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How To Swap a Ford 7.3L Power Stroke V-8 with a 5.9L Cummins I-6 and Allison 1000 Transmission

Big-3 Blend: Part 1 - How to Combine Ford’s Super Duty With a Cummins/Allison Powertrain

Bruce W. Smith
Oct 7, 2016
Photographers: Bruce W. Smith
Shawn Smalley and his family were 900 miles from home, pulling a 36-foot toy hauler, when the 7.3L Power Stroke engine in Shawn’s ’01 Ford F-250 let go. What had been a wonderful family vacation riding ATVs and enjoying other outdoor adventures had suddenly taken a very costly nosedive. It was decision time.
Like many other Super Duty owners over the years, Shawn had a tough decision to make: rebuild the blown engine or get rid of the truck and buy another—a costly proposition either way. His 4x4 Crew Cab had nearly 230,000 miles on the odometer, both truck and engine were well maintained, the body was straight, the interior comfortable, and the chassis tight.
Photo 2/16   |   Ford Cummins Swap
But Shawn had reservations about the 7.3L Ford and how it would hold up even after a complete rebuild. The repairs would cost more than $10,000 by the time the machine work was done and everything else was considered. That’s when he decided a third alternative would better suit his needs: replace the dead Power Stroke with a 24-valve, 5.9L Cummins powerplant and, at the same time, ditch the Ford automatic transmission for an Allison unit.
Mixing parts from other brands may seem sacrilegious to die-hard Blue Oval fans, but Shawn isn’t fazed by the crossbreeding. It’s all about practicality and long-term goals. “It makes sense,” says Shawn, who also is the owner of Mobile Diesel Service in Oakland, Oregon. “Ford’s Super Duty cabs are roomy and comfortable and don’t start rattling like Chevys and Dodges do as they get older. The 24-valve Cummins is simple and reliable compared to the Ford, cheaper to repair, cheaper and easier to hot-rod, and it should last a million miles.
Photo 3/16   |   Cummins F 250 Diesel
“And the Allison? It’s well known for its reliability and toughness. So, for me, doing the conversion instead of dumping money into rebuilding the 7.3L is a triple win. My plan is to keep this truck around for another 15 years, and the added cost of the conversion is cheaper than buying a used Super Duty.”
Getting It Done It’s not difficult to find used Cummins engines and Allison transmissions, because millions of each have been produced over the years. Shawn needed to make only a couple of phone calls to track down an ’01 5.9L engine, wiring harness, and an Allison 1000 automatic (all with relatively low miles), for a total cost of $5,500.
Since finding the engine and transmission is easy, what about all the parts needed to mate Ford with Cummins with Allison? That can be a little trickier. We called Aden McDonnell at GOS Performance to get the answers. His company in Livingston, Montana, has been doing Ford/Cummins/Allison conversions for more than a decade, and he knows every nuance of “de-stroking” a Super Duty.
GOS Performance is also the storefront for DeStroked, a supplier of parts and complete kits for diesel conversions, be it replacing a Power Stroke or Duramax powerplant with a Cummins or adapting an Allison automatic transmission to a Super Duty or a Dodge Ram.
That’s why Shawn turned to GOS Performance for his Ford/Cummins/Allison conversion, which we’re following from start to finish. It doesn’t matter whether you call the end result a “Fummins,” “Cord,” or even a “CF-250,” the bottom line is, we think it’s very likely that a truck built this way will be around for a lot of years.
Photo 4/16   |   Used Cummins engines are plentiful. Look for one with low miles and complete with the wiring harness. We picked this one up for $3,500, and it’s certified to be in good running condition.

