Duramax Build-Up

Turbo, Transmission, and CP3 Upgrades

Published in the August 2008 Issue August 2008 Build, Duramax

Building trucks the way we do is a long process. How would it be to drop off a stock truck at a shop and pick up a fully-built diesel rod a week later? You could go from stock power to more than double the output with just a signature on a credit card slip.

Actually, that's how a lot of people do it and rightly so. We wish we could, but that's not our job. Our job is to build one brick at a time. One part at a time. A few horsepower here, a few dozen pound-feet of torque there. That long, drawn-out process allows us to isolate and identify which parts are doing what, if for no other reason than to avoid making the same mistake the next time around. In the end, we also hope that it helps those guys who do drop off their trucks with a shopping list of mods to know better which parts they want on that list. After all, the real story isn't just the completed project, but the process that lies between the beginning and end.

So it is with this '05 Duramax LLY project. Once a bone-stock truck with 70,000 miles of hauling fireplace units around, we've transformed this crew-cab into a truck with 20,000 more miles on the odometer and the ability to haul fireplaces much more quickly.

The Rundown

From stock form, the first thing we got our blackened hands on was a Pacific Performance Engineering Hot+2 E.T. Xcelerator tuner. The tuner has eight power levels for LLY Duramaxes (10 for LB7s), but we stuck to the first four with the truck still stock. Level 4 cut the stock truck's 0-60 mph times by nearly four seconds, but the transmission didn't appreciate it. The longer (and harder) we drove it, the more it would slip fifth gear and put the truck in limp mode (DTC P0735: gear five ratio error). Fortunately, the PPE tuner allows for the user to clear the code and keep moving-a process we became very familiar with.

The next part to arrive for the truck was an MBRP Cool Duals four-inch turbo-back (down pipe-back, actually) exhaust system complete with the off-road cat-delete pipe. The improved flow of exhaust cut the truck's 0-60 times by another three-quarters of a second on level 4 (it cut the stock tuning's times by nearly 1.5 seconds). An aftermarket exhaust system is one of the best things you can spend your money on, with benefits ranging from increased power to lower egts and reduced cylinder backpressures.

Speaking of egts, a Spearco/Turbonetics intercooler from Black Widow Diesel dropped this truck's by an average of 100 degrees, depending on load and driving conditions. Digital gauges from Dakota Digital confirmed that.

We added a 56-gallon Titan fuel tank just before making the trip to Dynomite Diesel Performance to have the LLY's stock injector nozzles worked over to DDP's 50-horsepower specs. That netted a flat 60 hp across the board (yes, 60), stock PCM or tuned with the PPE tuner, confirmed by DDP's dyno. Changing injector nozzles on a Duramax is not a necessity, since there are trucks making up to 700 hp on custom tunes with stock nozzles. But from what we've seen, going with a smaller-bore aftermarket nozzle (like the DDP 50 hp ones) refines the injection process and provides a cleaner, more controlled injection.

You're probably wondering about the intake. It was scrapped for an AEM Brute Force HD intake kit, which is one of the cleanest intakes we've installed.

We also added a 100 GPH AirDog fuel preparator, which removes water and air from the fuel and pumps it to the CP3 at about 10 psi (we later upgraded to the AirDog 150, but we'll get to that in a few more paragraphs). That gave us a bit more fuel to the pump and upped the truck's horsepower by about 30 on Custom Auto's dyno. However, our problem with fuel was on the other side of the pump.

Another problem we had at this point was with airflow. The stock variable vane turbo couldn't move enough air to function with the PPE tuner's extra fuel. That and the stock fuel system had too many restrictions.

But the biggest problem was that the drive train was crying uncle. The stock torque converter couldn't hold the power anymore. The truck was like a group of 70-year-old canasta players: a lot of smoke and very little movement.

We now had three factors to address and they all needed attention more or less at the same time: (1) drive train, (2) turbo and (3) fuel.

Drive Train

We turned to Sun Coast Converters for help with the drive train. Without going full-blown for a built, crated transmission, they recommended their Stage IV kit and a new torque converter.

The Sun Coast Stage IV kit consists of new clutch packs that are much stronger and made of superior materials compared to the Allison's stock components. Our biggest problem with the stock Allison was that, with no load, we could slip fifth gear (overdrive) fairly easily. The more it happened, the easier it happened and things didn't get any better when towing a load. The stock clutches in the Allison simply cannot hold rear-wheel horsepower in excess of 350 for very long-let alone 500 plus.

We also went with the Transgo Allison Shift Kit. The shift kit eliminates the Allison's unusual tendency to hit neutral under load. It also prevents slippage at the 3-4 shift and burned C2 and C3 clutches. Another benefit of the shift kit is the new clutch control valves that make the Allison shift quickly with little clutch overlap. For high horsepower applications, this shift kit is practically a necessity.

We also got a Boost Modifier Valve from Sun Coast. This converter flow valve controls converter clutch fluid pressure and boosts pressure under converter clutch application from 145 psi to 170-190 psi. The additional pressure helps the clutch hold the power under high horsepower applications.

