Compound Turbos Bring New Life to a 5.9L Cummins Engine
Hopefully, you’re familiar with the “Big-Three Blend” series of Diesel Power tech reports in which we detail all the tasks for transplanting a mildy warmed-up 24-valve 5.9L Cummins engine and Allison 1000 six-speed automatic transmission into an ’01 Ford F-250. The result of that effort is a fun daily driver. However, as it is for most diesel projects, the taste of driving something out of the ordinary—like a “Fummins” (the diesel guys’ name for a Cummins-powered Ford rig)—only whets one’s appetite to take it to the next performance level.
In our case, that means moving up from a big single turbocharger to a compound-turbo system. The reasons for doing this are to reduce the high EGT we experience when towing a trailer heavier than 7,500 pounds, to quicken low-end spooling, and to push the I-6 engine as close to 500 hp as we can with its Cummins-based fuel supply (VP44 injection pump).
Our “stock” setup features a BD Diesel Performance exhaust manifold, Industrial Injection Diesel Performance’s PhatShaft 62mm turbo and Race2 Honed X4 120hp injectors, Fuelab’s 200-gph lift pump, the aforementioned VP44, Banks Power High-Ram intake, and an Edge Products Juice with Attitude programmer managing the electronics. We took it to Source Automotive in Clackamas, Oregon, where Robert Gates ran it on a Mustang chassis dyno to record baseline data. The truck made a respectable 316 hp at 2,700 rpm and 681 lb-ft of torque.
What Is Compound Turbocharging?In a nutshell, compounding involves using a second, larger turbocharger to blow into a smaller turbo. The larger primary “atmospheric” turbo of a compound setup sucks in the fresh air, compresses it, then force-feeds boost through piping directly into the smaller, secondary “high-pressure” turbo mounted on the exhaust manifold. At the same time, the smaller high-pressure turbo feeds exhaust directly to the larger atmospheric turbo, helping it spool quicker.
Reducing each turbo’s workload increases efficiency while dramatically increasing boost pressure. At the same time, EGT can be more than 200 degrees cooler under full load because the resulting air/fuel charge is denser, cooler, and of far greater volume than can be achieved with a single turbo.
Another big advantage of compounds over single turbos is keeping exhaust and boost pressures nearly equal. For example, in a single-turbo application, the exhaust pressure may be at 65 psi before entering the turbo, while boost pressure at the intake might only be 35 psi. A set of nicely matched compounds sees the same exhaust pressure, but boost is closer to 63 to 65 psi, or that ideal 1:1 ratio.
A person can get deep in the weeds discussing compound turbos and all the numbers that go into selecting the right combination for whatever a goal is. (For all you “numbers people,” when it comes to turbo selection, check out BorgWarner’s “Match-Bot” page: turbos.bwauto.com/aftermarket/matchbot.aspx.)
Let’s just say it’s a science best left to diesel-performance experts who deal with turbos day in and day out. They know what the pressure maps are for every ’charger they sell, how the size of the inlets and outlets for the compressors and turbines match up, what each turbo’s A/R (Area/Radius) is, which injectors and fuel pumps work best, and how all those combinations work for a particular engine.
“You have to be clear on your goals when selecting compounds,” says Dan Kizmann, Lead Technical Support at BD Diesel Performance. “If you are towing heavily on a regular basis, the turbos we combine are considerably different than turbos used on a street/strip truck. Compound turbos must be sized perfectly for each application so they complement each other instead of working against each other.”
For example, Dan says a small, high-pressure turbo with a low A/R mated to an atmospheric turbo with a high A/R will never spool high enough to get to the sweet spot on the efficiency map of the system because the exhaust gas from the smaller turbo will not provide enough drive energy to the turbine wheel on the primary (atmospheric) turbo.
“Let’s say you went with a 366 over a 480, or a combination sized along those lines. What you’d experience is a lot of lag in the bottom end, and the engine wouldn’t reach its power potential on the top. It would be a terrible setup for towing. You want two turbos that overlap and complement each other for a specific application,” Dan says.
For diesels like ours, daily drivers that tow on a regular basis, compound-turbo experts recommend the high-pressure turbo should flow roughly 50 to 60 percent of the larger primary turbo’s flow capacity in pounds per minute.
To that end, BD’s turbo engineering specialists paired a BorgWarner S358 (.80 A/R) with an S472SX-E (1.25 A/R) for our VP44-fueled 5.9L Cummins. This new compound-turbo tow package (PN 1045420; $4,629) is a build-on-demand kit to precisely match each buyer’s needs.
Our S358 features a forged-milled billet 58mm compressor wheel and 64.5mm turbine wheel, while the primary S472SX-E spins a 72mm billet inducer and an 87mm exducer.
“Individually, the S358X flows about 57 pounds per minute, while the S472SX-E moves about 110 pounds per minute. When used in a compound system, these airflow ratings change because of the maximum volume limitation of the smaller secondary turbo,” Dan says. “But that air charge is considerably denser (richer in oxygen) thanks to the compression of the primary turbo.”
To further match the two, BD set the high-pressure turbo’s wastegate to open at 45 psi for quicker low-rpm response while protecting over-speed failure on the top end. That setting would also provide strong pulling power in the mid to upper rpm, as the non-wastegated S472 overlaps where the secondary turbo pops off.
Using the correct turbos, injectors, injection pump, intake, and exhaust are all well and good—as long as the engine makes the right music when it’s all brought together. We reached out to Ben Brown at Accelerated Diesel Repair in Rapid City, South Dakota, for tuning help. He composes tunes for Cummins engines and uses MM3 Power’s programmer with accompanying touchscreen display and connecting hardware to flash calibrations into the ECM. The tunes are also compatible with Edge Products’ Competition Juice with Attitude programmer.
The Juice with Attitude taps directly into a stock VP44’s signal wire, which allows fine-tuning of the fuel metering and timing control solenoids, which the device also manipulates, potentially adding up to 120 hp and 380 lb-ft of torque according to Edge.
Ben spent hours developing the base tunes remotely, while Mobile Diesel’s Mat Johnson drove the truck and loaded the software updates into the ECM via the MM3 controller. Ben’s custom-tuning map includes additions and adjustments to the factory timing, injector pulse width, rpm, and fuel pressure. That enhanced “base” tune can then be paired with Edge’s seven levels of “competition” programming to make as much power as possible with our new setup.
A word of caution here: If you plan on pushing boost beyond 40 psi, which is something compound turbos will definitely do, use ARP cylinder-head studs and O-ring the head. “If that’s not done,” warns Source Automotive’s manager Bill Allen, “the head is going to lift—or worse. Guaranteed.”
Robert put our F-250 back on the dyno to see what the final numbers are with all the changes. The resulting 506 hp and 864 lb-ft made us smile. But what makes us even happier is the fact that EGT will probably never get anywhere near concerning levels when towing heavy loads.
Edge ProductsOgden, UT 84404
BD Diesel PerformanceSumas, WA 98295
Source AutomotiveClackmas, OR 97015
Diamond Eye PerformanceAthena, OR 97813
Mobile Diesel Service541-459-8939
Double R Powder Coating & Fabrication541-459-5662
Accelerated Diesel Repair605-391-4797