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August 2012 Top Tech Questions

You've Got Questions? We've Got Answers!

The Diesel Power Staff
Aug 1, 2012
Photographers: The Diesel Power Staff
Turbocharger Theories
Question: In a compound arrangement, as the engine speed increases under load, will the small turbo accelerate at a slower rate after the large turbo spools up due to the kinematic viscosity principles of air? Does this also account for why a turbo can overspeed a burst with a clogged air filter? I’ve noticed that if you blow a discharge pipe in between the large and small turbo, the small turbo will overspeed, often resulting in a burst compressor wheel. Those are my theories. How far off base am I?
Nathan Smith
Mesa, Arizona
Photo 2/4   |   Large compound turbos look cool, but in many cases are too much for the street. Sled pullers and drag racers generally need to leave the line at 3,000 to 5,000 rpm, and you can’t be doing that at every stoplight in your street truck.
Answer: Well Nathan, you’re pretty much right on all accounts! We’ll go into a bit more detail than that, though. The more pressure (boost) the larger turbo pushes into the smaller turbo, the more it will slow the smaller turbo down. Since the smaller turbo now has to act against a much larger volume of hot air, it (in theory) will spool a lot slower than if it just had to compress atmospheric pressure. We’re actually right in the middle of a turbo test to verify this is indeed the case.
In the instance of a blown boot between stages, you’ll have a whole lot of excess drive pressure from the exhaust side, but suddenly your intake side goes to zero. The lack of pressure on the intake side means a huge jump in compressor speed, and unfortunately a lot of times that jump in speed can result in a burst wheel.
Photo 3/4   |   Turbocharger theory is something we’ve been interested in for some time now. For a good read, check out our book review on Hugh MacInnes’ Turbochargers in the March ’12 issue of Diesel Power.
Finally, when turbos see a vacuum (as in a clogged air filter), they will try and spin extra hard to produce the same amount of boost. The higher pressure ratio will result in a lot of extra work for the turbocharger, and possible failure.
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 and we’ll do our best to answer it.
Mismatched Dodge
Question: Recently, I purchased a rare Quad Cab ’98 Dodge 2500 with the 5.9L Cummins 12-valve engine in it. When I bought the truck, it had a set of 220hp injectors, a 66mm turbocharger, and 4.10 rear gears. The problem is that for a street driver, the truck is kind of a pain. It spins 1,900 rpm at 60 mph and makes 0 pounds of boost while cruising on the freeway, but at wide-open throttle it can hit 42 psi of boost and 1,400 degrees EGT. I have since replaced the injectors with a smaller set, but I am still unsure what to do about my rpm and turbo issue. Should I regear or save up for a Gear Vendors overdrive? What size turbo would make my truck nice and driveable?
Eli Bass
Tallahassee, Florida
Answer: You’re one of the few people who realizes what a pain modified trucks can be. We’re sure it was fun to drive initially with the big injectors and big turbo, but having to keep the engine spinning 2,000 rpm all the time to make it work gets old after a while. For the turbocharger, you’ll have a few different options. You could replace your 66mm with a 62mm or 64mm turbocharger and gain a few hundred rpm of spooling, but what we’d actually do is build a small set of compound turbos using the 66mm turbo you already have. Using a stock or S300G turbocharger (57mm inducer) combined with the 66mm will give you the capability of 700 hp, but the small high-pressure turbocharger will start spooling as low as 1,500 rpm. We’d also expect to see your fuel economy improve by anywhere from 1 to 3 mpg. If you do switch to a single 62mm, you’ll be limited to power levels in the 500hp range, although spooling will improve. We wouldn’t worry about the 42 psi or 1,400 degree EGT; that’s within the normal range for a modified truck.
As far as gearing goes, you could replace your tires with larger ones, but that brings its own set of issues, namely fender clearance and worse fuel economy. A Gear Vendors overdrive could be used, but the installation will require driveshaft modifications. We would probably just switch the axle gears to a 3.73 ratio if you’re going with a smaller single, or a set of 3.55s if you decide to go with compounds. A small set of twins will be less likely to surge than a single, so you can get away with a numerically lower gear.
Idling Causes Insanity?
Question: Is there any reason why diesels need to sit outside of my building idling all night? I mean, you would think with all the technology they have today, they could let me sleep.
Mark Sayer
North Hollywood, California
Photo 4/4   |   Big rigs are the most common source of diesel idling, but that may be changing as pollution standards tighten considerably.
Answer: It seems that at one point or another, all of us have been victims of incessantly idling diesels and thought, “I wish they’d turn that darn truck off!” Well, unfortunately, it’s not always possible; sometimes trucks have to be left running. Tow truck drivers, for example, need the idling engine to operate the hydraulic systems in order to move or lift the vehicle they’re towing. Bus drivers often need to keep their cabin cool, and the easiest way to do so is by leaving the engine running. Finally, over-the-road big rig drivers often use their truck as a second home, watching TV, sleeping, even keeping pets in their cab—and doing so in extremely hot or cold weather usually requires heat or air-conditioning inside the cab. Finally, in very cold climates, the fuel in the lines can actually gel up if the engine isn’t kept running and warm.
This being said, there are other alternatives to idling a diesel engine for hours. In many motorhomes, for instance, auxiliary power units (APUs for short) are installed to comply with idling laws. Usually, these units consist of a generator with a battery backup and can be used to power the vehicle’s in-cab heating and cooling systems for a fraction of the cost in fuel, and with a large reduction in noise. In semitrucks, however, APUs can be costly and will also cut approximately 400 pounds out of the vehicle’s payload capacity; in other words, it’s 400 pounds of goods the company cannot ship.
As technology progresses, solutions are being found to incentivize drivers to use something other than their 15.0L engine for heat. Many states have passed laws that allow trucks with APUs to carry 400 pounds worth of extra weight, while others have passed grants that allow the purchase of APUs to be partially funded by the government. In California, where idling anything is frowned upon, we’ve already seen a massive reduction in idling trucks, buses, and delivery vehicles. The best part is, since virtually all the generators used in APUs still run on diesel fuel, it’s something we can get behind 100 percent.


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