How to upgrade ’14-’16 Chevrolet & GMC 6.6L Duramax for better power and mpg
AFE Power’s New Cold-Air Intake, Diamond Eye’s DPF-Back Dual Exhaust, and Hypertech’s Programmer Improve a 6.6L Duramax LML’s Overall Performance and Fuel Economy
In the first installment of this two-part series (“White-Glove Treatment - Duramax Power and MPG Upgrade"), we performed an experiment using Hypertech’s Max Energy 2.0 programmer on Chet Kokkeler’s ’15 GMC Sierra 2500HD Denali that pulls double-duty as both work truck and family transporter. With the programmer set on Stage 3, our test netted an additional 45 hp and 69 lb-ft of torque from the truck’s otherwise-stock 6.6L Duramax engine. We also saw an 8.5 percent jump in cruising fuel economy (from 15.3 mpg to 16.6 mpg, at 70 mph on flat interstate) while driving in the highest stage.
But there are a number of pickup owners like Chet who want as much power and fuel economy as possible from their diesels, without compromising the truck’s smog-legal status (for licensing, warranty, resale, or lease obligations). Deleting the diesel particulate filter is not an option, so we’re now focusing our attention on helping the engine breathe better.
While the Hypertech Max Energy 2.0 programmer definitely brings more power to the towing table, we want to take advantage of one of its built-in features: tuning compatibility with an aftermarket cold-air intake system and a DPF-back exhaust. And we’re doing this by adding an Advanced Flow Engineering (AFE) Stage 2 CAI and swapping the factory single exhaust pipe with a dual-tube setup from Diamond Eye Performance.
Hypertech’s chief engineer and software programmer, John Lambert, says they spent a lot of time on the dyno making sure calibrations are compatible with several popular air intake kits, including those from AFE.
Minding Your MAF
Chet Kokkeler’s 6.6L Duramax LML-powered ’15 GMC Sierra 2500HD Denali is a dual-role rig that serves as a work truck during the week and family hauler on the weekends.
Hypertech’s Max Energy 2.0 programmer can be loaded with tunes already designed to work with AFE Power’s cold-air intake systems, so receiving P0101 diagnostic trouble codes related to the changes in airflow past the MAF sensors is very unlikely.
Initially, one might think that installing a free-flowing air intake system airflow is a good thing. However, that’s not necessarily the case. The big concern with using aftermarket CAIs on Duramax engines is that the factory mass airflow (MAF) sensors are extremely sensitive to the most minute changes in airflow.
Any change in the way air flows across the MAF sensors changes the engine’s fuel delivery parameters, which can induce rough idling, loss of power, and other maladies. When there are vortices in the intake tube or other airflow fluctuations, the ECM will trigger a P0101 diagnostic trouble code and, in some cases, command the engine to run in “limp-home” mode, which, for a V-8, is typically on seven cylinders—or fewer.
This is especially true of the 6.6L Duramax LML. Even changing from GM’s style air filter to one with higher flow characteristics can alter how the engine runs, as would changing the air tube or any other components that affect airflow past the MAF.
That’s why Hypertech actually installs several CAIs, including AFE’s, on its test trucks and calibrates so the stock MAF sensors work in concert with a new intake system. Aftermarket CAIs also allow the tuners to take advantage of the increased airflow by adding more fuel to make more power across the entire range of programs, from “Stock” to “Stage 3.”
The same is true for the addition of an aftermarket DPF-back dual exhaust. Although there’s very little restriction in a GM diesel truck’s factory single-outlet tubing, upgrading to the 4-inch twin-pipe setup from Diamond Eye improves the overall flow of air.
Quick Shop Work
The Diamond Eye Performance DPF-back dual-exhaust kit doesn’t provide any noticeable power increase with the filter in place. But the dual black tips definitely make a statement in the looks department and produce a great sound, too.
