idea of putting in a Cummins.  This has been done before but this one is a little cooler than the typical one.  Instead of the simple and generic 12 Valve swap this 6.2 is being replaced by a 5.9L Common rail motor out of a 2006 Ram.

The Build

All Motor Swaps are a Puzzle, just sometimes you have to make the pieces yourself

An Idea is Born

Scott Came to us with his beautiful M998 Humvee and explained what he wanted to transform it into, the Humvee was great but with its 3l80 transmission and its non- turbo 6.2L GM Diesel Motor it was just not suited for much road driving.  He had already purchased a doner truck, a 2006 Dodge Ram 2500 Cummins.  He wanted to keep the Humvee a Humvee but wanted it to have more power and be more suited to drive around on the road.

Disassembling the parts truck 

The first step of this build was removing the Engine, Transmission, Engine wiring harness, and Body wiring Harness from the donor truck.  With the engine being a 2006 it is controlled by computer and the throttle is controlled by and electric petal so it was nessacary to remove the full body harness to control the engine.

Preparing to remove the 6.2  

It was time to tear down the M998 and that started with removing the front cowl.

With the 6.2 being set so far back in the chassis we had to prep the body to be lifted

Because of the motor sitting almost completely under the body and the rest of the drivetrain being tucked into the body for high clearance we had to lift the body off the frame to go any further, to do this the fuel tank and exhaust had to be removed.

Body is finally off!

With all the wiring, steering shaft, and other lines disconnected the body could finally be separated from the frame.

Removing the 6.2

Because of how far back the motor is set and how high it had to come to clear the air lift hooks a conventional motor crane just wasn't the right tool for the job .  Good thing we are prepared for these situations 

Positioning the Cummins

With the Cummins being a straight 6 apposed to the V8 it is much taller and longer than what was designed to be in the M998, No room for the massive stock fan of the Cummins motor.  With no fan in the way we were able to bring the motor forward to help with the added length.

Making Motor Mounts

With the Cummins where it needs to be we were able to make new motor mounts

Now was time for the 48RE and NP241

The 3l80 had to go, it wasn't suited for the power of the Cummins and 3 speeds just isn't enough to comfortably drive on the road.  So in comes the 48RE from the Dodge.  Well the AWD NP218 is great for off-road and combat situations,  it is not needed on the road, that being said the 4x4 capabilities of a Humvee are what makes it a Humvee so in goes an NP241 transferase to make it 2wd/4wd.  With the Doner truck being a RWD model we had to remove the tail shaft and install the proper parts to make the transmission 4wd.

Keeping it Cool

With the 6.2 being naturally asperated there was no intercooler originally in this truck, and with the body sitting so low on the frame the radiator is slanted back at a pretty steep angle.  With the addition of the front lift towers being located where they are there was no chance of running piping for a conventional style intercooler mounted in front of the radiator.  With Scott and ourselves wanting this to look as factory as possible from the outside we did not want to relocate the radiator or remotely mount the intercooler.  We decided to go with a water to air intercooler set up and mount it to the front of the radiator.  This allowed us to just run small diameter coolant lines up to the intercooler radiator, and mount the intercooler under the radiator.  These sytems use an electric water pump to cycle coolant through its own radiator then through fins that the air coming in has to run across.  In this process we also angled the radiator more vertically allowing more room behind for the engine because the body was now higher to allow it to stand taller.

4" Flat Exhaust 

From the factory a M998 had a 2.5" exhaust that ran down the drivers side, well this may be enough flow for the N/A 6.2L it would never provide enough flow for the 5.9L.  With the Cummins having its turbo on the passenger side we ran into two issues, no space for a proper flowing 4" exhaust and the fuel tank being located on the passenger side as well.  The solution was a custom in house built 4" Flat Stainless Exhaust.  To route the exhaust back over to the factory exit behind the drivers rear door and still maintain having nothing hang below the frame we had to route it diagonally over the transmission and transfer case.  We didn't want to cut down on the flow so it using pie cut bends and running it diagonally under the body was our best option.  After ben got all the pieces cut, fitted and Tig welded we wrapped the whole exhaust in a header heat material.

Driving the Front Differential 

With the Cummins oil pan hanging so low in the frame and the front differential being center mounted a conventional once piece driveshaft would not work.  Our Solution to this was a custom 2 piece driveshaft with a custom carrier baring being fixed to the drivers side engine mount.  We also had to have a new shorter rear shaft made with the dodge style yoke on one end and the Humvee yoke at the other.

Body Comes down for the Final Time

With space always being a concern the body was lifted and set back down all through the process of getting everything to fit right.  We removed the drivetrain for one last time to clean and paint it. All Modifications to the body for clearance were fairly minor as we wanted this to still feel and look as it came off the assembly line with this drivetrain in it. With the body now hard mounted we were able to fabricate the custom drop brackets for the fuel tank and filler neck to adapt it to the now 3" higher body.

Freshly Painted 5.9L

Before setting it in its new home for the final time we painted the engine and accessories with a high temp gloss black engine enamel.  

Wiring and Fitting the Intake

With the body back down we were able to fit coolant lines and mount the power steering reservoir, and oil catch cans.  We also made a custom intake to route into the factory M998 air Box.  We had to modify the air boxes high water foraging inlet to allow enough air in to let the Cummins breath. We also begun making our hood hinge spacer to account for the body lift and extending the hood lift points.  

  With the drivetrain being complete possibly the most complexed hurdle of the whole project was now here, Adapting the Cummins and M998 Wiring Harnesses to work with each other.  With the M998 being a 24 Volt System and the doner truck being 12 volt we had to convert the Humvee batterys to 12 volt the use two 12-24 volt step up converters to rung the lighting and interior electronics.  The Cummins throttle petal was mounted and used for the drive by wire throttle.  To make it run right the great guys up at Motorops wrote us an EFI-Live tune for the modifications.

The Final Pieces to the Puzzle 

With the M998C now running, driving and looking proper we had to hook up the 48RE and transfer case shift Linkages to the factory M998 Shifters.  After this was complete we had one last issue to address, the bulkhead in the cab.  The factory Bulkhead did not have the clearance needed to clear the back of the Cummins. A local sheet metal shop worked with us to fabricate this cover to mount to cut out bulkhead for it would look and fit properly.  We then covered the inside of the bulkhead with a fire wall heat and sound dampener matt and painted the bulkhead to match the rest of the Humvee.

Final Touches 

With the M998-C now complete we added the final touches, topped off the fluids and gave it a good final cleaning.  We had installed this custom round top billet valve cover with our logo machined into it as well. the stock 06 valve cover stood to tall to fit under the cowl so a lower profile was needed the logo was just a little added touch.

Ready for Pickup

The M998-C was now ready for its happy owner to enjoy for years to come.  Looking and preforming much more aggressively than when it arrived