Compound Turbocharged Diesel Engines 101
Not long after the diesel wars of the big three automakers began to heat up, truck owners began looking for ways to make them even more powerful.
Modifications such as programmers, air intakes and exhaust systems were added initially, with great end results. After the entry-level add-ons showed little to no detrimental side effects, many decided to push the envelope further, turning to larger injectors and aftermarket lift pumps for even more gains. Eventually, the time came when all of the added fueling needed to be paired with more airflow. With the stock turbocharger pushed to its max, aftermarket turbo setups became the next logical upgrade.
By installing a larger and higher-flowing turbo — or by adding another turbo into the mix — you can safely balance the air to fuel ratio, as well as turn out even more power. Today, compound turbo arrangements are almost as common as injector upgrades when it comes to diesel powered pickups. From hot-running street trucks to competitive sled pullers, they can be found everywhere. Even truck owners that work their rigs for a living are adding a second charger under the hood.
The primary benefits of compound systems are great low-end torque, strong mid-range and exceptional top-end performance. This is because a small (high-pressure) turbo is utilized to get things started at lower rpm, while a larger (atmosphere, or low-pressure) unit takes over at higher rpm. More importantly, instead of being stuck with a single turbo that shines during a specific period of time during the engine’s power band, you have optimum performance throughout the rpm range, as well as better drivability at any engine speed.
Here, we’ve examined some of the most popular compound turbo systems available and how each system is best utilized:
1. BorgWarner S400
Before we completely submerge into compound turbo territory, it’s important to call attention to the most commonly used atmospheric turbocharger in the diesel segment: the S400 based unit from BorgWarner. Available sizes (either from factory or aftermarket suppliers) typically range from 75 mm (labeled an S475) to 88 mm (labeled an S488) compressor wheels, along with 83 mm to 96 mm turbine wheels being the norm.
These turbos feature a T6 mounting flange, have no problem handling 60 psi of boost (or more), and can be had with a 360 degree thrust bearing assembly for added reliability. They offer (arguably) the best bang for the buck in the diesel industry, with affordability, durability and big performance gains being their key strong suits.
2. Add-a-Turbo
For budget-minded truck owners, add-a-turbo kits are frequently employed. This means you retain your truck’s factory turbocharger and simply add an atmospheric unit in front of it. These types of systems are extremely popular on Duramax powered Chevys and GMCs. The kit shown here is built by Wehrli Custom Fabrication for a ’15 LML Duramax and incorporates a billet S480 under the hood (i.e., S400 based turbo with an 80 mm inducer compressor wheel). These types of systems are good for the 550 hp to 750 hp.
(Find out how to make your Duramax diesel engine invincible.)
3. S362 Over S475
One of the most prevalent compound turbo setups in the diesel industry bundles an S362 (BorgWarner based S300 unit) on top of the aforementioned S475, which acts as the atmospheric turbo on 5.9L or 6.7L Cummins engines. While smaller in size, they are quick-spooling, great for towing, support 700 hp and can be had on a reasonable budget.
(The most budget-friendly Cummins engine is the 3.9L. Here's what you need to know about this 4BT.)
4. Daily Driver Compounds
Even though the larger displacement of the 6.7L Cummins (over the 5.9L) makes running a large single turbo more justifiable than it used to be, there is still no replacement for compounds. A Fleece Performance Engineering S362/S475 turbo arrangement resides under the hood of this Mega Cab and makes a very responsive, smoke-free 650 hp.
5. S364 Over S480
An age-old combo for making big horsepower with the 5.9L Cummins employs an S364 and an S480. These setups will support north of 850 hp, but unlike the S362/S475 arrangements listed above, aren’t ideal for towing anything other than lighter loads.