“Installing an AFE Stage 2 cold-air kit on one of these new Sierras is one of the easiest jobs to handle,” says Casey Castle, chief technician at Dunks Performance in Springfield, Oregon. “Everything is right out in the open, and the AFE air tube and filter housing are fairly easy to slide into place. The only modification for this effort is cutting off the front inlet on the new airbox in order to clear the truck’s aftermarket HID headlights.
Swapping the factory filter-back exhaust with Diamond Eye’s 16-gauge, aluminized dual exhaust (K4163A) is almost as easy, especially when the truck has a lift kit, which creates even more working space between the rear axlehousing and the body.
The new exhaust system comes in seven pieces and slips together without a lot of fuss or drama. The bends are smooth and the welded-on hangers are both sturdy and perfectly placed to slide right into the factory bracket grommets. The exhaust kit comes with new hangers for the driver side, which allows the new fork to hang like it was installed at the factory, clearing both the factory spare tire and the aftermarket helper air springs under the flashy work truck.
Back on the Dyno
The cold-air intake setup we installed is AFE’s all-new Diesel Elite Stage 2 SI, which the company says flows 44 percent more air than the stock GM airbox.
Dunks Performance technician Casey Castle removes the factory airbox lid and unplugs the connectors to the two mass airflow (MAF) sensors located on the air intake tube and box’s cover.
With improved intake and exhaust airflow for our test GMC’s Duramax, it was time to see how much of a difference the addition of the new parts made at the truck’s rear wheels. So it’s back to Garage 808 in Eugene, Oregon, where technician Tim Walton uses a Dynojet 424xLC1 chassis dyno to get to the bottom of performance gains…or losses.
Tim ran our ¾-ton GMC on the rollers, repeating tests in the same manner as he did in our first installment. He warmed the truck up then manually shifted the Allison automatic transmission into 5th gear to make the pulls, starting with the stock ECM calibration, and then using Hypertech’s Max Energy 2.0 to reprogram the processor with new Stage 1, Stage 2, and Stage 3 tunes that are suited to the mods.
In this second round of dyno tests, we see that AFE’s freer-flowing CAI (the dual exhaust is, for all intents and purposes, a cosmetic and aural upgrade) yields 24 additional horsepower and 27 lb-ft of torque over the engine’s baseline performance with the stock ECM calibration. With the new Stage 1 tune and cold-air setup, power and torque are about the same (27 hp and 30 lb-ft, respectively) as they were with the tune and factory air intake equipment. The Stage 2/new-air combination produces a 27hp/40-lb-ft gain over the second-level calibration without the CAI and dual exhaust, and Hypertech’s Stage 3 program with the new parts surprisingly brings the smallest power increase: only 11 hp and 26 lb-ft of torque at the rear wheels.
What the addition of these two upgrades shows us across the board is that the 6.6L Duramax LML definitely responds well to airflow improvements, and the Hypertech programs take full advantage of that by adding more fuel to the mixture, while still keeping EGT and other factors well within the engine’s and vehicle’s capabilities.
It’s also interesting to note that with the Stage 2 “economy” tune, the modified engine actually makes less power than it does in stock trim, below 2,000 rpm.
We also noticed all the horsepower peaks occur at approximately 2,800 rpm, while the torque peaks range from 1,900 rpm (stock) to 2,100 rpm (Stages 1-3), holding strong to 2,800 rpm where the speed limiter cuts in as wheel speed nears 103 mph. Maximum boost in stock trim is around 26 psi, while boost pressures with the Hypertech tunes vary slightly between 27 psi (Stage 1) and 28 psi in Stage 3.
A Bump in MPG
Tim Walton, Garage 808’s dyno engineer and tuning expert, breezes through the four different tuning stages (Stock, Stage 1, Stage 2, and Stage 3). Uploading each tune requires turning the ignition key to the “On” or “Off” positions, per prompts that appear on the Hypertech Max Energy 2.0’s screen.
The lower portion of the factory airbox pulls up and away from the base. It’s held in place by rubber feet that are plugged into holes in the metal base.
Hypertech’s programming also dials back the fuel at the lower “cruising” engine speeds. Our dyno data clearly shows that both the Stage 1 and Stage 2 tunes make less power than the Stock calibration does, between 1,600 rpm and 1,900 rpm.
