Installing a compound turbo kit for 6.7 cummins is easily one of the best ways to transform how your truck handles a heavy trailer or a highway merge. If you've spent any time on the forums or at the local diesel shop, you know the struggle: you want more power, but you don't want to wait three business days for a massive single turbo to finally spool up. That's where the compound setup—often called "twins" even though the turbos are usually different sizes—really shines. It gives you the best of both worlds by combining a smaller, quick-reacting turbo with a much larger one that takes over once you're moving.
The 6.7 Cummins is a workhorse, but it's got its quirks, especially with the factory Variable Geometry Turbo (VGT). While the stock VGT is great for exhaust braking and quick spooling, it tends to get choked out when you start pushing for more horsepower. It's also prone to soot buildup and eventual failure if you aren't careful. Moving to a compound setup isn't just about making the truck fast; it's about making it efficient. You're essentially letting the engine breathe better across the entire RPM range, which is exactly what these big inline-six motors want.
Why Two Turbos are Better Than One
When you're looking at a compound turbo kit for 6.7 cummins, you're looking at a system designed to solve the "lag vs. power" trade-off. In a typical setup, you keep a high-pressure turbo (often a smaller S300 frame or even the stock VGT in some budget kits) and add a massive low-pressure atmospheric turbo, like an S475 or S480.
The small turbo catches the exhaust gases first. Because it's small, it spins up almost instantly when you touch the throttle. This gets you moving from a stoplight without a giant cloud of black smoke and zero acceleration. As the engine revs up and produces more exhaust flow, the big turbo starts to take the heavy lifting. It compresses the air and feeds it into the smaller turbo, which compresses it even further. This staged compression means you can hit 50 or 60 pounds of boost without overworking either unit.
The biggest advantage for most owners isn't the peak boost number, though. It's the Exhaust Gas Temperatures (EGTs). If you've ever towed a 15,000-pound camper up a mountain grade, you've probably spent more time staring at your EGT gauge than the road, sweating as the needle climbs toward 1,400 degrees. A compound setup moves so much more air that it naturally keeps those temperatures significantly lower. You can keep your foot in it longer without worrying about melting a piston.
What to Look for in a Kit
Not all kits are built the same, and you'll notice a pretty wide range in pricing. When you're shopping for a compound turbo kit for 6.7 cummins, pay close attention to the plumbing. You want high-quality, mandrel-bent piping that doesn't have restrictive bottlenecks. Most of the top-tier kits use heavy-duty silicone boots and T-bolt clamps because standard worm-gear clamps will go flying off the second you hit 40 PSI.
Another thing to consider is the manifold. Some kits require an aftermarket exhaust manifold, usually a T3 or T4 flanged one, while others try to work around the factory hardware. Generally, getting a kit that includes a high-flow manifold is worth the extra cash. It makes the installation cleaner and ensures that the heat is being managed properly. Also, look at the oil lines. You're now feeding and draining two turbos instead of one, so the kit should include high-quality braided lines and the necessary fittings to tap into the block.
The Atmospheric Turbo Choice
The "big" turbo in the kit is usually where the magic happens. For a daily driver that occasionally tows, an S475 is a very common choice. It's reliable, relatively affordable, and provides plenty of air for most people's goals. If you're looking to build a dedicated street shredder or a competition truck, you might step up to an S480 or larger, but keep in mind that the bigger the atmospheric turbo, the more fueling you'll need to support it.
Supporting Mods are Not Optional
You can't just bolt on a compound turbo kit for 6.7 cummins and expect everything to stay together if you're pushing big numbers. If you're planning on running more than 45-50 PSI of boost, head studs should be at the top of your list. The factory head bolts are "stretch" bolts, and they simply can't hold the cylinder head down against the massive pressures a compound setup creates. Blowing a head gasket is a quick way to turn a fun weekend project into a multi-thousand-dollar nightmare.
Fueling is the next piece of the puzzle. While a compound kit will make your truck run cleaner by providing more air, you eventually need more fuel to make more power. You'll likely want to look into an upgraded Lift Pump (like a FASS or AirDog) to ensure the CP3 pump is getting enough diesel. If you're going for 600+ horsepower, you'll start needing larger injectors and possibly a modified or dual CP3 setup.
Don't forget the transmission, either. If you have a 68RFE automatic, it's already on borrowed time even with a mild tune. Adding another 150 to 200 horsepower with a compound kit will finish it off pretty quickly. A built transmission or at least a very good valve body and torque converter upgrade is almost a requirement if you actually want to use the power you're making.
The Reality of the Install
Installing a compound turbo kit for 6.7 cummins isn't exactly a one-hour job. It's a bit of a jigsaw puzzle. You'll be cramming a lot of hardware into an engine bay that is already pretty crowded. You often have to relocate the passenger-side battery and move some AC lines or coolant hoses.
Most guys can get it done in a long Saturday or a full weekend if they've got a decent set of tools and a buddy to help lift the heavy stuff. The trickiest part is usually the downpipe and getting the oil drain lines to sit just right so they don't kink. It's one of those jobs where you want to "measure twice and cut once," or in this case, "test fit five times and tighten once."
Tuning Your New Setup
Once the iron is bolted to the engine, you need to tell the truck's computer what to do with it. You can't run a compound turbo kit for 6.7 cummins on a stock tune or a basic "box" tuner. You need custom tuning—usually through EFI Live or EZ LYNK.
The tuner will adjust the fuel maps, timing, and, importantly, the VGT vanes (if you kept the stock charger) to make sure the two turbos are working in harmony. Good tuning is the difference between a truck that's a joy to drive and one that's constantly surging, smoking, or throwing check engine lights. A professional tuner can help you find that "sweet spot" where the truck feels like it came from the factory with two turbos.
Is it Worth the Investment?
Let's be honest, a compound turbo kit for 6.7 cummins is a significant investment. Between the kit itself, the head studs, the fueling, and the tuning, you're looking at a decent chunk of change. But if you plan on keeping your truck for the long haul, it's hard to find a better modification.
The driving experience changes completely. Instead of the truck feeling like it's struggling to breathe when you're lugging a trailer up a hill, it feels effortless. The power is linear, the EGTs stay in check, and the sound is incredible—there's nothing quite like the whistle of two turbos working together. Whether you're building a reliable tow rig or a street-beast, compounds are the way to go if you want the best performance the 6.7 platform has to offer.