Tech Q&A - Diesel World

Taller Gearing?

I have a 2005 Dodge Ram 3500 SRW 4×4 with 4.10 gears. Can engine rpm be reduced to duplicate that produced by 3.70:1 gearing and stock tires by simply using larger diameter tires? Would the gearing change require switching to 19.5 or larger wheels? Thank you for your thoughts.

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Ben Allain

Columbus, GA

Ben, your 2005 3500 single rear wheel truck was originally equipped with 265x75R16 tires, and according to the variables I worked with at F-Body.org/gears, upsizing from a measured 31.6 inches to 35-inch diameter tires will produce about the same effect as when changing your differential ratio from 4.10 to 3.70.

Now, you can get to that 35-inch diameter using a variety of tire/wheel combinations, depending on your tastes and what load range rating requirements you need. Upsizing to the 19.5-inch wheels would help you maintain the load range necessary when running at max GCVWR and still get the 35-inch diameter tires, and the larger diameter wheels will help to maintain the sidewall height. Keeping the sidewall height about the same as the stock tires would help maintain the handling qualities of your truck.

RicksonTruckWheels.com and SouthwestWheel.com among others offer 19.5-inch wheels for your Dodge. In addition, using the Internet search terms “19.5 wheels dodge srw” in Google or Bing will net you a lot of reading on this subject. For larger diameter tires, the website TireRack.com can help you locate the right tire.

A word of caution… Be sure to maintain the factory wheel offset when using aftermarket wheels. According to TireRack.com, “Offset” is defined as: “The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.” Wheel diameter and rim width are secondary factors when compared to offset ratio—for determining what effect a new set of wheels might produce on truck handling. And, you’ll need to evaluate whether the larger tire sizes will produce interference with fender corners. Good luck, and let us know what develops.

Fuel Economy

I’ve been following Diesel World for a while now, and have read the recommendation for 1,800-rpm as being ideal for fuel economy. Why is that? Thanks for the info.

Dylan Myers

Williston, ND

Dylan, Great question! The answer to your question involves fuel chemistry, engine geometry and thermodynamics. You can look at the power, torque and fuel consumption graphs for a wide variety of consumer turbocharged (and most non-turbo) diesels going back decades, and you’ll see a common theme in the data. Whether you’re driving a new Chevy Cruze or a Dodge ¾-ton pickup equipped with a 6.7L Cummins, all produce the best fuel economy near to 1,800 rpm, and all produce the maximum engine torque somewhere near to 1,800 rpm.

A big factor in the magic of 1,800 rpm is the volatility/combustibility of diesel fuel and how it affects flame propagation during the combustion process. Diesel fuel, even when finely atomized, doesn’t combust as quickly as gasoline. It takes time for the fuel charge inside the cylinder to completely burn. As it turns out, the burn time during combustion is nearly ideal with an engine turning 1,800 rpm. This longer burn time is partly why diesel engines produce more torque than a gas engine—diesel combustion results in more push on the piston over a longer period of time. To take advantage of this longer burn time, modern diesel engines can have a bore/stroke ratio that’s square or under square, meaning a diesel engine generally has a longer stroke than piston diameter. On the other hand, gas engines generally have an over-square piston/stroke geometry (larger piston diameter than stroke) because of the higher volatility of gasoline.

Knowing all this, manufacturers of diesel engines, turbochargers, fuel injection systems and engine control systems have all directed their efforts at producing engines that make the most of this 1,800-rpm phenomenon. So, drive your diesel at or near to 1,800 rpm if you’re looking for the very best in both engine torque and fuel economy. Thanks for reading.

Exhaust Brakes?

I own a 2007 Dodge Cummins 2500 that I bought new, and am concerned about braking power while towing a recently purchased travel trailer. I know that exhaust brakes have been installed on Dodge and Ford diesel pickups, but am wondering whether my truck needs one while towing farther out West. I don’t see a need for better brakes here in the flatlands of Nebraska, but I would not want to lose the brakes while descending a steep grade. Do I need an exhaust brake, and if so, which is better? Thank you.