The reduction of power in the lower rpm range translates to an 8.5 percent increase (roughly 1.4 mpg) in highway fuel economy, according to data compiled in our mileage runs (120 miles of cruising on flat interstate, at 70 mph).
“Stage 1 and Stage 2 power levels offer our customers a toned-down version of our Stage 3 tuning to suit their desired power increase and still provide this power under any towing condition that does not exceed the rating of the vehicle,” John says. We offer alternate power levels to allow our customers to choose the benefits each new tune provides.”
Overall, this experiment with 50-state-legal, bolt-on upgrades shows us that GM left a little performance on the table with its 6.6L Duramax LML engine, and with the right combination of aftermarket parts, it’s easy to bring that power to the front without compromising reliability or offending the smog police.
Casey swaps the drain plug from the stock box and installs it in the one-piece AFE unit.
For most applications, there’s no need to modify the new CAI. Everything fits perfectly, unless the ’15 GMC Sierra 2500HD (or Chevrolet Silverado 2500HD) has aftermarket HID headlight assemblies. The AFE box’s auxiliary air opening must be trimmed to clear the deeper passenger-side headlight.
Casey buffs the airbox-opening’s edge to ensure the plug cap fits properly. This step is only needed if the truck has aftermarket HID headlights.
Aligning the feet on the new box with the holes in the base of the factory piece takes a little maneuvering before they pop into place.
The AFE Pro Dry air filter should last the life of the truck when properly cleaned and maintained. It flows considerably more air than the GM filter it replaces.
According to Casey, aligning the air tube, filter, and the inlet at the back of the engine is the most difficult task of installation.
Once the tube is connected to filter and engine, Casey buttons everything up. AFE’s clamps are high-quality, as are the rest of the components in the CAI kit.
There are two sensors in the Duramax air intake: one that measures humidity and the other a combination of airflow and air temperature. Both have to be reinstalled in the new air tube using the special screws that are supplied with the AFE kit.
This MAF sensor serves dual purposes: one is to measure airflow and the other to measure air temperature. The Max Energy 2.0 programmer contains tunes designed to work with the new air system and prevents the MAF sensors from triggering diagnostic trouble codes.
After the sensors are reinstalled in the new air tube, Casey plugs in the factory connectors to complete the installation. It takes less than an hour to make the CAI conversion.
This is Diamond Eye Performance’s aluminized-steel, DPF-back dual-exhaust system for Duramax-powered ’15-to-present Chevrolet and GMC Silverado and Sierra 2500HD Crew Cab pickups.
When removing the original exhaust, Casey uses a strap to keep it from clattering to the floor once it is cut away from the truck.
Removing the stock exhaust requires cutting the DPF’s outlet pipe. Casey leaves 5 inches of the old pipe remaining on the particulate filter, so the new Diamond Eye exhaust pipe can be slid over it and clamped in place.
Once the pipe is cut at the DPF and the single exhaust is pried away from the rubber hanger grommets, the old pipe slides right out. Our truck’s lift kit also makes installing the new exhaust easier.
The dual-exhaust system’s single-to-double Y-connector fits nicely along the driveshaft and framerail.
The six pieces of the dual exhaust are clearly labeled, making it easy to fit them together. The kit comes with new driver-side exhaust hangers and brackets that bolt up to the framerail at the back of the hitch.
One trick Casey uses to ensure the exhaust pipes are even on both sides is using jackstands to prevent them from moving while the clamps are tightened.
The new exhaust fits the truck like the original system. It clears the factory spare tire, which is removed here only for photo purposes. The kit is available with either black or chrome exhaust tips.
The last stop on our intake-and-exhaust upgrade trip is on the Dynojet 424xLC1 chassis dyno at Garage 808. With Tim Walton manning the controls and the Hypertech Max Energy 2.0 set on Stage 3, the net performance gain is 56 hp and 95 lb-ft over the stock baseline.
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