Questions & Answers

Aden was kind enough to share his insights on the conversion process with Diesel Power. Here’s what he has to say in response to the same questions posed by owners like Shawn who are facing this major fork in the diesel pickup road:
DP: What is the relationship between GOS Performance and DeStroked?
Aden McDonnell: We’ve been buying from DeStroked for years, and we’ve developed some of its products. Last year, we partnered with DeStroked by taking over the sales and tech support while the company focused on the development and machining of new products.”
DP: What prompts Super Duty owners to “de-stroke” their trucks?
Aden McDonnell: There are a lot of different opinions on that. The Cummins is so tried and true, and nobody can deny that. It’s also twofold. Ford Super Duty owners in general are pretty die-hard. But with the advent of the 6.0L, Ford’s engine reliability came into question. The Ford trucks are still the Ford trucks. However, the 6.0L motors, to be quite honest, are a nightmare. With a 6.0L, you can address everything you think you can and something else is always going to rear its head and bite you in the butt. The Cummins is proven reliability.
DP: What’s the average cost to rebuild a 7.3L or 6.0L?
Aden McDonnell: It varies depending on who you buy it from and how extensive the rebuild, whether back to stock or upgraded. You can buy short-blocks for $5,000 to $6,000. That sounds attractive until you start putting everything else together. You’ve got an injection system, high-pressure oil system, and other areas that need to be addressed at the same time. Before it’s all said and done, you’re into them $12,000 to $16,000 to do it correctly.
DP: In terms of cost, how does that compare to swapping the Ford engine and transmission with a Cummins/Allison combination?
Aden McDonnell: You can skin a cat a million different ways. If a DIYer is going to do it from start to finish, the conversion parts will usually run between $4,000 and $6,000, depending on the package or parts he or she needs. That’s on top of the cost of the engine and transmission themselves. If you are paying a shop to handle it all, minimum entry into this ball game is $20,000. The Cummins/Allison is the most expensive combo. But it’s also the ultimate upper echelon of conversions.
DP: What is the most difficult mechanical aspect of this particular conversion?
Aden McDonnell: What I think people struggle with the most is approaching it in the correct order. Dimensionally, the Cummins is longer. To fit it in the Super Duty engine bay and to run the correct fan, the whole drivetrain needs to be shifted back approximately 2½ to 3 inches. That doesn’t sound like a big difference. But it really is when there’s so little room to work with. I’ll get DIY customers who think they’ll just pull the Ford motor out and go to stab that 5.9L engine in without moving the transmission. Nothing lines up and the fan is in the radiator. Get the motor, gearbox, and transfer case out of the way; set the Cummins into place; and then work front to back. If you do it in that order, this is not a hard job mechanically.
DP: Where does the electrical drama come into play when converting to a Cummins/Allison package?
Aden McDonnell: Getting the two to talk to the Ford electronics. A lot of these Cummins engines are mechanically compatible over a few different years, but the ECM systems are different with each one. You have to keep everything matched. I tell my customers to get the [Cummins] engine with its complete wiring harness. We have an offsite electronics guy who takes the Ford engine wiring harness and ECM and mates them to the Cummins’ harness and engine management, so what you get back is a new “plug-and-play” wiring harness and ECM. As for the Allison, we offer a new, standalone wiring harness and pre-programmed controller that interface directly with the Ford shifter and the operation of the Ford transfer case. We call it plug and play.
DP: What other key components are needed to swap the Cummins/Allison into the 7.3L engine bay?
Aden McDonnell: Across the board, I don’t care if this is an ’81 Ford all the way up to a ’10 Ford, the key components are always the engine mounts, engine adaptors, clutch kits, or flexplates, depending on which transmission you are running. A lot of the parts we offer through DeStroked you don’t have to buy; our parts are strictly a convenience for customers who don’t have the fabrication skills or shop to make the parts. You can build a lot of stuff out of a junkyard.
DP: What makes DeStroked Ford/Cummins/Allison kits attractive?
Aden McDonnell: We have really concentrated on refining our kits so you don’t end up spending $600 here, $400 there, and running back and forth to parts stores or calling some company to find a part you didn’t realize was needed to get that thing in the truck. Our kits are more expensive than others. But it’s a complete kit with everything required from hoses to adapters to manifolds. We are not leaving any stone unturned.
DP: Speaking of manifolds, what considerations should one make on turbocharger selection for an early 24-valve 5.9L Cummins that’s going into a Super Duty?
