Testing 12-Valve Twins

1998 Cummins Rebuild

April 2011 News, Cummins Ryan Harris

Our 12-valve Cummins project truck has been one of those trucks where the untold story is almost better than the printed version. But the transformation this truck has made over the past couple years underscores how much potential these earlier Cummins-powered trucks really have.

When we first picked up the truck, it was pretty much bone stock. It was putting out 215 horsepower-at the crank. That made for about 170 rear-wheel horsepower on the dyno.

Contrast that with the 618 rear-wheel hp the truck puts out now. We've picked up nearly 450 horsepower, and the truck is more reliable, tows better and is a heck of a lot more fun to drive and compete with.

In April 2010 issue of Diesel Tech, you can read about how Dynomite Diesel Performance gave this truck a work over with a single charger and some pump upgrades.

This time around, the truck went back up to Dynomite's shop in Monroe, Wash., but this time for a compound turbo kit and a full-race p-pump.

Compound Chargers

The DDP compound system consists of an S400 charger with a 74mm compressor wheel, an 88mm turbine wheel and 132 A/R turbine housing. The S400 feeds into an S300 with a 62mm Schweitzer compressor wheel. The S300 also has a DDP-modified 71mm turbine wheel and 1.0 A/R non-wastegated turbine housing.

Lenny Reed, owner of Dynomite Diesel Performance, says they choose non-wastegated housings because wastegated housings can have a tendency to crack. This kit is about reliability, so no chances are taken with the equipment. Besides, Reed says, with the right wheel and housing combinations between the two chargers, you really don't need a wastegate. Reed says DDP offers 75 different combinations with the S400/S300 compound kit.

Another custom modification made to DDP's compound setup is how the down pipe is mounted to the top charger's turbine housing. A lot of turbos use V-band clamps. V-clamps work great for holding two pipes together, but they aren't that great at sealing off air pressure. There can be 50 pounds of pressure coming out of the exhaust turbine, and the V-band clamp will leak. Instead, DDP machines the back of their turbine housings flat and taps them for six bolts. Then it machines a flat flange for the down pipe so there is a flat-to-flat surface mount.

Dynomite designed a plate that mounts between the top charger's turbine housing and the exhaust manifold. The plate acts as a mounting pad for the bottom charger. This design allows you to remove the top charger while leaving the bottom one mounted to the truck. It makes life easier for the do-it-yourself installer. The DDP compound turbo kit is also designed so that the engine's oil filter can be changed simply by removing the air filter.

The compound turbo kit comes with 5-ply, 220-psi burst pressure hump boots for the hot side. The hump boots allow for the flex that comes with any Cummins engine. Again-it's another key to a reliable system. The base kit uses the factory boots for the cold side of the charge system, but upgraded boots are available.

Pump the Fuel

The pump is a mirror image of the pump used on Dynomite Diesel's own 12-valve race truck. It began as a 215 hp injection pump. It features 12mm plungers and 5,000 rpm governor springs. As a side note to those who have followed this truck's progress: this is an all-new pump from the single-charger setup. This pump has a new fuel plate that is a little more aggressive, but very versatile. The current setup is capable of delivering all the fuel this compound setup can handle. In fact, this is the same pump that hit 1,450 hp on DDP's grey 12-valve race truck.

Reed has put a lot of R&D time into the aneroid, or AFC (air/fuel control) on top of the pump. DDP has a longer spring for the AFC that allows for more adjustability to fine-tune the truck for throttle response or low smoke. As Reed explained it to us, the pump lever which rides the profile of the cam plate through the throttle range first contacts the AFC finger at idle and at lower throttle position percentages. If there's no boost pressure in the charge air system, the pump lever won't depress the AFC finger and move up the cam plate (delivering fuel to the injection system). As boost builds, it pushes against the AFC spring and loads the spring, which pushes the finger back into the cam plate. Then the pump lever begins riding the cam plate profile.

What DDP is trying to accomplish is to improve control of the AFC finger for high-horsepower applications. In a high-boost setup like this truck, the stock spring is overcome so quickly and the AFC finger retracts so early so you just churn out black smoke. The fuel isn't delayed enough to let boost build at low throttle positions. By going to the longer spring, DDP can control that low throttle position fuel delay. It doesn't affect the engine's power delivery, because the AFC finger still depresses and the pump lever still follows the cam plate profile. With DDP's AFC setup, you can tune the preload on the spring for low-smoke (for emissions checks) or quick throttle response.

