Compound Turbo Kit For LML Duramax
When we began this journey, we wanted to present to you a CP3 injection-pump conversion and a Wehrli Custom Fabrication compound-turbocharger kit installation on a ’15 Chevrolet Silverado 2500HD, powered by a 6.6L Duramax LML engine. The bolt-on effort is usually simple and straightforward, requiring only three or four days in the shop to accomplish. In and out. Right?
As we all know, sometimes things just don’t pan out as expected. As we covered in the first report of this two-part series (“Pumped Up,” May 2019), replacing the truck’s stock Bosch CP4.2 high-pressure injection pump with a Fleece Performance Engineering CP3 750 conversion kit was executed without issue. However, when technicians at Dunks Performance pulled the cylinder heads to upgrade the engine with ARP studs (as insurance against the higher cylinder pressures expected from the compound turbos), we discovered seven pistons were cracked at the edges of their fuel bowls. Had the new turbos been installed without this discovery, the engine’s life would have been short-lived under the heavy towing missions in store for the Silverado.
The engine was pulled, and the truck’s owner decided to invest in a top-tier performance engine from DFC Diesel Rebuilders in Edmonton, Alberta, Canada. DFC’s Race Series 6.6L Duramax LML is a meticulously machined and assembled engine that’s capable of supporting more than 900 hp. The powerplant is balanced, blueprinted, and loaded with Wagler Competition Products’ alternate-fire camshaft, dual valvesprings, Carrillo connecting rods, Mahle Motorsports ceramic-coated pistons, ARP fasteners, an ATI Super Damper, a heavy-duty flexplate, and Dynomite Diesel Products 75hp injectors. The whole setup is covered by a five-year, 100,000-mile warranty.
Eric Dunks, ASE Master Technician and lead wrench at Dunks Performance, made short work installing the DFC Duramax LML and reinstalling the Fleece injection-pump setup and fuel system hoses and hardware, including the new injectors.
The next big step was to pull the pin connectors at the engine-control module and rearrange the firing order to work with the engine’s new alternate-fire cam. A modified firing order helps relieve the harmonic stress placed on the front portion of the crankshaft. That stress typically results in the crank breaking anywhere from the snout to the third rod journal. Changing the firing order from 1-2-7-8-4-5-6-3 to 1-5-6-3-4-2-7-8 alleviates that issue.
The stock engine was replaced because its pistons cracked after enduring long periods of high EGT. The Mahle Motorsports pistons in the new DFC engine are far more robust.
“Our cast performance DMAX pistons offer several benefits over the OE piston in this application,” says Mahle’s Justin Dossett. “The crown of the piston is thicker to accommodate higher cylinder pressures, and the inertia load capacity was increased for higher rpm capability. They maintain a cooling galley to help keep piston temperatures in check and utilize a Ni-Resist top ring insert to aid in top ring groove strength.
“The compression height of these pistons has been reduced 0.010 inch to allow for decking/trueing of the block and setting the desired piston protrusion. In addition, the bowl design features a de-lipped and heavily radius’d version of the OE bowl and maintains the OE 16.8:1 compression ratio.”
Justin says the de-lipped bowl allows for more aggressive injection timing, while the heavy radius reduces stress risers and potential for cracking in the bowl rim.
Also, a Thermal Barrier coating is applied to the piston crowns, says Brandon Burleson, who oversees this special process at Mahle. “The coating is durable, resistant to thermic shock, and reduces heat transfer to increase performance. It’s a combination we expect will work very favorably in high-performance Duramax LML engines used for towing heavy loads.”
With the CP3 installed and injectors’ firing order changed, Eric finally worked on getting the Wehrli Custom Fabrication compound-turbo setup installed. The BorgWarner S400/S300–based kit is remarkably complete. However, despite its thoroughness, there are no instructions. That’s just something to keep in mind if you are a DIYer and taking on this project for the first time. Fortunately, the technicians at Dunks are plenty familiar with it, so for them the installation was not much different than doing a turbo swap on a typical 6.6L Duramax.
Why do compound turbos work well on a street/work truck? There are several reasons. One is that compounding over the high-pressure turbocharger flows a lot more air through the engine, resulting in lower exhaust gas temperature across the entire rpm band. The second reason for adding an “atmospheric” ’charger to feed the high-pressure unit is to reduce turbo lag for faster spooling.
The end result is quicker throttle response and boost, with better efficiency and lower EGT across a wider rpm range. It’s a good combination for those who use their trucks for towing. This is all predicated, of course, on choosing the right combination of turbochargers.
The compounds we’re installing center around a BorgWarner S475 atmospheric turbo with a 1.15 housing, feeding a high-pressure S364.5 SX-E with a 73mm turbine. Both are fitted with billet compressor wheels, and their housings are powdercoated gloss black. The compound system is a stark contrast to the custom, Shocker Yellow–powdercoated aluminum tubes of the Mishimoto air-to-air intercooler that was already in place.
Installing compound turbos requires moving the passenger-side battery forward, replacing the stock coolant bottle (with WCFab’s piece that installs where the battery used to reside), removing the EGR system, and installing a new exhaust because the WCFab downpipe is much bigger (diameter) than the original tubing. It also requires a custom tune, which we will eventually tap the expertise of EFILive to handle.
We still have a few more things to check off before this project is complete. The stock Allison 1000 six-speed automatic transmission needs to be rebuilt and beefed up so it can handle the expected boost in power.
Breaking in the new engine before putting it on a dyno is also important. Our plan is to put between 500 and 1,000 miles on the powerplant, while also changing the oil at least twice during that time.
In “stock” trim, with only a tune and 35-inch mud tires, the original engine made 511 hp and 1,038 lb-ft. of torque on the chassis dyno at FTW Tuning. We expect those numbers to be higher with the compound turbos and EFILive tuning. How much more is a little bit of a guessing game at this time.
We can’t wait to spend a little time on the road with this one, and then put it on the rollers again to see what a compound-turbocharged 6.6L Duramax LML is all about.
Mahle MotorsportsFletcher, NC
Fleece Performance EngineeringBrownsburg, IN 46112
Wehrli Custom Fabrication630-277-8239