Once the Allison had gone through the relearn process, the improved transmission components showed a big improvement. The truck shifted much cleaner with no overlap. Shifts were quick, solid and clean, getting out of one gear and up into the next without interrupting the flow of power.

A big engine and built transmission has to have a solid torque converter between the two. Sun Coast's 1058 torque converter is an all-purpose, nearly bullet-proof converter capable of handling horsepower output in excess of 500 rwhp. It's the perfect converter for this truck.


The turbo was another story. Its low boost limits and small housing put the brakes on our power-building, egt-lowering quest. While the variable-vane technology of the stock turbo is good tech for quick spool-up and maximum output at boost, its design is limited to the parameters of a fairly stock engine.

It's no secret that LLY Duramax engines run hot. It's not unusual for a stock LLY Duramax to hold 1,300 degrees F on the egt gauge while towing a heavy load. These engines just run that way. The problem with that is when you start adding more fuel to make more power, it only gets hotter. Even with the larger intercooler, this truck would spike to 1,550 degrees F on a full-throttle acceleration run every day of the week. Cruising temps were manageable at about 1,000 degrees, but that's still hotter than we liked.

Aside from heat issues, the turbo simply couldn't flow enough air to match the added fuel we were putting to the engine. We deliberately did the transmission build before the turbo install to see exactly what the limiting factor was. With the new transmission components and torque converter, the truck would run better on the same tunes we were able to run previously, but the higher we turned up the tunes, nothing happened. We couldn't make more than 410 hp on the dyno and the truck didn't run any better on level 6 than it did on level 2.

Two things were happening here. First, the fuel rail wasn't getting the fuel the high-level-tuned PCM was asking for. We were throwing 1093 codes (low fuel pressure during power regiment) with the slightest tap of the throttle. Second, what fuel the engine was getting wasn't being cleared by the turbo and the egts were sky-high.

Under recommendation from PPE, we picked up a Garrett GT4094R ball-bearing turbo (our particular turbo is part number 751470-21). This large-frame turbo is a free-float, non-wastegated turbo that features both oil and water-cooled bearings. It also builds max boost around 32 psi, so head studs and fire rings were not an issue (saving money and more work). The stock head studs stress at about 40-50 pound of boost (depending on who you talk to), so we are well below that with the GT4094R.

Installing the new turbo requires a new pedestal, oil feed and drain lines, longer down pipe and compressor inlet. PPE's turbo installation kit includes everything necessary for the install except the air inlet components (you have to put that together yourself-and it's a tight fit).

The GT4094R turbo sits about an inch higher in the engine valley than the stock turbo, hence the new down pipe. The down pipe is also upgraded to a 3.5-inch round tube for maximum air flow compared to the flattened stock down pipe (for the record, we had been running a Dynomite Diesel Performance Duramax Down pipe, but had to exchange it for the longer PPE pipe to fit the new turbo).

The first thing we learned during the turbo swap was to strongly consider buying a Dodge next time. Getting the stock turbo unbolted and detached from the crevasse of the V-8 Duramax was on the same level as trying to replace the framing on an existing house. There's no easy way to do it, but there are a few tips to know if you go through the same process.

First, if you happen to have the transmission out prior to the turbo swap, reach up between the firewall and the back of the engine and break the six bolts loose that hold the exhaust manifold to the turbine inlet. You can still break the bolts loose otherwise, but that's just a tip.

Second, getting the oil drain line off of the stock turbo is a big pain best done by someone with small, flexible hands. Not so much a tip as a warning.

Third, before you bolt the new pedestal in place, make sure the manifold holes line up properly and that the pedestal sits in place without rubbing against the heads. Our pedestal needed some grinding to get it to sit in place correctly and we had to drill out the holes in the exhaust manifold to get it to line up with the threaded holes in the pedestal. Every engine has slightly different tolerances, so it's just part of the process.

To finish off the turbo install, we had to mate the AEM Brute Force HD intake to the compressor inlet. We picked up a 4-inch cobra-head rubber elbow from airflow.com and pieced it together, capping the valve cover breather line with a breather filter.


Combating the fuel issues always leads to the CP3 fuel pump. Rather than go all-out with twin CP3s, we opted first to try the less-expensive Modified CP3 from Industrial Injection and add a dual-feed fuel line. The Modified CP3 is claimed to push 42 percent more fuel than the stock CP3. We fully understand that for maximum output, twin CP3s are the way to go. But the modified single pump is half the price of twins and can still take you quite a way up the performance ladder. (For an even cheaper alternative, Industrial also offers a Stage I DIY CP3 mod kit, where you change a few parts on your truck's stock CP3.)

