When GM introduced the LS engine in 1997, it featured many design changes from the Gen 1 Small Block Chevy (SBC) engine that improved engine durability and efficiency. One of those changes was the evolution of the cylinder firing order from the well-known 1-8-4-3-6-5-7-2 to 1-8-7-2-6-5-4-3.

In a four-stroke engine, the crankshaft spins twice (720 degrees) to the camshaft’s single revolution (360 degrees) for the engine to complete one full engine rotation of an eight-cylinder firing.

The crankshaft timing sprocket is half the size of the camshaft sprocket. (Image/Summit Racing)

Typical V8 engine crankshafts phase the connecting rod journals at 90 degree intervals, meaning that two of the eight pistons will share the same position in the bore at any given time; these are considered “paired pistons.” Chevy engines share paired pistons on cylinders 1-6, 5-8, 4-7, and 2-3.

(Image/Summit Racing)

As the crankshaft rotates, the paired pistons will simultaneously arrive at top dead center (TDC). One piston will be at the top of the compression stroke, and the other will be at the start of the intake stroke. After the crankshaft spins 360 degrees from this position (one-half of a complete engine rotation cycle), the order of those two cylinders will trade off. Swapping the firing order between any of the paired pistons can be done within reason but requires a special camshaft to change the order of valve actuation; this will also change the overall engine firing order, including the ignition order. Additionally, paired pistons firing order sequences will always be 180 degrees from each other.

In the 1990s, Pro Stock and other racing classes began dyno testing 4-7 cylinder swap cams based on the theory of reduced intake reversion and better heat management in the two left-rear cylinders (5 and 7), which fired back-to-back and tended to run hotter. Moving these sequentially firing cylinders from the back of the block to the front (now firing cylinders 4 and 2 in order) placed them in a fresher coolant path. Engineers also observed reduced crankshaft torsional loads and improved induction tuning from more uniform cylinder temperatures.

Reduced crankshaft torsional vibration was the main reason GM adopted the LS engine’s 1-8-7-2-6-5-4-3 firing order (4-7/2-3 swap for SBC). According to GM engineers, computer modeling showed that the new firing order improved reluctor wheel accuracy and eased bearing loads. Using a 4-7/2-3 LS swap camshaft for SBC still moves the pair of adjacent firing cylinders from the back to the front of the block, but to cylinders 3 and 1 instead of cylinders 4 and 2 like the 4 -7 swap cam. It also swaps the firing order of paired cylinders 2 and 3. While many variables come into play with the benefits of swapping engine firing orders, the 1-8-7-2-6-5-4-3 firing order is a clear advantage for the LS and potentially beneficial in SBC engines, as well.

Until now, if you wanted to run your SBC with an LS firing order, it meant purchasing a custom-ground cam—if a core was available. However, the team at Summit Racing has stepped up with a new line of American-made SBC hydraulic roller cams with the LS firing order without the custom cam price—click here to see the Summit Racing Pro SBC LS Firing Order Hydraulic Roller Camshafts.

Summit Racing also offers hydraulic flat tappet Pro SBC 4-7 Swap Camshafts as well.

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Author: Dave Matthews

Dave Matthews was a mechanic for the U.S. Army, a Ford dealership, and served for many years as a fleet mechanic for construction companies. Now a technical content producer at Summit Racing, Dave has spent decades working on everything from military vehicles to high performance race machines.