Firing order refers to the ignition sequence of cylinders in an internal combustion engine. For gas engines, this means spark plug operation order. For diesel engines, it means the order of when fuel is injected into each cylinder. Firing order affects engine vibration, sound, and power output. It also heavily influences how the crankshaft is designed.
There are a number of common firing orders depending on the type of engine. Some engines have more than others. For example, a common firing order for a straight-six engine is 1-5-3-6-2-4.
However, there are numerous firing orders for V8 engines, which often differ even under the same manufacturer.
Some engines can also operate under different firing orders from stock, which may increase the power output.
The firing order for a stock 6.6-liter Duramax diesel is 1-2-7-8-4-5-6-3. If the engine is left completely stock, there isn’t much of a problem with this firing order. However, the crankshaft on the 6.6-liter Duramax is known for being on the weaker side. Many diesel owners tweak their engines for more power, which affects engine harmonics and stability. Higher RPM and more power cause the front end of the crankshaft to take quite a beating. The stock firing order then may become a liability and weaken the crankshaft even further, or cause it to weaken sooner.
A very common solution to the above problem is to change the firing order to 1-5-6-3-4-2-7-8. This spreads the power throughout the entire crankshaft so that the front part doesn’t take all the force. The alternate-firing camshaft moves a portion of the power to the rear of the crankshaft, which allows it to be absorbed by the flexplate. Unlike with gas engines, the alternate firing order doesn’t really promote any major power gains. However, it can potentially prolong the lifespan of the crankshaft and stabilize the engine for additional power mods.
A common question on upgrading a Duramax diesel by changing the firing order is whether it should be done on a crankshaft that has already been used under the original firing order. The argument is that the used crankshaft is already stressed at this point and will become more stressed when subjected to the alternate firing order. This is where you might consider upgrading your Duramax diesel parts with a new crankshaft. Even a brand new stock crankshaft with an alternate firing cam is better equipped to deal with the stress than a used one.
However, an even better solution is to upgrade your Duramax parts with a billet, narrow-rod crank, which increases the strength of the fillet radius. Other diesel truck parts you might need to reinforce your engine includes extensions for your factory injector wiring harness to adjust your electronics for the alternate firing order. Another solution is to swap pins in the injection harness.
To sum up: an alternate firing cam’s main goal is not to increase raw horsepower in the Duramax engine, but to make it more durable and stable during further horsepower increases with other upgrades. The alternate firing cam also yields a smoother idle.
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