How It Works

Duramax Alternate Fire Camshaft

For almost 15 years now, the 6.6L Duramax Diesel found in the 2001-15 GM trucks has been proving it’s one of the most durable and dependable engine platforms found in the 3/4 and 1-ton truck market. Throughout its lifespan the Duramax has gone through a few revisions to improve performance and drivability while reducing emissions. Some internal changes like different connecting rods and pistons along with external additions like emissions equipment and a variable geometry turbocharger have formed the LML Duramax (2011-15) into the best version yet, producing nearly 400 hp. Through those years and multiple improvements, the crankshaft has remained virtually unchanged and can be subjected to undue stress causing premature crank failure, especially in a high-performance or competition application.

Beefing It Up

Since the Duramax engine’s introduction in 2001, the aftermarket has developed some incredible things to take its performance places never imaginable. It’s not uncommon to see competition sled pull and drag race trucks with 2000-plus horsepower these days. Obviously, the stock engines really must be beefed up inside with stronger rotating assemblies to handle the cylinder pressure, boost and torque they’re producing up there, but who would’ve thought an engine engineered for 300 hp could be pushed so much further? One of the more common failure points in a 1000+hp build turning higher rpm is the crankshaft. Broken snouts on the front of the crankshaft back to the #3 rod journal started becoming a more frequent issue as the performance barrier continued getting pushed further and further around 2008-09. The aftermarket had to find solutions, and a few places looked closer into the firing order of the Duramax engine and the harmonics it created within. While there is no proven theory behind the idea, some determined that the firing order (1-2-7-8-4-5-6-3) could be putting major stresses along the crankshaft’s plane and adding undue stress throughout the crank, leading to fatigue and failure over time.

They’d hoped that by creating a new camshaft profile to change that firing order, maybe it was possible to more evenly distribute the stresses of combustion events across the crankshaft, thus reducing or eliminating the broken crank problems. By redesigning the camshaft profiles and changing the firing order to 1-5-6-3-4-2-7-8, they could change those internal harmonic issues and make parts and engines last longer at extreme power levels. Changing an engine’s firing order isn’t a new science, as GM gas guys have been doing it for years before the Duramax crowd considered it with the popular LS engine “4-7 swap.”

What all does swapping to an alternate-fire camshaft entail? Changing the camshaft out in a Duramax isn’t quite as simple as it was in the old small block Chevy. It’s basically the first hard part installed in the bare engine block, so it’s basically like rebuilding the entire engine to do it. To go along with changing the mechanical side of things, like changing the cam lobe positions to physically change the cylinders’ firing order, the electronics had to be figured out as well. How was the engine’s computer going to know the firing order changed? The injectors were also going to need to open and close in a different sequence. After some investigation, it was discovered all the ECM needed to initiate the firing order of the injectors was when it saw signal from the trigger on the engine’s tone wheel. This trigger told the ECM when to fire the #1 cylinder injector; after that, the ECM didn’t care what order the remaining seven injectors fired in. If an injector signal was moved from its original intended firing location and if that injector signal matched the opening/closing strokes of the valves, the engine would run like factory, based off that #1 cylinder signal.

Once that was determined, there were two easy ways to fix the electronics side for an alternate fire camshaft. One would be extending and rewiring the factory injector wiring harnesses and the other would be moving pin locations on the injection harness itself. There was talk about being able to adjust tables within the ECM (via performance tuning) to just signal the injectors at a different time. But while this would be easier, making the change by doing the pin swap in the harness would be much safer. There would be risk of causing major engine failure from the increased possibility of loading the incorrect tune not set up with that alternate firing order.

Changing an engine’s firing order isn’t a new science, as GM gas guys have been doing it for years.

Since the first alt-fire cams were developed and tested, the few aftermarket companies offering them have had great success keeping the Duramax crankshafts and engines alive. Early engine dyno testing in 2009-10 revealed a much smoother idle along with a noticeable difference in engine tone at higher rpm, and some customers think the reverse firing order better distributes air through the intake manifold for better combustion and engine efficiency as well. Camshaft profiles have become more aggressive for competition builds and are now offered for street or even towing applications. Making this conversion in any engine build, from a mild towing and street application to an all-out 1,500+hp competition build, has shown its advantages for many.