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Top Diesel Tech Questions

You Asked, We Answer

David Kennedy
Sep 1, 2006
Photographers: David Kennedy
All right, you asked for it. And now you're going to get it.
Diesel Power is already the leading diesel enthusiast magazine, where readers come to learn about all things diesel-powered. Now we're going to ratchet it up a notch. Starting with this issue, we're going to field all your diesel-related tech questions. If you have a problem with your 6.9L, can't find a part number for your LLY, want to know how to make 400 lb-ft with your VW 1.9L, or just want to know what's the best transmission fluid to use in your 48RE, then this is the place for you. We'll begin this month's installment with some of the more basic questions we've already received. Where we go from here is entirely up to you. The best way to get your questions answered is to e-mail them to If you're the type that's more comfortable with the U.S. Postal Service, send your questions to:
Top Tech
C/O Diesel Power
6420 Wilshire Blvd
Los Angeles, CA
Now the disclaimer: Please understand that we can't return anything you send us and that we reserve the right to edit your letter for length and content. You should also realize that we can't answer every letter we get, but we do promise to read everything you send us. Please try to limit your letter to one question, and, obviously, the more information you give us about your problem and vehicle, the better our chances of solving your problem.
How Much Torque Will The Allison Handle?
Q: I just bought my first diesel truck, an '06 Chevy 2500 HD, and I had heard the transmission will only handle 650 lb-ft of torque. I was wondering if you knew what it would really handle? I want to put a performance exhaust and an air filter on it but keep the transmission stock for now. Will I have problems with the Allison?
Mike Mullahey
Watervliet, NY
A:The torque capacity rating of the Allison transmission is tied to three things: how much power your engine makes, how much load you put on the transmission, and how much traction you get when putting the engine's power to the tires. Theoretically, the Allison 1000 can handle 650 lb-ft of torque towing it's maximum rated gross combined vehicle weight rating (up to 23,500 pounds) on stock tires. General Motor's durability testing should ensure that if you were to run your truck at maximum GCWR for 36,000 miles (the length of the powertrain warranty), you shouldn't have a problem. So it stands to reason that you could run more than 650 lb-ft through your transmission if you're not running at max GCWR all the time. We're hard-pressed to say exactly how much more power your particular Allison can handle, figure around an extra 100 lb-ft. Rest assured it will live just fine with the added power gained by the intake and exhaust improvements.
Foreign Diesel Invasion
Q: I went a different route, and instead of a domestic diesel truck, I imported a '90 Mitsubishi Pajero 4x4. It is the Asian version of the Montero/Dodge Raider. It has a 2.5L turbocharged and intercooled diesel. It is automatic with air conditioning and limited slip-a great little 4x4. I put in a larger exhaust from the downpipe on back, added a K&N air filter, and a block-off plate on my EGR valve (a part that I had to order from England). These vehicles are common over in Europe but quite rare over here.
I was wondering if there was anything else I could do to produce more power? Unfortunately, it has mechanical fuel injection and they don't sell a power programmer for it in England-only for electronic fuel injection engines.
I enjoy reading all your articles and seeing if I can cross-reference any tips to my Mitsubishi. Maybe you guys can do a story on imported diesels that have made it here in the U.S. They're here, just not in the quantities of the Big Three.
David Maness
Via e-mail
A: Imported diesel vehicles are cool. The downside (as you've found out) is that the fewer of them there are here, the harder it is to find repair or upgrade parts for them. The Internet is the best way to tap into performance parts from other countries, as well as online forums dedicated to your particular vehicle. While we're not intimately familiar with your 2.5L engine, there's a good chance shops that are used to dealing with Bosch, Stanadyne, or Mitsubishi mechanical injection systems will be. If they can't help you get more power out of your diesel, you can always try retrofitting a propane injection system.
Dodge Launch Shudder
Q: My family drives an '03 Dodge Cummins Quad Cab two-wheel drive pickup. We have a full K&N system, a MotoBlue chip, and a MBRP 5-inch turbo back exhaust. When going from 0-45 mph or so, the automatic transmission has a shuddering effect that seems to start from the rear axle. Is that the rapid shifting it has or are the clutch plates slipping?
Brian Robert Bauman
Via e-mail
A: It could be transmission slippage under load or it may be driveline vibration as a result of poor carrier bearing alignment. There's really no way to test the rear driveshaft scenario at home in a two-wheel drive truck. With a four-wheel drive truck, you could remove the carrier bearing-to-rear axle portion of the driveshaft and shift the truck into four-wheel drive to essentially drive the truck in front-wheel drive only. If you still feel the vibration, chances are good it's in the transmission. If you don't feel the vibration, we'd suspect the carrier bearing and U-joints.
