Back to Basics: Dodge 12 Valve

Part Five: Twin Turbo Efficiency

Published in the August 2011 Issue August 2011 Feature, Turbos

Twin Turbo EfficiencyOur Back To Basics project continues as we are in the process of building a 1998 Dodge 2500 Quad Cab, powered by the Cummins 6BT (12-valve) engine. Our goal with this truck is to build significant horsepower and torque, making this truck a lot of fun to drive, while at the same time tuning this truck so that it can be driven as a daily driver with good fuel economy and practicality. Normally with 12-valve engines, because they're not electronically adjustable, you have to make a choice: either drive a tractor with great reliability and economy, sacrificing excitement, or drive a hot rod with big power, big smoke, but with limited drivability. Unfortunately, life's full of compromise. Sometimes Step 1you just have to settle. However, in this case, we demand more. We want the best of both worlds. With the right tuning, we think we can have it all.

Project Recap

So far, we've replaced the transmission with a full billet, Fat Shaft trans from Ultimate Transmission. It's firm, shifts quickly, runs cool, and Ultimate Transmission assures us that it will hold up to anything we throw at it. After the tranny was done, we took the truck to Big Twin Diesel in Boise, Idaho, to rebuild the top end of the engine. They installed a new Hamilton street/strip camshaft and did a complete valve job, including new 165-pound valve springs, also from Hamilton Cams. BT also milled and cut the head for a fire-ring head gasket and ARP head studs. We also installed a Baja bumper with an integrated Warn HD PowerPlant winch and (four) PIAA ATP (All Terrain Pattern) beam lights. Our next project on the mighty 12-valve is a fuel makeover.

In our last issue, Big Twin Diesel installed a Dynomite Diesel built P7100 injection pump that will flow up to 900 cc's of fuel, a FASS Titanium Series lift pump and regulator, and 90hp Dynomite Diesel injectors.

Step 2Twins!

It's common to hear people refer to compound turbo sets as "twin turbos," but as you probably know, a twin turbo set would be a matched set of turbos that would most commonly be used on an application that would utilize separate turbos that operate simultaneously with separate exhaust supply and in turn separate compressed air on each bank of the engine, such as a V-type engine. An inline engine such as the Cummins will generally use compound turbos where a smaller turbo spools first as exhaust leaves the engine, providing quick charge air to the engine, in turn generating more heat and exhaust. Then a second larger turbo will spool as the increased exhaust pressure continues through the small turbo and drives the large turbo's turbine. At this point, the turbos work together to provide high charge air volume and pressure to the engine. This combination provides the quick spool-up of a small turbo, which would normally be accompanied by high EGTs and a flat power curve at the engine's higher RPM, while the larger secondary turbo provides the high air volume to provide low EGTs and big power on the top end of the RPM range-normally accompanied by slow turbo spool, smoke, and decreased drivability. Compound turbos are great on most any diesel truck. They'll be especially useful on this mechanical 12-valve engine in providing the linear Step 3power curve we'll need to meet our requirement of great drivability and big power.

Getting Started

On one of our trips to Boise, Idaho, we were introduced to Pat Liskey, owner and founder of Big Twin Diesel. As we toured his shop and viewed a few of his twin turbo builds, we began to understand that some of our projects trucks needed to have his name on them. We were impressed by the level of precision Step 4that they put into their builds. At first glance, we liked the fact that his twin kits all use steel tubing, without any silicone boost tubes. Every connection is made with close tolerance fittings, grooved, o-ringed, and then sealed with hard V-band clamps, eliminating the potential for leaks or pressure loss until the air enters the intercooler. We also noticed that the BT turbo kit on 12-valve and 24-valve engines would allow the use of the factory transmission fluid-to-engine-coolant heat exchanger. Most twin kits require deleting this heat exchanger, which if deleted, severely decreases the truck's ability to cool the transmission. Fluid-to-fluid heat transfer is far more effective than fluid-to-Step 5atmospheric transfer. We knew that Liskey could not only build a set of twins, but we also found that he knew his way around setting up and tuning an engine after we drove one of his 12-valve trucks as well. Big Twin has been the key to building the tightest, most drivable, and efficient high horsepower 12-valve trucks we have ever driven.

The first component that will get booted out of the engine compartment for our new twin setup is the factory exhaust manifold. As we turn up the heat with our upgraded fuel delivery system, this manifold will be quick to surrender to cracking as it expands and contracts under varying degrees of heat. We chose Step 6to run a new manifold that's made by a company called Steed Speed. The Steed manifold is the next progression in exhaust manifolds. It's a one-piece manifold that's CNC-machined out of mild steel and then ceramic-coated for decreased heat soak and as a result, more efficient energy transfer. This manifold flows more exhaust and has much less restriction than the commonly used two- or three-piece manifolds we've been using for years. All of this adds to quicker turbo spool and a reduction in EGTs. These manifolds are 50 percent more expensive than the old two- or three-piece cast manifolds you see today, but our philosophy here is that you get what you pay for.

Step 7Keep It Moving

After the exhaust manifold, we should note that all of the plumbing in this kit has been ceramic-coated and fiberglass-wrapped where possible to stop heat soak and keep the energy (in the form of heat) contained and moving through the pipes, pushing the turbines, and out through the exhaust as quickly as possible. We also really like the fact that all of the plumbing is welded together and any connections that are made are accomplished with V-band clamps. No nuts or bolts to come loose or leak after we put some miles on this truck during work, Step 8play, or race.

