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February 2013 Top Tech Questions

You've got Questions We've Got Answers

Diesel Power Staff
Feb 1, 2013
Photographers: Diesel Power Staff
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.
Question: I have a question concerning extended idling of my diesel trucks during winter. I plow snow commercially, so my trucks may sit for an hour or more sometimes while I shovel or blow snow. I always leave the blade down with the auto trans in neutral, so the trans cools off. Do you think this is best? Should I turn them off if I plan to be out more than 30 minutes? I can spend up to about 18 hours straight working the truck, sometimes they run for closer to 20 hours without shutting off. I’m in very cold weather—20 below zero or more sometimes—so I will not shut the truck off then. I have both an ’05 Duramax with some aftermarket parts, and an ’01 Cummins, bone-stock with 300,000 miles. I worked for a construction company before, and one of the old loaders blew up because of extended idling. No oil pressure without higher pump rpms. Any thoughts?
Roger Nocerini
Frozen Northern Wisconsin
Photo 2/4   |   In freezing or below freezing conditions, make sure to warm your truck up as much as possible before driving it. If the engine and transmission aren’t up to operating temperature, blown gaskets, leaky seals, or even internal damage can result.
Answer: So far, it sounds like you’re making all the right moves. Feature Editor Mike McGlothlin mentioned a while back that every 60 minutes idling is the same as driving 25 miles down the road. It’s something that most people forget, and if the oil degrades and doesn’t lubricate properly, then engine damage can be the result. Newer diesels have very good oiling systems, even at lower rpm, and we wouldn’t worry about seizing up an engine that is newer, and in good shape. Just keep in mind that with extended idling, your 300K-mile engine could get the equivalent of 50K a year or more, even if just a small portion of that is actual driving. This means weekly oil changes, or installing a centrifuge. If you want to keep heat up in your engine, you can always install and activate an exhaust brake or high idle setting to keep the engine and EGT hot and prevent the whole system from freezing up due to the excess cold.
Over-Geared IDI
Question: I own an ’88 F-350 dualie with a 7.3L IDI motor and a five-speed. I want to know how I can reduce the rpm while cruising at 70 mph. I know I would get better mileage if the rpm were lower. I would love some help and would like to know where I can get aftermarket parts for an IDI, as most companies only list parts for the newer motors and not anything older.
George Abbott II
Via email
Photo 3/4   |   If the weight difference isn’t too great, taller wheels and tires can actually help fuel economy, rather than hurt it. Gains of 5 to 10 percent are not uncommon.
Answer: You’re right in thinking that lower rpm will probably equal better fuel economy. In the case of the first IDI you mentioned, you’re probably nearly 1,000 rpm past your desired fuel economy rpm range if you’re spinning 2,600 to 2,800 rpm at freeway speeds. Since diesels are built stout, the extra rpm really isn’t all that hard on the engine, but it probably isn’t good for it, either. Increased rpm will generate more heat in the engine and transmission and is also hard on the old eardrums, depending on your level of cab insulation.
To get your engine to spin slower, you have a few options. The easiest to do is probably buying taller tires. While putting 44-inch mud tires on your truck will undoubtedly make fuel economy worse, going from say, a 30-inch to 33-inch highway tire can give you a good rpm drop, without much of a weight or rolling resistance penalty.
While swapping transmissions could work as well, the real issue is probably in steep rearend gears. In 1988, you could get an F-250 diesel with either 3.55 or 4.10 gears, and by the sound of it, you have the 4.10s. If you’re able to find a set of 3.55 gears at a junkyard, the axle should simply swap over. You’ll have an rpm drop of about 600 rpm, and you’ll be right in your powerband. However, because of the lack of torque multiplication, you might have trouble keeping speed up while pulling heavy loads. If you don’t tow that often, just swap gears (or complete axles) and have fun with your mileage gain! In your case, you don’t even need the aftermarket—your solution is lurking somewhere in a local junkyard!
Over-Boosting Dodge
Question: I am looking to spray nitrous on top of my existing compound-turbo Cummins combination, which has already made more than 1,000 rwhp on the dyno. The problem is, I am having trouble keeping boost less than 100 psi, and I am concerned about engine reliability if I use nitrous with that much boost. I’m already starting to lift the head as it is. I have a wastegate that I haven’t messed with much yet, which sends boost from the engine, past the small turbo, and into the big turbo. Will opening this wastegate more help? I really need to get my boost down!
Mark Barry
Houston, Texas
Photo 4/4   |   Pressure can be added to the top of wastegates to keep them from opening and keep boost pressures up. For bleeding off exhaust pressure and limiting boost, air must be run to the bottom of the diaphragm to open it properly.
Answer: That’s an interesting one! Most people complain about having too little boost, but we’re always interested to hear about someone who has the opposite problem. Without knowing turbine and compressor sizes, as well as housing dimensions, we’ll have to take somewhat of a shot in the dark, but our guess is that you’re not wastegating enough. We’re assuming you have large and huge turbos to make that kind of power (say, S475 and S595), so to illustrate our point, we’ll take estimated exhaust flow numbers from each of those turbos. An S475 might take about 35 to 45 lb-min of air to drive, while an S595 will take about 50 to 60 lb-min of air. That means, if the air is routed past the smaller turbo and into the larger one, it will do less work because the flow volume of the exhaust housing and turbine is so much larger. So, instead of super speeding the smaller turbo, you’ll be driving the larger turbo just a tad harder.
If that doesn’t drop your boost any, then you just have too much drive pressure throughout the total system (small and large turbo). In order to lower it, you’ll need to vent the wastegate to the atmosphere instead of using it to drive the large turbo harder, which should drop total boost by quite a bit—below 80 psi would be our guess. From what we’ve seen, 60 to 80 psi of boost and nitrous will usually make as much or more power than 100 psi on fuel only, and be more reliable to boot.


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