Kerosene vs. Diesel Fuel
Q: I have found kerosene for $1.99 per gallon and diesel for $2.79 per gallon. The pump is located in the fuel island as "unlabeled fuel," not off-road fuel. Can I use kerosene in my '97 Ford Power Stroke?
Scott T
Via e-mail
A: Yes and no. Your 7.3L will run on kerosene, but it will produce less power and the Caterpillar-designed HEUI injection system will not have the benefits of #2 Diesel fuel's lubricity. Even though it's cheaper, kerosene (which is similar in formulation to #1 Diesel) has less heat energy per gallon than #2 Diesel fuel. That means less power at the rear wheels and fewer miles driven between refueling. Plus, without being taxed for road use, it's illegal to use it in your street-driven truck.
Warped Super Duty Rotors
Q: I have an '02 Ford F-250 Super Duty that I've put 57,000 miles on over a 20-month period. I drive long distances (500-700 miles) one way with the factory brake pads and rotors with 0.01-inch clearance (factory specs) between them. I'm warping my rotors about every 12,000 miles, and, no, I don't ride my brakes. I've already had my rotors turned four times, and I am about ready to have them replaced now. No one can give me any helpful ideas that will keep these rotors cool on long hauls, and I can't find non-metallic brake pads that fit F-250 Super Duty calipers. Got any ideas for me?
John Wadlow
Via e-mail
A: You'll want to replace those front rotors with new ones as soon as possible. In our experience, resurfacing warped rotors is not a solution for trucks like yours. Every time you remove material to make the rotors true again, you've reduced the amount of iron available to dissipate the heat caused by the brakes. That means the rotors will warp faster than they did the first time.
The next time you're on one of your long-distance trips, consider re-evaluating your braking techniques down long grades. Try applying your brakes firmly to bring the vehicle under control and then downshift into Third gear and coast at a safe speed with your foot off the brake pedal to allow the rotors time to cool. You could also improve brake life with an aftermarket exhaust brake kit.
If neither of those ideas solve your problems, you might also consider Off Road Unlimited's (818/563-1208, Ford brake upgrade kit. For $800, you get a new set of rotors that have been slotted for and cryogenically treated to stay cooler and run truer than your current rotors. The ORU brake kit also includes new brake pads to complement the heavy-duty rotors.
Wandering Dodge Frontend
Q: I have a '95 Dodge 3500 dualie 4x4 with the 5.9L Cummins automatic transmission and power steering. The problem I am having is that the steering seems to wander all over the road when it is driven. I constantly have to adjust the wheel back and forth to maintain a straight line. The truck only has 70,000 miles on it, and it tows my 32-foot Haulmark race car trailer without any effort. I have replaced the steering damper-and even added an additional damper-had the frontend aligned, and installed new tires, all with no luck. Is there some sort of adjustment to take the play out of the steering box, or is there another cure for this problem?
Don Kanka
Trenton, NJ
A: The first thing we would check is the track bar. It's prone to wearing out on heavier Dodge trucks that tow a trailer or see off-road use. When the ball-joint end at the frame wears out it can cause all sorts of handling and steering issues. It's not as much of a problem on 1/2-ton trucks, but on diesel and 1-ton trucks it can wear quickly, where the 1/2-ton sized part is not up to the task. Moog offers a Problem Solver track bar upgrade for Dodge trucks (PN DS1256) for trucks built prior to February 7, 1994, and DS1413 for trucks built from February 7, 1994 to 2002. The track bar on the '03-and-newer Dodge Rams seems to be a far stronger design that is not prone to the same problems.
What's The Largest Tire I Can Fit?
Q: I have an '00 7.3L Ford Excursion with 265/75R16 tires and stock wheels with no suspension lift. I was wondering what the biggest tire I could fit under my truck is.
Mexico City, Mexico
A: We've ridden in a V-10 Excursion with 285/75R16 Goodyear Wrangler AT/Ss (approximately 33.1 inches tall and 11.3 inches wide) on stock wheels. The owner of the truck is pretty sure he could fit 305/70R16s (approximately 33 inches tall and 12.2 inches wide), too. Anything bigger than that and the front tires might rub.
Stroker vs. Smoker
Q: I have a '79 1/2- ton GMC pickup that I'm working on and want some advice to what engine to put in it. I have assembled all the parts on paper and wrung out the combination on a desktop dyno to see what it would give me. When I began my project, I was planning on a gasoline 383 small-block stroker with a roller cam, and fuel injection to give me what I want in power and torque. But being a diesel mechanic and knowing what diesels can do, especially the Cummins turbodiesel ('94-'98), I don't know what to put in it. I am planning on a 14-bolt rear axle to handle 37-inch tires, the power, and a manual transmission. But as I add up the cost of building a stroker gas engine, it equals out to buying a used Cummins that would give me all the pulling power I want in the truck. But if I figure in the cost of converting the truck to diesel and making the powertrain withstand that kind of torque I want, I'm right back to the price of a good hard-driving gas stroker motor. What would you guys do?
