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December 2011 Top Tech Questions

Cummins Crew Cab, Big Tires, and Bio Fuel

Jason Sands
Dec 1, 2011
Photographers: Jason Sands
Welcome to Top Tech Questions. Oftentimes, readers contact us with questions about articles, or to praise us on what a good job we are doing. But our favorite form of reader communication is tech questions. Our Top Tech section is a place where you ask what’s on your mind, and we answer it. Got a trouble code? Wondering how to get your engine to make more power? Send us an email at jason.sands@sorc.com and we’ll do our best to answer it.
Cummins Crew Cab
Question: I’d love to have a four-door diesel, but being on a budget and just out of college, I can’t afford one of the newer ones. Plus, I like the simplicity and lack of smog on the older trucks. I really like the looks of the ’80s Chevy and GMC Crew Cabs, and I was wondering how hard it would be to sneak a 5.9L Cummins into one. Should I go mechanical or electronic on the engine? Should I install a manual transmission, or maybe an Allison? I’m just looking for an initial direction to take for my project. I’d like it to be about 500 hp at the rear wheels and have very little smoke.
Matt Bono
San Luis Obispo, California
Photo 2/4   |   Michael Thissell of Monmouth, Oregon, told us swapping all the parts over from a ’93 Dodge Cummins donor truck into his ’85 crew cab Dodge was pretty straightforward. The transmission crossmember had to be moved, and some driveline changes were needed, but mostly the swap just required a lot of time and heavy lifting.
Answer: We’ve seen four-door Dodges, Chevys, and Fords, all with Cummins power, and almost every one of their owners was very happy with his or her truck. We here at Diesel Power are actually building a four-door of sorts. Project Doomsday Diesel is Editor David Kennedy’s effort to create a Cummins-powered Suburban.
You’re also in luck with the smog issue, because GMC and Chevy made 1-ton dualies with 6.2L and 6.5L diesel engines, which means they don’t have to be smogged in California like a gas conversion would. Even if the vehicle was gas, the transplant should still be legal, as long as the factory emissions devices (catalytic converter, stock turbocharger, and fuel system) are retained.
As far as engine choices, we would stick with simple and reliable. A common-rail diesel engine will be more expensive to buy, get running, and maintain. So if you like minimal wiring and longevity, we’d go with a 5.9L Cummins 12-valve engine. For a transmission, we’d swap in an NV4500 five-speed found in later trucks, as the swap would be relatively easy, considering it already came behind the Cummins engine.
Right now, a rebuildable Allison transmission core will run $750 to $1,000, so by the time you rebuild the transmission and buy a performance converter and standalone controller, you’ll be looking at about $6,000 in parts and labor. Another option is to just use the TH400 that came in the truck, along with a low-stall converter. But not having an overdrive gear would mean 2,000 rpm at 60 mph on the freeway—even with tall tires. If the choice was ours, we’d shift our own gears and opt for the manual transmission.
As far as 500-low-smoke horsepower goes, we’d say that should be perfectly attainable. Our own ’95 Dodge project truck puts out 433 rwhp (see “Pre-Packaged Performance,” Oct. ’11) and that was with a modified HX35 turbocharger. With compound turbochargers and little AFC tuning, we fully expect to hit 500 hp. The key to the low smoke is running stock delivery valves in the injection pump, along with efficient injectors, and a quick-spooling turbo.
No matter which direction you take, it sounds like a fun project to us. Make sure to take pictures when you are finished!
Transmission vs. Tires
Question: I put a 6-inch lift and 37-inch tires on my ’07 6.0L Ford. If I don’t swap out the 3.73s, is my transmission on borrowed time? I’ve heard some say the gearing should be changed immediately, while others claim I should leave it alone since I don’t tow frequently. I don’t need to snatch a house off its blocks, I just want what’s best for my transmission and fuel mileage.
Josh Dockstader
Via email
Photo 3/4   |   Since diesels were designed for heavy loads, big tires and wheels aren’t much of a strain on the drivetrain. However, the larger you go, the worse your fuel economy will become, and the slower the truck will be due to the added rotating weight.
Answer: Re-gearing your axles with 4.10 or 4.30 gears will help save some transmission life, but since diesel trucks are designed to be able to tow heavy weights, the transmissions should be able to handle the extra load from the larger wheels and tires. One thing we might do is purchase a lower-stall converter, so the transmission creates less heat. Heat is one of the biggest enemies of transmissions; as temperature rises, the lifespan of the transmission falls. A transmission temperature gauge would also be a good idea, as anything more than 230 degrees is bad news. Be especially careful when towing with large tires, and try to limit your full-throttle blasts to when you absolutely need them.
Gearing for fuel economy with larger tires is kind of a crapshoot. If the engine speed is too low, the truck will be struggling along naturally aspirated, and fuel economy will suffer. If engine and turbocharger speed are up, you run the risk of using more fuel than you need to move the vehicle. One of the best indicators of fuel economy we have seen is exhaust gas temperature. With larger tires, we’d expect cruising EGT to be around 700 to 900 degrees as measured in the exhaust manifold. If you had gearing any higher than 3.73s (such as 3.55s), we would probably suggest a re-gear. But in your case, we’d say the 3.73s and 37-inch tires are just fine, as long as you’re conscious of your driving style and try not to be too abusive to your transmission.
Bio, or Not to Bio
Question: I am looking into buying a GMC with the 6.6L Duramax engine, and I have heard of people heating used cooking oil and running that as a fuel after some major filtering. Is this possible? I have an unlimited amount of used cooking oil, but nowhere to store or convert it into fuel while driving on long trips or even for daily use.
Brian Williams
Via email
Photo 4/4   |   Plenty of people burn waste vegetable oil in their diesels, which requires lots of filtering and works best in temperate climates. The trucks with the least repairs due to waste vegetable oil still seem to be the older vehicles with simple and sturdy injection systems, such as this early Mercedes.
Answer: The truth of the matter is all sorts of oils can be burned in a diesel as fuel. Cooking oil, motor oil, transmission fluid—all these oils can make a diesel truck or car go down the road. How long the engine and injection components will last with these energy sources is another matter. You have to remember, cooking oils aren’t diesel fuel, no matter how much you want them to be, or how much you filter them. We’ve heard of people running their trucks for many years on vegetable oil, but for every one of those stories, we’ve heard just as many about people who have had to replace fuel systems, injectors, and injection pumps. Whenever anyone asks what kind of truck they should buy to run on vegetable oil, the answer should be: a cheap one. In the case of the 6.6L Duramax engine, which has a very costly set of injectors, we’d suggest not running vegetable oil.
For running straight vegetable oil, transmission fluid, or waste motor oil, lower injection pressure engines such as ’89 to ’93 VE-pump Cummins, and ’82 to ’91 6.2L and 6.5L Chevys and GMCs are good calls. Older ’94½ to ’03 7.3L Ford Power Strokes (with their less-expensive injectors) could be good candidates as well.
While a lot of people see biodiesel and vegetable oil as the same thing, they are actually very different. Biodiesel has many additives and has to pass tests before it can be sold as a fuel. While biodiesel is a lot easier on injection parts than vegetable oil, most manufacturers only recommend about 20 percent biodiesel, blended with 80 percent regular diesel fuel.