Aden McDonnell: When I’m talking to customers about turbocharger selections, the very first and foremost question I have is: How are you going to use your truck? The second question is: What type of fueling or engine upgrades are you going to do? You have to match the turbo to your use and the fueling. I’m not saying you can’t run a bigger turbo and not tow with it, but the fueling needs to be there to make that turbo work. This is especially important if you are going to run a single turbo on the Cummins. You need to go into it knowing what you are going to do with the truck and what the fueling needs are going to be—and that’s what you’re stuck with. That’s why the compounds are my go-to turbo choice: You can have big airflow and good EGT control with towing ability. With a single turbo, you set it up for street performance or for towing. You can’t have both.
DP: Are there any special needs for the VP44 injection pump and injectors if a customer is staying with the single turbo and doing a lot of towing?
Aden McDonnell: On a VP44 motor, I always recommend going at least one size injector over stock. There are a couple of reasons why. Power, of course, and good fuel economy. By running a bigger injector, we are not relying solely on the VP44 to provide the fuel source. What I have realized over the years we’ve worked with Industrial Injection is that with 50hp injectors (at least), a VP44 lives longer, because the pump is not doing all the work. I believe in matching fueling to air, and in order to do that you need to put the right combination of parts together on the pump, with the injectors, and with the air. They all need to be matched. The days of blowing smoke are coming to an end, and if we don’t get a handle on it, it’s going to ruin our industry.
DP: Dodge Ram HVAC hardware is behind the firewall, while a Super Duty Ford’s is in the engine bay. How do you get the 5.9L Cummins’ turbocharger to clear the Ford HVAC system?
Aden McDonnell: That’s an issue we figured out early in doing these conversions. The 5.9L Cummins engines run a center-dump exhaust manifold that literally positions the turbocharger right on the Super Duty’s heater box. You can kind of get by with using the stock exhaust manifold, but the downside to going that route is you’ll end up with about ½ inch of clearance between the turbo, the exhaust side section, the downpipe, and the heater box. We’ve seen trucks with this setup come into our shop with the heater box melted. The other downside to it is the stock turbos that come on those vintage of trucks require removing the wastegate in order to get them to fit in that location. I don’t believe in that: Wastegates are there to prevent overdriving of the turbo. What we recommend and supply in our complete kit is a BD #1045987 manifold from BD Diesel Performance. It has the offset turbo mount so you can retain the factory Ford air conditioning by relocating the turbo to clear the A/C components.
DP: What years are the best for the GM Allison 1000 automatic transmission?
Aden McDonnell: We can support ’01-to-’10 Allison five- and six-speed automatic transmissions from GM HD pickups. I suggest ’06-to-’10, because they don’t require swapping the five-speed valvebody for the six-speed, which is what our standalone wiring harnesses and modules are designed to control.
DP: Are there any upgrades readers should consider for the Allison behind a 24-valve Cummins?
Aden McDonnell: The first and foremost upgrade we recommend is the torque converter. It’s the key to everything. We’re putting an inline motor in that has substantial amounts of additional torque at a lower operating range compared to the engine it’s replacing. We always recommend a low-stall converter for the Cummins/Allison conversion; between 1,600 and 2,000 rpm. That’s ideal for daily street and towing applications. Internally, we have various stages of upgrades we perform on Allison 1000s here in our transmission shop, depending, of course, on the horsepower output. My rule of thumb for engines making up to 700 hp is a standard build that includes good clutches, a shift kit, and a converter. We are totally fine with all the other internals in an Allison. If the Cummins is making between 700 and 1,000 hp, the transmission definitely gets billet input and output shafts.
DP: When someone spends this kind of money to swap a 24-valve Cummins 5.9L engine and an Allison automatic transmission swap into their Ford Super Duty, what’s the payback?
Aden McDonnell: Reliability and performance. The Cummins has always been touted as a million-mile motor. I 100 percent believe in that. I have a local customer with a truck that we did the 24-valve, common-rail swap with an Allison on. The rig’s got 267,000 miles on it, and we never touched the truck since that installation was done. Good maintenance, correct fueling, keeping an eye on EGT, a well-built transmission, ensuring transmission temperatures are kept in check with good coolers—it all plays a big role in the overall reliability and longevity of the new drivetrain.
Photo 11/16   |   Cummins I-6 engines are known to have some cooling issues with the rear two cylinders. We’re eliminating those concerns by installing this Fleece Coolant Bypass kit.
Photo 15/16   |   Another item in the conversion package that helps makes the swap easier is this lower radiator hose kit, which eliminates the need for running to the parts house to track down the correct fittings and hoses.
Photo 16/16   |   Trying to figure out where to place a 5.9L Cummins engine in an ’01 Ford F-250’s frame would be a real headache if it weren’t for these custom motor mounts. They are labeled and ready to bolt into ’99-to-’07 Super Dutys.


BD Diesel Performance
Sumas, WA 98295
Wheat Ridge, CO 80033
Fleece Performance Engineering
North Salem, IN 46165
Mobile Diesel Service
GOS Performance