6. Towing Turbos
Here is an example of a compound arrangement intended for work. This system integrates the factory variable geometry Holset HE351 VNT (60 mm) up top, with an S475 beneath it. The two snails combine to allow this ’09 Ram 3500 to tow 20,000 pounds through the mountains on a daily basis. Compound arrangements that utilize a variable geometry turbo are ideal for getting loads up and moving quicker. This is because they are capable of varying the exhaust housing A/R depending on engine speed. At low speeds, the exhaust housing is more restrictive (i.e., more drive pressure to get the turbo’s compressor spinning) while being less restrictive (higher flowing) at increased engine speeds or wide open throttle.
7. Compounds on a Budget
We’ve seen tons of budget compounds pieced together using a 74 mm Holset HTB3 (shown) and the venerable, time-tested Holset HX35 found on ’94-’02 5.9L Cummins engines. The HX35/HT3B combo was actually one of the first compound turbo arrangements ever devised, supports 500 hp and is most commonly found on 12-valve versions of the mechanical 5.9L offered in ’94-’98 Dodge Rams. Both chargers are extremely tough, with the HX35 alone being infamous for handling in excess of 40 psi of boost on a regular basis. If you have the know-how to fabricate your own piping, a system like this can literally be put together for a few hundred bucks.
(For those in the market for a used diesel pickup, we have a Cummins Catalogue to help you out.)
8. 6.4L Power Stroke Compounds
Being that the 6.4L Power Stroke (produced for ’08-’10 model year Super Dutys) left the factory with compound turbochargers (coined “sequential” turbos by Ford), it’s no wonder why these trucks performed so well right out of the box. Add a tuner and you could see 500 hp to 580 hp immediately, thanks to the great airflow. And, for those seeking more power, it only makes sense to upgrade the factory turbos before throwing money at a completely different compound system. One company that pioneered modified stock turbo arrangements on the 6.4L was Elite Diesel Engineering.
Specifically, the company’s Tow-Power compounds entail the high-pressure (small) charger receiving a billet 59 mm compressor wheel vs. the cast 52 mm stock wheel, and a billet 71 mm wheel being fitted in the atmosphere (large) turbo vs. the cast 65 mm factory wheel. This setup can support 700 hp, while a slightly larger Tow-Power Plus features a 72.4 mm billet wheel in the low-pressure (atmosphere) turbo, and can support north of 700 hp. Compound arrangements for the 7.3L and 6.0L predecessor engines remain rare, as a lack of aftermarket interest and the engine’s ability to perform sufficient enough with a single turbo in the mix (the 7.3L’s case) kept the market from ever taking off in that respective segment.
(Looking to buy a used power stroke? Take a look at our Ford Diesel Pickup Buyer's Guide.)
9. Complete Duramax Kits
Although the add-a-turbo option mentioned above is a popular route to go for a lot of ’04.5- ’16 Duramax owners, the factory IHI turbo found on the LB7 engine (’01-’04 GMs) is less prone to survival in such arrangements. This pushes a lot of first generation Duramax owners toward S300/S400 kits. The system shown here is built by Wehrli Custom Fabrication and is coined its Big Twin kit. While various turbo options are available with its Big Twin kit, the one shown here replaces the valley (stock location) charger with a billet S366, and mounts a billet S480 where the passenger side battery used to reside. This setup is said to be good for up to 900 hp.
10. Big Power Compounds
While not as common as stock-over-S400 and S300/S400 compound turbo kits, large Garrett-based arrangements can also get the job done, such as this dual GT55 setup on an LB7 Duramax. However, due to higher costs (namely from ball bearing center cartridges), the BorgWarner chargers remain most cost-effective.
11. Compound Turbo Systems vs. Twin Turbo Systems
Compound turbo configurations are often referred to as twin-turbo systems, which is not technically accurate. In a compound system, two stages of boost production occurs (one turbo blowing into another), and the system is made up of a larger turbo, called the atmospheric or low pressure unit, being used in conjunction with a smaller (high pressure) charger. Twin-turbo systems are parallel turbo systems, which is best illustrated on a V8 engine, where each bank is being fed air by its own turbo — with both turbos being the same size.
