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.
Exhaustive Possibilities
Question: Why do mufflers for diesel exhaust systems always feature a straight-through pipe whereas gas engines have such a variety of baffles, chambers, and deflectors? What’s different about the diesel exhaust pulse?
Tracy Norris
El Paso, Texas
| Perhaps the biggest reason gasoline engines run convoluted, chambered-style mufflers is because they simply need more muffling than diesels do. On diesel engines, the turbo does most of the exhaust muffling, and the traditional muffler is only needed to tune out drones and annoying sounds.
Answer: The answer has to do with a lot more than just the diesel exhaust pulse. And to our knowledge, there’s really no reason why a chambered muffler for a gasoline application wouldn’t work on a diesel. Perhaps the main reason most diesel mufflers use a free-flowing pipe through their center is that it’s the easiest solution to many problems. Straight-through mufflers are cheap, and since there are very few space constraints, the mufflers can be made to be very large (4- or 5-inch) without difficulty. Since the turbocharger muffles the exhaust note, the muffler itself doesn’t really have that much more sound damping to do, so a complex design isn’t needed. Straight-through designs are also tough to clog with soot, make the muffler lighter, and can be less prone to rust.
It’s also almost universally accepted that diesel guys like to hear their trucks running, so quieting their compression-ignition pulses has never been a priority, simply because the market doesn’t ask for it. The final reason is that even if the exhaust is quieted to the point of being barely audible, the engine itself will still be loud.
Heavy, Heavy-Duty Diesel
Question: My brother and I are each purchasing a military M35A2 deuce-and-a-half for a project and will likely bob the tail (shorten the frame and remove one of the rear axles) to make a super-rugged, heavy-duty 4x4. It seems to me that you can’t get a tougher off-road diesel vehicle for the amount of money these things sell for. We have looked on the Internet and found many great resources covering the M35 but have not found much information on tuning the engines. Most sites discuss turning up the injection pump, but that’s about it. I would be interested in seeing what an intercooler, bigger turbo, bigger injectors, and water-methanol injection would do to these engines.
Brett Miller
Fleming Island, Florida
| Military 6x6 trucks are way cool, but don’t try and change their nature. They’re loud, heavy, and slow. Turning up the fuel on the engine helps, but they’ll never be hot rods without an engine swap. Off-road they’re pretty darn fun, though!
Answer: It’s funny you mention it, because we were just talking about Four Wheeler Editor John Cappa’s Jeep M35A2 that he owned a few years back when he worked for JP magazine. He reported that it got along pretty well, although someone had turned up the injection in the past. Unfortunately, the 7.8L multi-fuel inline-six that powers these trucks has a few power-producing limitations. Since it was designed for military use and intended to run on a variety of fuels, its operation actually differs from most diesel engines. While most diesels inject fuel as a finely atomized mist (which starts burning immediately), a multi-fuel engine sprays the fuel as a stream into the piston bowl and allows the swirling air to pick up fuel particles and burn them over time. This results in a much slower combustion event but allows the engine to run on a variety of fuels in combat situations.
Unfortunately, the extended burn time doesn’t give the quick cylinder-pressure spike needed to produce big power, which is why the 7.8L engines are only rated at 135 hp. Turning up the fuel will help, but only to a point, and in a 10,000-pound vehicle you’re not going anywhere fast. That’s why most people install gauges, turn the fuel up, and leave it at that. If you and your brother do go past basic bolt-ons, make sure to drop us a line at jason.sands@sorc.com to let us know how you did, power-wise.
Dyno-Proven Duramaxes?
Question: When are you going to do a dyno-proven power combos article on the Duramaxes like you did for Fords in the Nov. ’11 issue?
Bob Lobsiger
Gretna, Maine
| For more Duramax power recipes, check out our recent Sleepermax and Streetmax project vehicles. The Sleepermax made 744 rwhp with compound turbos and dual CP3s, while the Streetmax checked in at 588 hp to the wheels on a single turbo and pump.
Answer: Requests for advice on making power is still one of our most commonly asked tech questions, and we’re trying to accommodate these queries with an ever-increasing Internet presence. Perhaps one of the best ways to check out different power levels is to search for feature vehicles on our website, www.dieselpowermag.com. Not only will these features provide a template for engine modifications, they will also note the type of transmission upgrades needed for various power levels, and the type of performance one can expect. We actually did a Duramax-specific power recipes article in the July ’10 issue of Diesel Power, which can be found at: http://www.dieselpowermag.com/tech/chevy/1007dp_duramax_power_recipes/index.html.