Dave Weryler
Omaha, NE
A: When you told us you were a diesel mechanic, you pretty much convinced us that the Cummins engine swap was the way for you to go-assuming you can find a good running 12-valve Cummins engine that doesn't need a rebuild. As you know, these engines are not cheap to repair, so it's worth paying more for a good one than trying to piece together a basket case.
Currently, we know of no conversion kit to put the 12-valve Cummins into a solid axle GM truck. But don't let that stop you. We've seen it done by adapting '95 Dodge truck engine mounts to the GM frame and then mating the Cummins engine to the GM transmission with an adapter, starter, and flex plate available from Cummins for the four-cylinder 4BT engine. If you run a TH400 (non-overdrive automatic transmission), you'll want to fit the axles with 4.10 or 4.56 gears to keep the engine speed in a usable rpm range with 37-inch tires.
Your other option is to run a Dodge NV4500 manual transmission and mate it to an older (passenger-side front output) Dodge NP205 or NP241 transfer case.
You'll have to create your own exhaust, cooling, intercooler, and air intake to work with your chassis, but the Dodge parts have proven to be quite adaptable.
Which is Stronger?
Q:I always argued that manual transmissions were stronger than automatic transmissions. I've always proved my point by explaining to people that every tractor-trailer you see on the highway uses a manual transmission. But it seems like whenever someone buys a new diesel pickup, they always get the automatic, and whenever I see a truck sled-pulling or drag racing, it's almost always an automatic. Do they know something I don't?
Paul Dubay
Hemet, CA
A: In general, manual transmissions are more durable than automatic transmissions because they use a more basic design with fewer moving parts. But automatic transmissions have advantages in performance applications (like sled-pulling or drag racing) because they can shift when the engine is at full throttle. This is possible because of the nature of an automatic transmission. Basically, automatics transmit torque from the engine to the transfer case in a constant state of controlled slip, so they are designed to deal with it. If an automatic transmission receives torque loads that are too much for the transmission to handle, dozens of circular clutches can slip to relieve the stress on the planetary gears and internal steel shafts. The clutches are constantly being cooled and cleaned by filtered ATF, so the temporary overload can often be coped with.
Now if you exceed the torque capacity of a manual transmission, either the large air-cooled clutch disc slips, the dual-mass flywheel gets ripped apart, or you break a shaft, bearing, or gear. Manual transmissions don't have the hydro-mechanical cushion that automatic transmissions do.
As of the '06 model year, the Dodge, Ford, and GM 1-ton diesel trucks were all rated to tow more with the automatic transmission option than they are with the manual transmissions that are also available. But in the past, some diesel trucks (Dodge Cummins-equipped trucks, in particular) were rated to tow more with the manual. In the end, which one is better (or stronger) may really be up to you and the way you drive.
Cat or No Cat on a Diesel?
Q: I have an '03 F-250 Super Duty with the 6.0L hooked to the five-speed TorqShift transmission. Thus far, I have installed the leveling kit to raise the front end, installed a turbo timer, rear air bags, 35-inch tires, and a K&N air-intake system along with an auxiliary bypass oil filter. My next project is to work on the exhaust system. But I am getting mixed signals as to whether I should get a turbo back system or an after-cat system. I live in central Utah and have to retain the catalytic converter to be legal. A lot of the aftermarket systems eliminate the cat all together, but I have found a few after-cat systems. No one I've talked to can give me a horsepower difference between the two types of systems. My local exhaust shop told me that if the cat has to be retained, the factory 3 1/2-inch downpipe is just as good as any aftermarket 3 1/2-inch downpipe with the factory cat. What's the deal?
John Andrews
Via e-mail
A: Here's the deal: Catalytic converters are more restrictive than a section of straight pipe of the same diameter. There are very few states that emissions test diesel vehicles on a yearly basis at this time, so some diesel owners remove the factory catalytic converter to improve exhaust flow. It's illegal, but it happens. Diesel trucks used in competitions should remove the cat because the excessive fuel that gets pumped out the exhaust (that makes the black smoke) could overheat the cat and cause it to plug up.
That said, we think you should keep your cat and add an after-cat exhaust system to your truck. It's up to you whether you replace the factory downpipe you have now with an aftermarket piece. The aftermarket downpipes typically will flow more, but as your exhaust shop said, you probably won't feel the difference with the cat in place.


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