Top Tech Questions: Diesel Transmission Swaps, OBS Fords, Built Ram

You’ve got questions? We’ve got answers!

Jason Sands
Nov 28, 2014
Photographers: Jason Sands
Welcome to Top Tech Questions. One of our favorite forms of reader communication is tech questions. Our Top Tech section is a place where you ask what’s on your mind, and we answer. Send us an email at dieselpowertech@enthusiastnetwork.com and ask away!
Three Questions
Question: I have an ’01 Dodge Ram with the NV5600 manual transmission and, due to worsening disabilities, I am interested in having an automatic transmission installed in place of the six-speed manual. I drove one with a four-speed auto and just can’t abide with that, which is why I bought the six-speed in the first place. I was wondering about installing a 68RFE in place of the NV5600. Is this a viable swap? Has it been done and what would I need in the way of controls? I don’t want to get a newer truck; I like my ’01 a lot. It’s mostly stock and will probably remain that way other than changes to the intake and exhaust. I run a Smarty on power setting 3 or economy mode, depending on what I’m doing. I’d like to put compound turbos on it as well, mostly for EGT control. I understand it would also probably increase my fuel economy. I’m not interested in maximum horsepower. Instead, my goal is usable torque for towing, and I prefer not to exceed about 2,400 rpm—and even that is more than I like. Gear spacing on the NV5600 pretty much mandates that I use it in Fifth to shift to Sixth while meeting the requirement of the owner’s manual to not tow heavy loads below 1,600 rpm. Do you have any idea why they put that restriction on the operation of the truck?
Chris Baker
-via email
Photo 2/4   |   We’ve seen lots of transmission swaps in diesels, and we can tell you that if there’s a will, there’s a way. Check out this unique adapter (arrow) that’s been used to mount a Dodge 47RH transmission to a 6.0L Power Stroke engine.
Answer: Sounds like you have quite a few different things on your mind. We’ll do our best to address them. First off, yes, an automatic transmission swap is possible, although we’d steer away from the 68RFE due to the cost, lack of power-handling capability, and lack of aftermarket controllers to run the transmission. Since you’re not looking to bump up the power a bunch, an Allison 1000 five- or six-speed even in factory form would fit the bill nicely. They’re cheaper than the 68RFEs, can handle more power, and a few different aftermarket controllers exist to run them. We’d contact a company like Destroked.com or PPE to get info on a controller, which should be used to run it behind your Cummins. Some trial and error will be needed to dial it in, although since the newer Allisons have an extra gear (or two in the case of the six-speed), we can assure you you’ll almost always be in the right gear. Now, a word about cost. If you’re the type of guy who can find a wrecked truck and do all the wiring yourself, this could be a cheap swap. If you’re like most of us, you’ll have to find a used core, get it rebuilt, and then find a computer to run it, which will cost around $7,000 to $10,000, depending on options.
Yes, running twins will result in more boost at cruising engine speeds, theoretically leading to better usage of the fuel that’s available at light loads and low rpm. This is the reason why we have seen mileage go up when running compound turbos, although a small, quick-spooling single turbo (like the one you already have) will be good when it comes to fuel economy as well. Also, when we say compounds can give a fuel economy advantage, we’re talking more like 1 to 3 mpg, compared to the 5 to 10 mpg some people seem to think. If it were us, we’d stick with the stock turbo unless you want to add some extra power into the mix, and tow heavy and often.
Finally, diesel engines are designed like any other engine, to operate in a specific rpm range, and going too low or too high outside of that range can be detrimental. When revving an engine too high, problems such as valve float can occur. But trying to make power too low in the rpm band can result in blown head gaskets or broken turbochargers from excess cylinder pressure and compressor surge. Whenever towing, it’s best to drop down a gear. Remember, Cummins marine engines were designed to operate all day long at 2,500 rpm, so you shouldn’t be afraid to do the same with the engine in your truck.
Mirror Image OBS
Question: I have been following your articles on the ’97 Ford F-350. You have also helped me in the past on the Power Stroke forums. I have the same truck and like everything you have done to yours. I already have a DP tuner and 3-inch downpipe to 4-inch exhaust. What mods would you recommend I start with to gain more power? Maybe a new turbo or new injectors? If you had to do it over again, is there anything you would change up? I have 210,000 miles on the engine, so I don’t want to push it too hard just yet without having the money to go all out. Thanks for your time and input. I would be starting with about $1,500. Great work.
