Tripling The Airflow Into a Duramax With Compound Turbos

The quest to build the most powerful Duramax engine on earth is a lofty goal. While Dmitri Millard (2010 Diesel Power Challenge winner) has never actually said that’s his mission—he’s clearly in the running for the title.

Maximum Flow Highly Modified Duramax Engine

Dmitri Millard’s Chevy race truck began life with compound turbos (a 106mm Garrett GT5541 and an 80mm GT4508), a PPE Dual Fueler, and a Duramax long-block from his street truck. Millard wanted more power, so PPE recommended upgrading the primary turbo to a GTX4508R.

Dmitri Millard’s Chevy race truck began life with compound turbos (a 106mm Garrett GT5541

Some of you may be familiar with Millard’s 1,200hp, ’01 Silverado street truck. It was powered by a max-effort LB7 Duramax featuring compound turbochargers from PPE. We showed you that setup in “The Truth About Twin Turbos” (July ’10), and Millard successfully campaigned the combo on dragstrips and dynos across the country. But for 2012, he swapped that engine into a regular-cab pickup—and this one is a dedicated race rig.

Millard’s race truck is called Overkill, which you caught a glimpse of in our July ’12 issue. This Silverado started life with his street truck’s compound turbos, but virtually every part of that kit has been massaged from what we showed you two years ago. For this article, PPE added new up-pipes and a second wastegate, and upgraded the primary turbo to a Garrett GTX version.

Maximum Flow Garrett Gt5541 Turbo Removal

Millard is seen here using an engine hoist to remove the original Garrett GT5541 turbo while it was still connected to its support frame and exhaust inlet manifold.

Millard is seen here using an engine hoist to remove the original Garrett GT5541 turbo whi

We followed along while PPE’s Corey Macha transformed Overkill’s engine compartment days before Millard competed at the NHRDA Diesel Texas Nationals where he made a record run. Millard’s Overkill now holds the quickest e.t. record for an Allison-transmission-equipped truck at 9.776 seconds at 148.6 mph. Once Millard gets ahold of a solid track, look for Overkill to go even faster!

Using 2-inch stainless steel tubing, PPE’s Corey Macha positioned the primary wastegate to the riser block of the pedestal. Once happy with the result, the pieces were TIG-welded together. Note that the major fabricating was done on a stand-mounted engine and then the parts were transferred to the engine in the truck.
Using 2-inch stainless steel tubing, PPE’s Corey Macha positioned the primary wastegate to
The first section of 3 1/2-inch stainless steel tubing was TIG-welded to an interlocking V-band flange.
The first section of 3 1/2-inch stainless steel tubing was TIG-welded to an interlocking V
Every piece of the intake and exhaust system required forming, such as this piece for the turbo connection flange, which was hand-hammered into shape.
Every piece of the intake and exhaust system required forming, such as this piece for the

To facilitate more airflow into the engine, PPE modified the factory Duramax intake to use a 3-inch inlet (right) rather than the standard 2 1/2-inch inlet the Duramax comes with (left). PPE offers this modification as its High Flow Intake Manifold.
To facilitate more airflow into the engine, PPE modified the factory Duramax intake to use
To keep the exhaust piping from cracking as it expands and contracts (from the extreme exhaust gas temperature), PPE fits aircraft-spec, stainless-steel bellows (arrow) between the two stages of turbocharging.
To keep the exhaust piping from cracking as it expands and contracts (from the extreme exh
When completed, the exhaust tubing and wastegate plumbing (also fitted with a compliant stainless-steel bellows to prevent cracking) looked like industrial art.
When completed, the exhaust tubing and wastegate plumbing (also fitted with a compliant st

The turbo installation on the truck began with bolting the primary turbo’s turbine housing and pedestal assembly to the engine. Note: The internals of the turbo were removed, as the turbo was so big, it couldn’t be bolted on as an assembly with the intake manifold in place.
The turbo installation on the truck began with bolting the primary turbo’s turbine housing
PPE then fit a set of its Hi-Sil Moly ductile-iron Race High Flow Exhaust Manifolds to the engine. As part of its manifold kit, PPE offers these 11-gauge (0.120-inch thick), stainless-steel exhaust up-pipes equipped with stainless-steel bellows to allow movement without cracking.
PPE then fit a set of its Hi-Sil Moly ductile-iron Race High Flow Exhaust Manifolds to the
The location for the secondary wastegate had to be determined on the race truck, as it had to fit close to the turbo and still allow the hood to close.
The location for the secondary wastegate had to be determined on the race truck, as it had

A 2-inch hole saw was used to cut into the exhaust, and then a short piece of stainless-steel tubing was TIG-welded into place between the wastegate and the main tube. TIG-welding is preferred for this type of work in order to limit the amount of welding slag and splatter inside the tube—which if it were to come loose could ruin a turbo.
A 2-inch hole saw was used to cut into the exhaust, and then a short piece of stainless-st
With the wategate on, the turbo system was assembled again, and measurements were taken for the 4-inch-diameter turbo crossover tube.
With the wategate on, the turbo system was assembled again, and measurements were taken fo
To help prevent the PPE silicone boots from blowing off, Vibrant Boost Brace turnbuckles were fitted to the joints that see high pressure.
To help prevent the PPE silicone boots from blowing off, Vibrant Boost Brace turnbuckles w

During the fabrication process, multiple bungs were welded into the crossover tube. These were added to mount nitrous nozzles and to read boost pressure. The unused ports were plugged but will be there if needed.
During the fabrication process, multiple bungs were welded into the crossover tube. These
Due to the size of the GT5541R primary turbo, a small notch had to be made into the PPE aluminum coolant overflow reservoir just to get it to fit.
Due to the size of the GT5541R primary turbo, a small notch had to be made into the PPE al
The new Garrett GTX 4508R turbo is water cooled, so PPE plumbed it into the engine’s cooling system.
The new Garrett GTX 4508R turbo is water cooled, so PPE plumbed it into the engine’s cooli

Both turbos were equipped with heavy-duty heat shields to help channel the exhaust energy through the turbine housings—rather than allow it to radiate outward as heat.
Both turbos were equipped with heavy-duty heat shields to help channel the exhaust energy
The primary wastegate was installed. Its exhaust is vented through the hood of the vehicle when it opens.
The primary wastegate was installed. Its exhaust is vented through the hood of the vehicle
By routing the wastegate through its own exhaust stack (arrow), Millard will be able to visually tell when the secondary turbo’s wastegate opens.
By routing the wastegate through its own exhaust stack (arrow), Millard will be able to vi

The wastegate is opened when the boost pressure in the crossover tube between the two turbos reaches a predetermined level.
The wastegate is opened when the boost pressure in the crossover tube between the two turb
It was tight, but PPE was able to fit the Garrett GTX4508R in the lifter valley of the Duramax. Note that heat shielding was placed over the boost-reference tubes coming from the secondary turbo’s wastegate.
It was tight, but PPE was able to fit the Garrett GTX4508R in the lifter valley of the Dur
With barely an inch of space or a minute of time to spare, Millard’s Overkill truck was loaded onto a trailer and headed to a race. With the new turbos, horsepower is estimated in the 1,300 to 1,500hp range without nitrous.
With barely an inch of space or a minute of time to spare, Millard’s Overkill truck was lo