Alex Schofield

Lincoln, NE

Alex, Yes, losing the brakes on a downhill descent can make for a hair-raising ride. However, you shouldn’t need additional braking beyond your factory truck and trailer brakes while towing a 25-footer. Generally, those who tow the 30-foot or above or 12,000-lb+ GVW trailers are who would benefit more from an engine exhaust brake, and even then only on the steepest of grades. Descending US-212 out of Yellowstone National Park into Cook City, Montana, with a heavy trailer illustrates the reason why brakes are important.

Banks, BD, Pac-Brake and others all produce effective and reliable exhaust brakes for the big-three diesel pickups. These can be installed by a competent home mechanic, but a professional installation might help to ensure safety—for both brake operation and engine protection. All of these exhaust brakes are designed to work within the engine’s design parameters and still produce efficient braking.

Before installing an exhaust brake, we recommend you test the truck and trailer brakes to ensure they are operating as designed, and that your trailer brake controller is properly adjusted. This is crucial to safe trailering. Lastly, those new to towing frequently descend hills at too high a rate of speed. The mountain passes in Montana usually have a semi-truck speed limit of just 25 mph on the downhill side. Pickups towing travel-trailers don’t usually need to drive that slowly, but it does illustrate the need to drive more carefully and sometimes more slowly while descending steep grades.

Emissions Control Delete

I recently purchased a 2007 new body style R-titled Chevy Silverado 2500 Duramax. (Editor’s note: An “R-titled” vehicle is a rebuilt/reconstructed vehicle that has been repaired and restored to on-road operation.) I live in Pennsylvania and have been driving it for a month with no problems until the other day. The check engine light was already on when I bought the truck. I’m now getting the “reduced engine power” message displayed on the dashboard and it’s confirmed by the truck having no power. The truck will not exceed 15 mph.

I took it to local Chevy dealership and they reported that the reduced engine power is due to a soot accumulation in the exhaust filter has exceeded 50g—it’s actually showing 71g of soot. Soot in the particular filter? What? There’s an aftermarket MBRP exhaust on the truck that has no catalytic converter or exhaust particulate filter! In addition, it appears that the EGT sensor is also showing over 1,800 F.

Service techs at the dealership are saying it needs either custom programming or new original equipment exhaust system installed for $6,000! The dealer I purchased the truck from had to install a new GM computer module to even get the truck to start. Please help. Do you have any ideas or suggestions on the information provided here? I sunk all my money into purchasing truck and installing a new plow. $6,000 for a new exhaust system is not an option for me right now.

Thank you,

Todd Mitchell

Via Email

Todd, Hindsight is always 20/20, so any suggestions I might have had about what to do about this truck before you bought it is a moot point. For others reading this, I recommend never buying a truck with an illuminated service engine soon light or one that has been modified in ways that violate the EPA rules.

I imagine that a previous owner or salvage dealer removed all of the original factory exhaust system, which also removed several key components involved in the emissions system. For this to work, the computer must be running a modified program to allow disabling the emissions system. This is likely why the truck is running the way it is now—the programming in the dealer-installed computer has not disabled the emissions system.

You have two options….

1- Reinstall the factory exhaust system and factory computer.

2- Buy an aftermarket programmed computer, but this could be a problem. The EPA has begun seriously cracking down on aftermarket manufacturers who sell emissions-disabling components or programming.

For the best long-term value of the truck and keeping the truck in a condition where dealerships can help you with maintenance, I recommend returning the emissions control system to factory specifications. New emissions control components are expensive, but there’s a large automotive salvage industry that should be able to cut the costs in half.

For new or like new GM salvage parts, contact:

 DW

Schram Auto Parts

Waterford, MI

800.462.9292 or 800.292.1032

SchramAuto.com