7/17/2008 - Tech Articles

Dieselus Maximus

How to Improve Your Diesel Truck’s Performance and Mileage

When it comes to hauling your work gear, towing a trailer, or transporting a horde of Little Leaguers or ballet dance company, you can’t beat a diesel-powered truck or SUV. For sheer torque, longevity, and yes, fuel economy, nothing really comes close to an oil-burner. Even the price of the fuel we fed our trucks was lower than gasoline, making diesel an even better deal.

Until recently, that is. In a clear sign of a world gone mad, diesel fuel—arguably the most basic fuel made from crude oil—is more expensive than gas, which requires far more refining to produce. You could panic and either park or sell off your truck, but that’s not the performance enthusiast’s way. We’d much rather find parts and methods that will improve both mileage and performance, compromise be damned.

Fortunately, there are plenty of products on the market that can give us healthy doses of go-fast and fuel economy. We can’t promise they will make your Powerstroke run 12 second quarter-miles and get the mileage of a Toyota Prius, but you will notice the extra power—and the measurable improvement in fuel mileage.

Since the majority of the diesel trucks we drive are fuel-injected and have a computer-controlled engine management system, we’ll concentrate on those types of engines. Here are the key factors we will talk about:
• Air and fuel
• Exhaust
• Tuning
• Drivetrain
• Maintenance

Air and Fuel
Diesels are compression-ignition engines (no spark ignition), so they use air and fuel a little differently than their gasoline-fueled cousins. In basic terms, they take in air and compress it at very high ratios (ratios of 17:1 to 20:1 are not uncommon). Once the air is compressed—and thus very hot, around 700°-800° F—a small amount of highly pressurized fuel is injected into the combustion chambers. The fuel mixes with the hot air, ignites, and begins to burn, creating combustion.

Unlike a gas engine, some diesel engines have no throttle body or throttle plate to control the amount of air entering the engine; in theory, airflow is unrestricted. Instead of opening a butterfly to let in air, pushing on the throttle pedal adds fuel. The more fuel you feed an oil burner, the more power you will make. On late model engines, the pedal sends an electronic signal to the engine computer (Electronic Control Module, or ECM) to deliver more fuel (i.e. drive-by-wire). Older diesels have a throttle cable connected to a regulator valve inside the high pressure fuel injection pump.

In the old days, fuel delivery was less than optimal. Any fuel the engine could not consume went right out the stacks—the source for the infamous diesel “black smoke.” Today’s computer-controlled systems meter fuel more precisely so both power and fuel economy are maximized. Adding fuel to a modern diesel is a function of adjusting delivery or fuel injector timing by reprogramming the ECM with a tuner or programmer (we’ll cover those shortly). Physical upgrades to the fuel system, like larger injectors or high-capacity injection pumps, are not necessary unless you tow very heavy loads or get into drag racing where fuel economy is not high on the priority list.

One thing you can do is keep your fuel system clean. System cleanliness is uber-important with a diesel. If the fuel injector opening pressures become low and the spray patterns are poor, the fuel will not atomize and burn as efficiently as it should. Injection pumps will wear out or fall out of calibration, throwing off both fuel delivery and injector timing. Again, engine performance and fuel economy will suffer. Regular use of a high-quality fuel additive or adding a permanent fuel catalyst will boost cetane (a hydrocarbon in fuel; more cetane promotes better combustion and detonation resistance), improve lubricity, and pull moisture out of the fuel so the water separator can trap it more efficiently.

On the airflow side, diesels love cold, oxygen-rich air as much as gas motors do. A high-flow air filter or better yet, a cold air intake system is a no-brainer. The engine computer will compensate for the extra airflow by adding fuel and/or modifying fuel injector timing, resulting in more power and often a modest fuel mileage increase.

The additional cold air has the added benefit of helping reduce the horrendous exhaust gas temperatures (EGTs) in the combustion chambers. Unlike gas engines, where adding more fuel can help lower combustion chamber temperatures, adding more fuel in a diesel raises temperatures. As you add more fuel to increase power, you run the risk of creating enough heat to literally melt pistons and otherwise cause engine damage. More cold air can help moderate that internal combustion hell.

Power Adders
One proven method of boosting power in a diesel truck is propane injection. Injected into the engine at an approximate 1:4 ratio to diesel fuel, high octane propane burns slower and longer to help diesel fuel burn more completely to increase power and mileage. Propane also helps clean out carbon buildup and keeps the engine oil cleaner. And because you control the activation of a propane injection system, you can use it only when needed.

Water/methanol injection is another way to boost power and fuel economy. Injected into the air inlet, it acts like a liquid intercooler, drastically reducing air inlet temperatures. That increases air density (more oxygen) and helps lower both combustion chamber and exhaust gas temperatures, allowing you to safely add more fuel and increase power. The methanol acts as a high octane fuel additive (similar to propane) and also helps lower combustion chamber temperatures. Other benefits include a reduction in emissions and better mileage thanks to improved combustion efficiency.

Nitrous oxide does for diesels what it does for gasoline engines—add more oxygen and significantly lower air temperatures. Diesel nitrous systems inject the gas into the incoming air stream. The combination of additional oxygen and colder, denser inlet air is like diesel nirvana. It leans out the air/fuel mixture to help burn excess fuel. That produces more power and reduces exhaust smoking. Turbodiesels especially love nitrous because it acts as a chemical intercooler, allowing more boost by keeping exhaust gas temperatures in check. Like propane injection, nitrous oxide is an on-demand power adder—use it when you need the extra kick, leave it off when cruising or towing to maximize mileage.