Although Industrial Injection's Dual Feed Fuel Line is designed for LB7 engines, it can be installed on LLYs, too. The dual-feed fuel line eliminates the stock fuel line that crosses from the left side common rail to the right side common rail. On a stock Duramax (LLY and LB7), the fuel leaves the CP3 pump and enters the front of the left common rail. The four injector lines on that rail are fed and a fifth fitting allows fuel to travel from that rail, across the top of the engine and into the right side common rail. The dual-feed line feeds the right side common rail straight from the CP3 pump, so both rails are being fed simultaneously with identical pressure. The theory behind it is that while the left side common rail has plenty of fuel, the last two injector lines on the right side common rail can starve for fuel under load and with hotter tuning. By feeding both rails, that problem is eliminated. (A twin CP3 system works the same way, only each common rail has its own fuel pump and high supply of fuel.)

While on the subject of fuel, we also found a couple of other problem areas once the turbo and modified CP3 were installed. We were still getting 1093 codes like crazy and even got the occasional P0087 code (fuel rail/system pressure too low). We ported the fitting that connects the feed line from the CP3 to the left side common rail to a 0.125-inch bore (PPE sells ported fittings). That helped, but didn't eliminate any codes.

At this point, we turned attention to the AirDog 100 GPH fuel preparator that was on the truck. The AirDog 100 (recommended for stock and slightly modified trucks) was pushing 10 psi of fuel to the CP3 pump (we had a gauge sending unit mounted in the fuel filter manifold). Before these latest mods, that fuel supply pressure would drop to 6-7 psi under hard acceleration. With the new turbo and modified CP3, we could pull the pump (drop the gauge reading to 0.0 psi) with relative ease. That meant we were running out of fuel on the supply side of the CP3 pump. We immediately upgraded to the AirDog 150, which supplies 150 GPH at 19.8 psi static pressure and added a half-inch suction tube to the fuel tank and bypassed the stock fuel pickup. Those changes made a huge difference, keeping the fuel supply up to the demands of the modified CP3 (the CP3 pulls the new AirDog 150 down to 10 psi under hard acceleration). While we had fixed one problem, we still had issues that we wouldn't discover until later. But before we diagnosed the real problem, the next logical step was to shim the fuel pressure release valve (another kit available from PPE). That made a difference, keeping closer to the desired fuel demands when the truck shifted to second gear and on the top end, but we still got the 1093 code on rare occasions (an indication that we had a bigger problem).

We changed the fuel pressure release valve out for PPE's Race Fuel Valve, which completely blocks fuel from leaving the rail.

In the mix of all of this lay another issue: What we were facing was a slow drive-away response time from a dead stop. Imagine trying to cross two lanes of traffic to turn left onto a street. The gaps between cars had to be pretty big, because there was a long period of time from when you pressed the accelerator and when the engine finally revved up. The truck would not build boost on a brake stand, instead building up to about 3.5 psi at about 1,800 rpm before something in the system seemed to "dump," preventing any higher rpm or boost. The other issue came in fifth gear (overdrive) at speeds between 50 and 75 mph. When you rolled into the throttle, nothing happened. A little smoke would flutter out, but the engine would not rev and the turbo would not build boost. Unless the transmission would downshift or the road would roll downhill, the accelerator pedal was worthless. This specific circumstance almost always results in a 1093 trouble code.

The culprit? A bad fuel pressure regulator on the modified CP3 pump. The pump would begin to build fuel pressure to meet the PCM's demands, but would dump it before it got anywhere close to the desired rail pressure (why we were getting the 1093 code). Once we had pinpointed the problem, we pulled the bad regulator off of the pump and put the stock regulator back in. All of the issues we were battling went away. Industrial Injection had a new modified fuel pressure regulator to us the next business day and we were back to a fully-modified CP3.

After that issue was corrected, the truck ran better than ever. It's just one of those freak things that can happen when you get into heavy modifications of such a complex fuel system. But it showed us how great Industrial Injection's customer service is.

End Result

Diesel tuning is a whole lot different than gas engine building. There are so many more recipes to a hot running diesel engine. While there are a dozen different ways to approach what we've done with our LLY Duramax, our formula resulted in a reliable, powerful and above all drivable vehicle. We have a daily driver capable of putting out rear-wheel horsepower in the high 500s and plenty of torque for towing. The GT4094R turbo is a hard-working turbo for trucks that want to build good power without nearing the 40-psi boost mark (where installing head studs are necessary). The components provided by this project truck's sources and installation and dyno-testing help from Colby Hulse and the Custom Auto staff turned a sunbird RV-park ornament into a smoke-blowing dyno day regular.

While the project is completed, we'll still keep the truck around long enough to test a few more products on it and maybe play with some different tuning and fueling formulas. We'll also work on putting together our next Duramax project truck, which means yet another long, tedious process.

Maybe we should go the credit card route.

While we were more concerned with function rather than fashion on this truck, we did make a few changes to the Chevy. We picked up the black SS-style Silverado HD grille and billet bowtie emblem from LMCTruck.com.


Pacific Performance Engineering


Black Widow Diesel


Garrett Turbochargers

Industrial Injection

Dynomite Diesel Performance

Dakota Digital

Sun Coast Converters

Titan Fuel Tanks

Extreme Sounds

Custom Auto

Pure Flow Technologies


AEM Brute Force HD

Royal Purple Oil

LMC Truck



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