Compounding Turbo Crash Course

Do You Need More Than One Turbo?

Feb 1, 2013
Photographers: The Manufacturers, Diesel Power Archives
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?
Photo 2/8   |   1302dp 01 Compounding Crash Course Ats Diesel Compound Setup
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.
Photo 3/8   |   1302dp 02 Compounding Crash Course Upgraded Ford 6 4l Turbo Setup
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.
Photo 4/8   |   1302dp 03 Compounding Crash Course Custom Lb7 Duramax Diesel
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.
Photo 5/8   |   1302dp 04 Compounding Crash Course Huge Single Turbo Cummins Diesel
Trailer Towers and Snowplowers
Three-quarter- and 1-ton trucks are designed to tow very large trailers or push snow with factory equipment. Modern variable-geometry turbos are built to have the personality of both a small and large turbo, but drivers who tow or push heavy loads on a regular basis may benefit the most from having two turbos under the hood. Towing and/or plowing puts the engine under a constant load, and that means a lot of extra exhaust gas heat. And what better way to use it than to spin an additional turbo?
Photo 6/8   |   1302dp 05 Compounding Crash Course Custom Cummins 5 9l Engine
There are also single-turbo setups designed just for these jobs, but since they are generally larger than a stock turbo, you may have to live with some additional turbo lag when you drive unloaded. Along with the extra power to move your fifth-wheel, flatbed, pile of snow, or whatever else, when you’re not hooked up to a trailer, a two-turbo compound system should allow you to still enjoy the extra power of your upgrades without the penalty of turbo lag. This is because you’ll still have one stock VGT, or a relatively small aftermarket primary turbo, to spin up at normal street speeds, filling the low-speed gap that exists with a single big turbo. Also, if you’re able to resist the temptation to lay down your right foot, the increase in induction volume may actually help improve your fuel economy during daily driving.
Maximum MPG Masters
Owners who use their truck mainly for commuting are likely to be those most concerned with the miles they get per gallon of fuel. While a two-turbo setup, which uses a stock VGT and a larger unit for higher rpm, may be able to achieve higher mileage thanks to the extra power on tap to move the truck, it will require a lot of self control. It’s not impossible, but you have to be honest with yourself. If you know that a big atmospheric turbo is just ready to spool up any time you squash the pedal, will you really be able to resist?
Photo 7/8   |   1302dp 06 Compounding Crash Course Garrett Turbo On A Ford 7 3l Diesel
Also, if your ultimate goal is to save money by getting better fuel economy, you need to do the math to figure out your return on investment. You’ll have to compare how much a two-turbo system and the installation (and the drivetrain improvements necessary to deal with the extra power) will cost you compared to how much you’ll save on potential fuel economy improvements during the years you expect to own the truck.
What Do YOU Really Need?
We definitely don’t intend to rain on your upgrade parade, because the reason this magazine exists is because using diesel power is so much fun. We understand your dilemma. We’re hooked on power, too, but our goal is to help you make the best decision with your upgrades. We know it’s a bummer to want to take off from a dead stop in your daily driver and end up leaning forward in the driver seat until your turbo(s) catches up with your intentions and pins you into the seat.
Photo 8/8   |   1302dp 07 Compounding Crash Course Borgwarner S366 Turbo
If you’re at stock power levels, or even a few hundred horsepower more than stock, a mild single turbo upgrade is probably all you’ll need for daily driving. For towing, a small set of compounds is the way to go and will keep exhaust gas temperature in check. For a hot street/race truck, running a big single will make good power, although you should plan on spinning your engine up to about 2,000 to 2,500 rpm before any measurable boost is seen. Finally, for an all-out race vehicle, either a very large single combined with 4,000-rpm-plus engine speeds or compound twins or triples is recommended.


Elite Diesel Engineering
Pueblo West, CO 81007
Auburn Hills, MI 48326
Pacific Performance Engineering
Fullerton, CA 92831
Ram Trucks
Aubum Hills, MI 48321
General Motors
Cummins Turbo Technologies (UK)



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