Simplifying the Gen IV Chevy by Removing Active Fuel Management
Written by Steve Magnante on January 4, 2017
Way back in 1981, Cadillac, reacting to the same pressure to maximize fuel economy automakers face today, came up with America’s first mass-produced, variable-displacement, gasoline, V8 engine. Known as the Modulated Displacement L62 V8-6-4, this 368ci (6.0L) powerplant was an outgrowth of the massive 500-, 472-, and 425-inch Caddy V8 but with a smaller bore. Collaborating with automotive component subcontractor Eaton, the Caddy made 140 hp and 265 lb-ft of torque (in V8 mode) and featured computer-controlled solenoids that deactivated the fulcrums under select rocker arms. Depending on load, it automatically toggled between V8, V6, and V4 modes to save fuel.
Though the design was mechanically sound, the electronics of the day weren’t as sophisticated and reliability issues quickly surfaced. Still, Caddy was so dedicated to the V8-6-4 engine that it was installed in hundreds of thousands of cars, though multiple recall campaigns to fine-tune the software haunted buyers. In the end, most of the Modulated Displacement engines were deactivated at the dealership—a simple matter of disconnecting a few wires—and run as fulltime V8s.
Junkyard spotting is easy: just look for any 1981–1984 Caddy V8 with a pair of cigarette-package-sized rectangular blocks affixed to each rocker cover. They’re big and blue—you can’t miss ’em. But keep walking if you spot one. Beside the freak factor, there’s zero car-crafting potential.
Today, the cylinder-deactivation concept has returned as a standard feature on many 2005-up General Motors Gen IV V8 engines as a way of having our cake and eating it too. But unlike the old days, modern electronics have far more computing power, and instead of defeating rocker-arm motion at the fulcrum, they rely on juggled oil pressure to “collapse” certain lifters to close the valves and seal the deactivated chambers.
Once sealed, the trapped air acts as an air spring while the computer shuts off the fuel injectors and ignition coils, and the deactivated pistons and rods are just along for the ride. When more power is demanded, events are reversed and the engine goes back to V8 function within milliseconds. In fact, the Gen III Mopar Hemi engine also employs similar technology, Fiat-Chrysler calls it the Multiple Displacement System (MDS), and the majority of 5.7 Hemis and many 6.1 and 6.4 Hemis also have it. Ford—so far—hasn’t equipped any of its OHC V8s with this magic.
GM’s Displacement On Demand (DOD) system (also called Active Fuel Management) has many millions of miles of proven, reliable service. Most importantly, when in V4 mode, fuel economy improves by as much as 20 percent, helping meet the stringent Corporate Average Fuel Economy (CAFÉ) standards and keeping Uncle Sam and the EPA happy.
All of that said, car crafters generally don’t need cylinder deactivation, and since the factory-installed camshafts found in DOD engines are pretty tame, swapping up to more aggressive grinds for better power is hindered by the presence of the oddball DOD lifters and related parts (lifter buckets). There’s also the reliability/maintenance factor. The DOD system relies on many small oil passages and orifices that are easily clogged by deferred oil changes. Once this happens, lubricant flow to the roller lifters is curtailed and leads to wheel and lobe failure.
In this story, we’ll watch as Josh and Eric Buzzell, co-owners of NextGen Performance in Spencer, Massachusetts, convert a boneyard-fresh L76 from DOD to FUN using Summit Racing’s specially formulated kit (PN CMB-09-0026). Priced at just under $400, the kit includes eight roller lifters, four lifter buckets, a stock LS3 valley cover, and an LS2 head bolt kit. If the engine is out of the car, the process takes just a couple of hours.
Our Displacement On Demand (DOD) conversion subject is a 6.0L (376ci) L76 from a 2008 Pontiac G8. The NextGen guys tell us this is an exception: they mostly work with more common—and nearly as potent—pickup-truck and SUV-sourced 5.3s, most of which also have DOD. GM also offered a 4.8L version of the Gen IV, but they’re non-DOD and always fire all eight cylinders. Since the smallish 4.8 is typically found in pickup trucks and SUVs, GM likely predicted unacceptable V4 power levels when in economy mode. Thus, this article doesn’t apply to 4.8s.
After stripping the induction and valve covers, rocker-arm shaft removal is the first step. These items aren’t affected by the DOD elimination and can be reused, as can the pushrods. Note the offset rocker arms. Because the L76 uses the Corvette’s LS6 big-port cylinder heads (casting number 243)—minus their sodium-cooled valves—the standard pushrod trajectory must be redirected to clear the fatter outboard port walls. The realigned rockers do the trick. If they’re disassembled for cleaning, be sure to obey correct orientation during reassembly.