Reed also explained that it used to be fairly common for high-horsepower Cummins builders to just take the cam plate out completely to get as much rack movement as possible. But there is a pin inside the rack that-with no cam plate-becomes a stop pin. It can shear off and cause the rack to hang. They do that thinking the more fuel, the better. But with DDP's AFC setup, they can actually lean out the fuel plate, clean up the bottom end and maintain peak horsepower. It really improves the truck's drivability.

"You've got to have the right amount of fuel at the right time," Reed explains. "Too much and too late and it doesn't have a chance to get the chargers lit. As old and arcadic as p-pumps are, there are still new things you can do with it."

PureFlow Technologies supplied an AirDog II fuel preporator/lift pump to keep fuel properly supplied to the p-pump and the DDP Stage 3 injectors.

Ring of Fire

To keep the 200,000-mile engine's head gasket from blowing out, DDP had it machined for a fire ring setup. What's the difference in a fire-ring and an o-ring? For an o-ring, you usually machine a groove and insert a brass or copper ring that compresses against the factory fire ring. Reed says that a head gasket in original form is 0.063 of an inch thick, and when the head is torqued to spec, the gasket compresses to 0.058 thick. When o-rings were first adapted to Cummins diesels, engine builders figured that the ring should have a 0.018 protrusion. In that case, as Reed explained to us, there is so much pressure just trying to squeeze the o-ring against the factory fire ring that the gasket doesn't seal. By the time the o-ring was reduced enough to seal the gasket, the o-ring's protrusion was only 0.004. It's just not worth the cost of machining at that point.

But when ARP came along with a good set of head studs, the factory fire ring holds a lot better-better than o-ring setups at that point. So, like on our truck, DDP takes a wire steel ring that's 0.105 in diameter and machines a groove large enough so that when the ring is compressed between the head and cylinder, it expands to fill the groove. It becomes a pretty solid part of the head, and doesn't wiggle or walk around. This method reduces the chances of a head gasket failure to nil.

While working on the head, DDP also installed a set of 90-psi in-seat valve springs.

Since the DDP compound kit was installed in DDP's shop, they also installed a few of the other parts we included in the build. Like the PDI exhaust manifold, TS Performance Torque Converter Lock-UP, AirDog II and the ARP head studs. When Dynomite installs a set of head studs, their procedure is to install the studs using a moly lube and torque them to 140 ft-lbs.

Then the engine is run long enough to get the head hot and let everything move and settle. The engine is left to cool completely, and then everything is broken free and re-torqued to 140 lb-ft again. Reed says you will always get another quarter- to half-turn out of the fasteners this way. You can repeat the step again for added insurance or for race applications.

Final Verdict

We have towed with this setup, used it as a daily driver, used it for road trips and used it as a hauler for anything and everything. Granted, it is a 1998 Dodge, so it handles pretty much like a wheelbarrow. But the reliability has been amazing since DDP's latest makeover. And that carries tremendous weight with us, mainly because when we first bought the truck originally, just saying "reliable" within 15 feet of it would cause it to break. We've fired it up on minus-20 degree mornings (with it plugged in, of course), driven it 180 miles through snowy conditions, let it sit for 8 hours and driven it back home in one piece. We would have never relied on it for a trip like that before.

And we are impressed with the overall design of the kit. We've been around compound kits that take a three-man crew to install. We've seen kits that come undone on their own. There's nothing more frustrating than having a turbo system that causes more problems than it fixes. But we literally don't open the hood on our Dodge other than to check the oil or move the fuel plate for a dyno run.

Like we mentioned early on, this setup had laid down 618 horsepower on Custom Auto's SuperFlow dyno. That is with the fuel plate all the way forward. We do most of our driving and towing with the plate just forward of middle. By tuning the AFC, and with some transmission and gearing tuning from Adrenaline Performance, the truck is very drivable with that kind of fuel. It takes a controlled throttle foot to manage the boost and throttle position without blowing smoke with that much fuel, but when the boost comes on-and trust us, the boost gauge jumps from 15 psi to 60 psi in a nanosecond-all that fuel is just a party waiting to happen.




Dynomite Diesel Performance



PureFlow Technologies


Automotive Racing Products


PDI Diesel

Performance Diesel Inc.

TS Performance


Custom Auto


Adrenaline Performance


Level 10

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