The Big Twin kit we chose for this build uses a Garrett GTX4202R ball bearing turbocharger for the bottom (larger) turbo with a 75 mm inducer compressor and a 105 mm inducer turbine. The top (smaller) turbo is a Garrett GT3788R ball bearing charger with a 64 mm inducer compressor and a 68 mm inducer turbine. Part of Big Twin Diesel's secret recipe for this twin kit lies in their exhaust housings. Apparently, they run housings that are not specific to Garrett and even still will be further modified to Liskey's specification. Ball bearing chargers will provide significantly less resistance than thrust bearing Step 9chargers, which will decrease turbo lag and greatly improve drivability around town. You can actually hear the chargers spinning for 30 to 40 seconds after you shut the engine off. If ball bearing turbos have a negative side to them, it's that they are more expensive than traditional thrust bearing turbos and require not only oil lubrication, but must be cooled with engine coolant additionally. However, we've given up many creature comforts by driving a 13-year-old truck. We're walking the diesel purist's line by going back to basics. We've been fiscally conservative with our money in our original truck purchase. We've done this so we can afford the coolest hardware under the hood and in the Step 10drivetrain. So your spouse should either be able to relate to this sensible logic, or never take the time to peek under the hood and see what the old truck really has there. This kit has two wastegates. The top charger does not have an internal wastegate, so BT uses an external wastegate before the turbo that is actuated solely off of spring pressure to release exhaust drive pressure when boost levels reach a predetermined level, which will depend on the application. The bottom charger has an internal wastegate that's operated with a boost reference line and an actuator as seen on most turbochargers.

Step 11The Fine Details

As they installed the kit at Big Twin, they were fanatical about every detail. They made sure everything was fitted, aligned, and torqued perfectly. The turbos, flanges, and piping fit perfectly, nestled together as if it were an apparatus you'd find under the hood of a high-end German car.

For the exhaust on the truck we chose to give the truck an unmistakable muscular look by installing an MBRP Smokers single stack kit with an 8-inch Step 12black powder coated stack. BT built the 5-inch exhaust from the lower turbo back to the bed, where the 5-inch pipe expands up to 8 inches in diameter. This stack is also helpful for those occasions where we get hasty and forget to "drive the turbos" before we "pour on the coal." In other words, as with any modified diesel with a decent amount of fuel, you have to make sure the turbos spool before you give the engine a lot of fuel; otherwise, you'll be belching black smoke until the turbos spool and match the fuel with the required amount of air. If this does happen every so often, it helps to have the smoke go up above the truck instead of in front of somebody's super-cool tuned import car.

Step 13Coming Together

Now that the turbos and exhaust are installed, we can finally drive this truck with all of our modifications working together in harmony. The upgraded fuel system, advanced timing, hot camshaft, built top end, compound turbos, big exhaust, race worthy transmission, and full complement of gauges in the cab are everything we need for this truck to perform like we intended. On the dyno, the truck laid down 539hp and 1042 ft./lbs. with a stock fuel plate on the injection pump. With a modified fuel plate, it put down 612hp and 1,259 ft./lb. We've Step 14limited our numbers by limiting our fuel supply with 90hp injectors, but we'll keep it here for now so we can test the drivability of this truck. We may bump it up with 120hp injectors to see how drivable the truck is with more power, but as is the drivability is great. The turbos spool quickly and effortlessly. Like we mentioned earlier, the throttle is sensitive and the DDP P7100 pump supplies a ton of fuel, so you can't just mash the accelerator and go. It takes a little restraint to caress the throttle to drive the turbos, but little movements with your foot equal big movements with the truck. Normal driving produces a slight haze from the stack or less. Spirited, quick driving around town produces little Step 15puffs of smoke, not unlike a 12- or 24-valve Dodge with 350hp. Exhibition driving can produce excessive amounts of smoke (enough to make a black cloud that hovers about 8 feet above two lanes of the road) at first until the turbos light, but after they're spooled, the smoke cleans up and the stack puts out a gray haze through the duration of the full throttle run. At 70 miles per hour, with overdrive locked up, you have to be careful with the throttle, because rear tires will break loose when throttle is applied too quickly.

The EGTs run fairly cool. Under full throttle acceleration, EGT readings will get up to 1,500 degrees by the end of the run. When towing at freeway speeds, Step 16we saw EGTs ranging from 900 to 1,200 degrees, depending on the grade. Our favorite thing to do in this truck is to light the turbos and dump a bunch of fuel into it at about 30 mph and make two of the nicest-looking rubber tracks for about 100 yards down the street. It's effortless, satisfying and a quite a rush. So for all you kids out there, don't do drugs, do diesel. (That phrase not to be used without the expressed permission of Nate King.)


ARP Fasteners
1863 Eastman Ave
Ventura, CA 93003

Big Twin Diesel
670 E King Street
Meridian, ID 83642

Step 17Buckstop Industries
1125 Industrial Pkwy
Newberg, OR 97132

Dynomite Diesel Performance
13675 Roosevelt Rd SE
Monroe, WA 98272

16240 Highway O Ste B
Marthasville, MO 63357

Hamilton Cams
Step 18925 Loop 232
Liberty Hill, TX 78642

2515 NE Riverside Way
Portland, OR 97211

14718 Arminta Street
Van Nuys, CA 91402

315 Old Ferguson Road
Huntsville, ON, Canada P1H 2J2

Ultimate Transmission
220 W 37th Street
Boise, ID 83714

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