Exhaust
A free-flowing exhaust system performs two valuable functions on a diesel truck. First, it reduces backpressure, scavenging exhaust gasses more efficiently and allowing the engine to pull in fresh air and fuel more quickly. That means more power. Second, a free-flowing exhaust helps reduce exhaust gas temperatures—upwards of 200 degrees depending on the engine’s power level and exhaust system size (pipe diameter). For turbodiesels, the added exhaust flow helps the turbo spool quicker, increasing power in the low and midrange.

Another exhaust component that affects mileage is the catalytic converter on trucks so equipped. The converter’s primary function is to turn combustion byproducts into relatively harmless CO2 and water. Over time, a converter’s core will plug up with contaminants or break apart from the intense heat. The resulting backpressure and contaminant buildup in the exhaust will have a definite impact on engine efficiency.

Tuners and Programmers
There’s no doubt about it—a tuner or programmer will give you the biggest bang for the buck on a diesel truck. While the price of admission may look steep, it really isn’t when you consider what these wonder devices offer. Just plug a tuner into the OBD-II (diagnostic) port under the dash, press a button, and you’ll be rewarded with big gains in horsepower, torque, and yes, even mileage. With some tuners, you even get a choice of programs optimized for economy, towing, or all-out horsepower.

Tuners and programmers do much more than just boost power. Most can optimize an automatic transmission’s shift points and firmness, recalibrate an electronic speedometer for new tire sizes or axle gear ratios, adjust or remove the top speed limiter, and read and clear diagnostic codes. Some units even let you monitor engine functions (especially EGTs), manage power adders like nitrous and propane injection, and log performance data. It’s like having a garage full of parts and tools in a little black box.

Drivetrain
Since it’s easy to add more power to a diesel, it’s also easy to make more power than the rest of the drivetrain, particularly the transmission, can handle. Fortunately, there are plenty of upgrades available to improve drivetrain performance and durability. Since diesel trucks are often used as tow vehicles or to haul heavy loads, most upgrades—torque converters, clutches, deep sump pans, valve bodies—are designed to withstand the large amounts of torque and heat generated by a modified engine pulling lots of weight. These parts also improve drivetrain efficiency, which leads to—you guessed it—better fuel economy. This is especially true of automatic transmissions.

Maintenance
It’s a no-brainer: change your oil and oil filter regularly. Thanks to the high compression ratios and operating temperatures, oil in a diesel engine works much harder than it does in a gas-fed engine. Consider switching to a good synthetic oil. It reduces friction and wear better than conventional oil, and won’t break down as quickly in extreme temperature environments. That can help improve power and fuel mileage gains. Synthetic oil has the added benefit of longer drain intervals. Running high-quality synthetic transmission fluid will provide the same benefits to your transmission.

Don’t ignore that Check Engine Light when it comes on; it often signifies a problem with an engine sensor. These include sensors for oil and coolant temperature, exhaust oxygen level and exhaust gas temperature, throttle position, and knock. A bad sensor can cause a loss of performance and fuel mileage to suffer. If left unattended, major engine damage could occur. While most sensors are simple “remove and replace” deals, a good diagnostic scanner or code reader is required to track down the offending device.

The condition of your vehicle’s tires and suspension also affect performance and fuel economy. For example, tires that are under-inflated by as little as two psi will increase rolling resistance. Since the engine has to work harder to overcome the resistance, it will consume more fuel. Follow the manufacturer’s recommended tire pressures; you can find them in the owner’s manual or on the decal on one of the door jams.

Worn front suspension/steering components and wheel misalignment will also increase rolling resistance and reduce fuel mileage. Things like uneven tire wear, sloppy handling, pulling, and “darty” steering are signs that you have out-of-balance wheels and tires, need a front-end alignment, or require other suspension repairs.

Don’t forget to check things like accessory drive belts, thermostats, hoses, fuel and vacuum lines, glow plugs, and PCV valves, and replace them as needed. Make sure all fluids are clean and at proper levels. Small things like these have a big impact on engine efficiency. Don’t skimp on the quality of your replacement parts and fluids, either. Brand name products really are better than the no-names. Spend the few extra dollars on the good stuff—you’ll be glad you did.

Driving Habits
One of the biggest changes you can make to increase fuel economy is changing your driving habits. Here are just a few things you can do:
• Go easy on the gas and the brakes. Accelerate and stop gradually, and avoid sudden stops and long periods of idling. If you anticipate a lengthy wait, shut off the engine
• Limit warmup times in the winter whenever possible
• Use the air conditioning only when absolutely necessary, primarily on the highway where open windows creates fuel-consuming drag. In stop-and-go traffic or city driving under 30 miles per hour, roll down the windows or use the fresh air vents
• Don’t haul around unnecessary cargo. Each additional 100 lbs. of weight you stuff in your vehicle reduces fuel mileage one to two percent

• If you have a pickup, consider getting a tonneau cover. It can improve mileage by smoothing out airflow over the bed, which reduces drag
• Always use the highest transmission gear possible based on the speed you’re traveling. If your vehicle is equipped with an overdrive transmission, shift into overdrive as soon as your speed is high enough
• Plan your trips wisely, especially around town where your fuel mileage is typically lower. Combine as many small trips (store, gas station, etc.) as possible into one big trip

The glory days of inexpensive diesel fuel may be behind us, but that doesn’t mean we should park our wheels. Performing regular maintenance and paying attention to your driving habits will go a long way to maximizing every tank of fuel. And as our list of goodies shows, you can have better performance without sacrificing fuel economy. That’s not a bad deal at all.