Dave Ladd
-via email
Photo 3/4   |   OBS Fords are getting more and more popular to modify, as being more than 20 years old officially makes them a classic among diesel trucks.
Answer: The old “If you had to do it all over again...” scenario is perhaps one of the tougher questions to answer about one’s own project truck. We posed this question to Mike McGlothlin a while back, and he indicated that although his 7.3L is a lot faster now than it used to be after its last round of modifications (which brought the horsepower total to 555), the truck was more fun to drive all around when the engine had the GTP38R and 238/80 injectors. It was nearly smokeless, and the small turbo spooled a lot quicker. It’s not that the new setup is bad, it’s just that you reach a certain point where tradeoffs must be made in the quest for speed.
With OBS trucks being so severely fuel limited, one of the first things we’d spring for is a set of injectors. The size of the injectors will depend on the type of power you’re looking to make, although something close to a 200cc injector with a 100-percent-over nozzle would be a good choice (especially if you’re going to upgrade the turbo later). Remember, with larger injectors, you can always back down the fuel via tuning, whereas it’s very hard to get a large turbo to act like a small one. After injectors, we’d start mirroring Mike’s path on his truck and go as far as your wallet will allow.
I Want It All!
Question: I bought my first Cummins new in December 2011. It’s a great truck, but I seem to have the itch to improve everything I own/buy to a certain extent. The only things I have managed to upgrade so far are a 4.5-inch suspension lift from Fabtech, Bilstein shocks, A-pillar gauges, a Reunel bumper, and I bought an H&S XRT Pro (though I have not installed the tuner yet). My plan is to get better fuel economy, more throttle response, and make the engine run like it should. My proposed upgrades are as follows: ARP head studs, S&B cold-air intake, FASS fuel system, install the tuner, MBRP exhaust, and H&S programming. I am also looking at possibly installing an AFE BladeRunner intake horn, and I have been thinking about doing a turbo swap. Which turbo would you recommend? I was leaning toward a Stainless Diesel second-gen manifold, BD turbo, and inline exhaust brake upgrade. Can you steer me in any other route and add or take away products that may not help me or be worth the money for what I’m trying to achieve? In the end, I want a reliable truck, a little more power, and no black smoke (or very little)—something I can jump into and drive without a worry.
Robb B.
-via email
Photo 4/4   |   The Dodge 68RFE transmissions require lots and lots of dollar signs to make them live, but we have seen some last in high-performance applications. We spotted this silver third-gen Ram at the Scheid Diesel Extravaganza making pass after eighth-mile pass in the 7.7-second range (11s in the quarter) with a built SunCoast 68RFE.
Answer: First of all, congratulations Robb on the purchase! The 6.7L Cummins engines are beasts, with awesome torque numbers and low-end response when you hop them up. We do have a few suggestions for you, though. First, you can’t dismiss the transmission, as you’ve most likely purchased an automatic. If you do have the 68RFE transmission, then it’s something you’ll need to modify—even with only the mild power upgrades. H&S also offers Overdrive tuning, but we’re starting to see the 68RFEs go south at about 150,000 miles or less, even at stock power. We’d skip all the turbo, manifold, and brake stuff for now and start saving for a new transmission. Stock transmissions are upward of $4,500 from Ram, and modified versions can be more than $7,000. While we have seen some of the built 68RFEs handle more than 700 rwhp, they’re not cheap. If you have a manual transmission, then you lucked out, because all you’ll need is an upgraded clutch, provided you don’t abuse it. Another bit of advice concerns head studs. We’ve seen more than one 6.7L engine blow a head gasket, even with studs, if it’s used very hard. We’d recommend going with the ARP 625 head studs for your 6.7L, as they have a much greater clamping force than the basic ARPs. They cost twice as much, but it’s not a job you’d want to do twice.
Once your engine has good studs and the truck is outfitted with a stout transmission, we’d say it’s OK to drop the cash for a turbo, tuning, and exhaust brake. Billet 64.5mm turbos (a variety of companies make them) are good, responsive turbos that can make as much power as your stock injectors and CP3 will support. With going to a larger turbo, it’s also advantageous to have a custom tune created that will kill some of the low-end fueling and smoke. We’ve had good luck with Hardway Performance for this type of tune.

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