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Compounding Turbo Crash Course

Do You Need More Than One Turbo?

Text By Trevor Reed, Photography by Diesel Power Archives, The Manufacturers

Are two turbos always better than one? How about three? The answer depends on what you plan to do with your diesel. While some setups can be used for multiple purposes, others may only be great during a competition. With the wrong upgrade, you might end up tormenting yourself with turbo lag at low engine speeds during daily driving, or running out of “oomph” during a contest. So, what do you really need?

While it may be simple to just assume “bigger is better” or “more turbos equals more fun,” you’ll actually need to find the correct balance of spin-up speed and top-end performance to meet your specific needs. If your vehicle is only used for daily driving, then a gigantic single turbo may actually reduce performance, except when your accelerator is on the floor and you are well beyond the speed limit (even in Montana or Texas).

On the other side of the spectrum, even though a Duramax with a stock turbocharger won the first Diesel Power Challenge—our competition that combines quarter-mile blasts, trailer drags, dyno runs, and sled pulling, along with a real-world, public-street mpg battle—it took generous levels of nitrous oxide and some luck to make it happen. If you have a truck that gets trailered to the dragstrip or the sled pull arena, you probably don’t need to worry about fuel mileage or instant acceleration, because you may be better served by a drivetrain package that really kicks in while you are deep into a run or pull. So, in this crash course, we hope to steer you toward the right direction in choosing the best turbo setup for your needs.

Daily-Driving Weekend Warriors
Most Diesel Power readers fall into the daily-driving weekend warrior category. During the week, these trucks are used for commuting to and from work. And on the weekends, they are driven to the dragstrip, hooked to a sled, or used to tow a large trailer. Massive power may not be needed on a daily basis, but you diesel soldiers want to be ready for battle with just a twist of the knob or tap of a touchscreen on the programmer.

Since these folks have the most diverse plans for their trucks, they also have the most options for powertrain enhancement. This includes reliance on the stock turbo (along with specialized programming), upgrading the factory turbo, or investing in a multiple turbo system. Plus, with large turbo upgrades, you’ll definitely need to improve your fueling system, enlarge the exhaust system, beef up your transmission, and improve the cooling, along with other modifications, to deal with the massive increases in power and torque.

If you want to upgrade your personal truck to be a lion on the weekends but a wild lamb during your daily drives, you’ll have to make some compromises. Your build plans are really quite open, with your upgrade options ranging from the following: installing a larger compressor wheel inside your stock turbo to move more air; throwing on a bigger wastegate to safeguard against overspinning your turbo; installing a completely new, larger primary turbo; stepping up to a two-turbo system, which uses a small compressor (either the stock, variable-geometry turbo, or an upgraded primary turbo) for low-end power along with an extra-large turbo, which kicks in when the truck is being pushed toward its limits; or even a compound turbo system that uses a larger turbo and an even bigger secondary charger.

While none of these setups will necessarily max-out the ultimate power potential of your truck, they will still dramatically increase the performance of your engine while keeping your truck streetable, by minimizing the increase in turbo lag, or even decreasing the time it takes the turbo(s) to spool up. In order to make your decision, you’ll have to find out whether you prefer the attributes that are best for commuting, or your desire to be the king of competitions.

Competition Only
Unlike a daily driver, a pure dragstrip or sled pull truck owner will not necessarily be concerned with turbo lag, because these vehicles are designed to operate in a specific rpm range. As opposed to a truck used for commuting, a dedicated drag or sled truck’s main job is maximizing the amount of fuel that can be injected into the engine, which requires as much air as possible to be forced into the intake.

In addition to the large amounts of smoke produced by pushing massive doses of fuel into the engine, the turbo setups that are most effective for these specialized trucks are very large and take a relatively long time to produce maximum boost levels. While these dedicated setups create hundreds of pounds per square inch of intake pressure (using either a gigantic single turbo; large, two-turbo systems; or even three turbos), it takes a lot of fuel and high rpm to get the turbochargers to reach optimal boost levels.

That amount of time until full boost kicks in may be just a couple of seconds during a competition, but in normal traffic situations, that’s considered massive turbo lag. Plus, you can’t be flooring the accelerator, injecting huge amounts of nitrous, or using a race tune on public roads, which is what you’d need to do to get track-sized turbos spinning up fast.

So, although it’s always tempting to install the biggest and the baddest parts (much like a large-bore carburetor or high-lift camshaft for a hot-rod gasser), slapping the largest turbo, or turbos, onto your diesel-powered truck may be counter-productive, unless you plan to use it exclusively for competition. If you take one of these trucks and expect it to act like a Corvette on city streets, you’ll be sadly disappointed, because you may never get the turbos to spool in a civilian setting.

By Trevor Reed
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Just finished reading the article about Various turbos for the Dodge cummins in the November issue and wondered how a twin would stack up compared to the singles you tested. My towing consists of a 30' toyhauler and I stick to the speed limits. I drive a 2001 Dodge dually with the Cummins 5.9 and an NV5600 with 3.73 gears. That puts me right at the factory minimum of 1600 at the California towing speed limit which I do not violate except for Very special circumstances. I typically keep my rpm between 1600 and about 2250 since I must rev to that RPM in 5th to get into 6th at just the minimum allowed by the owners manual. I have no idea why they don't want you to tow below 1600 but I abide by it unless all I'm towing is my utility trailer. I also have a MADDS Smarty set to power setting 3, because it runs much smoother than the stock tune and gives me a little more power. I seem to have high EGT with towing and I find it very noisy and uncomfortable to tow above 2,000 RPM. I'd like to be able to pull steeper grades without blowing my EGT above 1200 or needing to downshift. I hate the idea that I can produce torque that I can't use. Per the article the stock turbo is actually best for my driving practices. Would a dual with the stock as the smaller produce enough extra boost that it would lower my EGT enough to pull the hills?
Or at least use full throttle without exceeding 1200, downshifting only if there wasn't enough torque to maintain speed?