The Buzzell brothers have been wrangling LS engines for several years. Eric shared this tip: “I often buy Gen IV–powered pickups and SUVs at auction for less than they’re worth because the instrument gauge shows a low-pressure warning light. Fearing costly bottom-end repairs lurk, other bidders shy away and I grab them for cheap. The culprit is often nothing more than a clogged screen below the sending unit to the gauge [seen here in its black plastic cage]. No real damage is done.”
GM’s adoption of one-time-use head bolts (and other varied fasteners) is controversial. But Eric says, “I know some guys reuse the head bolts, but I think it’s a false economy. If the customer or engine comes back with blown head gaskets in a week, are you really ahead of the game?” NextGen says the $60 cost of new factory bolts is money well spent and that studs are only really needed with boost or heavy nitrous use.
Unlike other Chevrolet V8 engine designs, the Gen III–up series requires cylinder-head removal before the lifters can be accessed for replacement. Note the LS6-style rectangular intake-port windows (taped to prevent debris entry). The “lowly” 5.3L engines’ cathedral intake-port configuration is pretty decent and outflows many classic heads from the muscle-car era.
On the driver side of the engine, DOD deactivates cylinders 1 and 7 and cylinders 2 and 6 on the passenger side under light-load, highway-cruising conditions.
Like previous Gen III engines, pairs of lifters are located within plastic buckets and are quickly removed by removing a central hold-down bolt. Here are the lifters for the driver-side leading cylinders (1 and 3). When deactivated, oil flow is curtailed and the external auxiliary spring takes over, allowing the roller to follow the cam without moving the valve off the seat. The plastic lifter buckets are specific to DOD applications and are not reused.
At 3.791 inches, the active roller lifters used at cylinders 1, 2, 6, and 7 are longer than the 2.603-inch-long, non-active roller lifters used at cylinders 3, 4, 5, and 8. A sweet detail is that the non-active roller lifters are standard items, and for those on a tight budget, can be reused with the non-DOD camshaft. The other four lifters and the DOD lifter buckets must be exchanged, as we’ll see.
The pen points to the raised guide key machined into each active lifter to keep its roller aligned with the camshaft lobe. Note the different designs of the DOD lifter bucket (top) and Gen III bucket (PN NAL-12595365). The Summit Racing conversion kit includes four of these necessary lifter buckets.
The underside of the Gen IV/DOD valley cover reveals the integrated oil-flow control solenoids. When bolted down, the cover aligns to the eight raised towers cast into the valley. A single external connector at the firewall end of the cover plugs into the vehicle’s electrical system to activate the solenoids—and cycle lifters.
Eric points out the earlier Gen III block’s (background) lack of DOD oil-feed towers. When defeated, the openings at the ends of the towers must be sealed to maintain the oil circuitry. The next picture shows how it’s done.
Summit Racing’s DOD delete kit includes a stock LS3 valley cover (foreground). The eight O-ringed pads seat against the oil-feed towers in the Gen IV block to seal them shut. The Gen IV valley cover (background) is replaced.
The replacement valley cover even comes with fresh hold-down fasteners. Eric says, “The O-rings and integral gaskets used in modern engine designs, not just the Gen III–up V8, makes working on them much easier. I never want to go back to the days of cork gaskets and two-piece seals.” Amen.
With the DOD oil passages closed off, the Gen IV block stands ready to accept any Gen III camshaft. NextGen keeps customer take-out Corvette and Z/28 LS1 cams for budget reuse in 5.3s where they add power for free. As always, any camshaft upgrade must be accompanied by matching valvesprings. Attention, fleet owners, the DOD cam can be reused if its mild specifications aren’t a concern.
The Gen III lifter buckets slip right into the Gen IV block. Similar in basic shape to Gen IV DOD buckets, they’re not interchangeable. Again, the Summit kit includes them.
The engine will run after DOD elimination, but the check-engine light will be triggered as the PCM searches for a function that’s no longer there. Any GM-authorized programmer can re-flash the PCM. This also holds true if a hotter camshaft is installed. NextGen Performance works with SMG Motoring in Mendon, Massachusetts, when computer tuning is required.
The NextGen Budget-Beater Shop Truck
NextGen whipped up this nifty 2002 GMC Sonoma budget-beater shop truck for $2,277 and use it as a calling card to promote the company’s LS conversion abilities. Born with a sleepy V6, the four-speed automatic transmission, brakes, and suspension are all bone stock.
Here’s a budget recipe:
2002 GMC Sonoma bought with bad engine but rust-free body and frame: $1,100
4.8L Gen IV from parts truck: $0
Used LS1 cam from 2002 Camaro SS: $0
862 head castings with porting and machine work: $325
LS6 valvesprings installed: $60
Used Chevrolet Performance Muscle Car swap oil pan: $120
Ceramic-coated Sanderson headers: $315
Motor mounts from Current Performance: $77
Corvette high-mount alternator, power-steering pump bracket from ICT: $130
New Corvette harmonic damper and water pump: $150
Stock 7.625 rear axle with used 2002 Camaro SS 3.42 posi and gears: $0