November 2010 Top Tech Questions

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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. Large Tires = Bad Fuel Economy
Question: I have an '09 Silverado 2500HD with a 6.6L Duramax. I recently upsized the factory tires from 245/75R16 to a tire with a 265mm section width and it changed the fuel mileage for the worse. How can I fix this?
Vince Sam
Sanders, Arizona   |   Jacob Aziza's IDI Ford gets more than 30 mpg and features manual steering, no A/C, and a variety of aerodynamic modifications. Although they're not for towing use, modifications such as grille blocks can usually yield about a 10 percent increase in fuel economy. Answer: Well, the short answer is you can't. A vehicle will always get better fuel economy with lightweight, skinny wheels and tires. What you can do, however, is make up some of the difference by making your truck more efficient elsewhere. The reason your fuel economy changed is because your vehicle now requires more horsepower to move down the road at the same speed. Wider tires not only have a larger contact patch (more friction), but they will also have more rotational weight, so running larger tires penalizes your fuel economy in more ways than one. The only way running a taller, heavier tire will help is if your gear ratio is less than optimal, but the '09s already have a pretty tall overdrive. Since you're using more horsepower at the same speed, you can either: A) slow down, or B) make more horsepower with the same amount of fuel. While slowing down is the quick solution, many people refuse to compromise speed for fuel economy. If you're this type of guy, then it's time to make your vehicle (or your engine) more efficient. Running more injection timing will help fuel economy, as it will create more cylinder pressure and therefore more power under light-load conditions. We've also seen good results with adding another type of fuel, such as propane or water-methanol injection. Although, with these devices, keep in mind there is an extra tank to refill. Running Dyed Diesel Fuel with Common-Rail Injection
Question: I currently own an '06 GMC with the LBZ Duramax that I use off-road fuel in with no problems. I am thinking about buying an '11 or '12 GMC with the LML Duramax. Will this engine run on the off-road fuel without any problems?
Nick Powell
-via email   |   If you're going to be driving off-road, it's OK to use red dye diesel, even in the newer trucks. If you venture out on public highways, however, watch out, as the fine for running dyed diesel on the street can be $2,000 or more in some states. Answer: We did some research on this one, and basically, it would be more expensive to make more than one type of fuel (say, 15-ppm diesel and 500-ppm diesel) than it would be to just make ultra-low-sulfur diesel and add red dye to it. So the off-road fuel that you'd be running is likely to be the exact same stuff you'd be running if you got it at the pump, minus all the road taxes. Keep in mind that although this type of fuel may be safe for your engine, if you're caught running it on public roads, there's a hefty fine to pay. Horsepower vs. Torque
Question: Why is it that two diesel engines of the same origin or displacement will make very different torque numbers for the amount of power they produce? In other words, why is it that some will make about 1 hp for every 2 lb-ft, and others will make 1 hp for every 3 lb-ft of torque?
Joe Davis
Haliburton, Ontario, Canada   |   Variable-geometry turbos (VGTs) can spool very early, resulting in good torque production. For a great example of a street truck with a VGT retrofit, check out "Work and Play" on page 94. Answer:
The first thing you have to remember is that you really can't separate horsepower and torque. The fact that something has more torque, means it is making more horsepower at a lower rpm. In the case of factory diesels, peak torque will be a figure roughly double that of peak horsepower, because of their low (less than 3,000 rpm) powerband. For instance, a manual transmission-equipped '96 Dodge Ram was rated at 215 hp and 440 lb-ft from the factory. Fifteen years later, the '11 Fords are rated at 400 hp and 800 lb-ft. Even with all the changes throughout the years, the 1 hp per 2 lb-ft ratio is pretty consistent. Larger engines (such as those in big rigs) will make even more power at even lower rpms, which makes the torque-to-horsepower ratio even higher. Since the engines are so large, they can handle the added stress without much of an issue. Now that we've said that, once you modify a diesel engine to start producing more power, the 1:2 ratio gets thrown out the window. While bore, stroke, and cylinder head flow are all contributors, in modified applications, turbo selection is the determining factor in the torque-to-horsepower ratio of the diesel engine. For example, in this year's Diesel Power Challenge, Dustin Mintern made 753 hp and 1,325 lb-ft of torque. On the same dyno, Mike Corsilli made 735 hp and 1,570 lb-ft of torque. Both engines made about the same horsepower, were roughly the same size (6.6L vs. 6.4L), and both were the same configuration (V-8). So why did Corsilli's engine make 245 lb-ft more than Mintern's? The answer lies in the fact that the 6.4L truck had a set of quick-spooling stock turbochargers arranged in a compound setup, whereas the 6.6L Duramax had a large, 80mm single turbo. Almost all diesels have plenty of fuel down low, so the point at which the turbo begins to make boost is the defining factor in torque production. It's also interesting to note that all-out competition vehicles actually try to limit torque production in order to keep cylinder pressures under control. Too much power at a low rpm can cause turbo surging, connecting rods to bend, and head gaskets to blow. So, in these vehicles, torque and horsepower can be nearly a 1:1 ratio. Small turbochargers can also act as an airflow restriction at high rpm, which will limit peak power. The engine in Erik Stacey's Smoknya HD puller made 2,000 hp but only 2,300 lb-ft-and can spin more than 5,000 rpm. In this case, even if it were possible to make 4,000 lb-ft from a 5.9L engine, it would probably result in breaking almost everything, including the driveline of the vehicle.