Racing teams spend many hours looking for the competitive advantage. These teams tweak the intake manifolds for that air/fuel mixture distribution perfect for the individual cylinders for best overall power. Getting the ignition timing tuned for each cylinder presents another opportunity to improve the total average power output of the engine.
Essentially, a multi-cylinder engine is a collection of individual engines operating at different combustion and mechanical efficiencies. If we can optimize the power output from each one, the overall total power will increase. When the typical multi-cylinder engine's ignition timing is tuned through a distributor, it is being set at a compromised value that is the most total ignition timing allowed by the weakest (defined by its engine knock limit) of the individual cylinders. For example, if the number two cylinder of a Chevy V-8 only tolerates 29 degrees BTDC (before top dead center) of total ignition timing before damaging engine knock in that cylinder begins, we have to set a conventional ignition distributor to that overall value of ignition advance for all of the other cylinders. We could be giving up power gains from those other cylinders that could tolerate more total ignition advance, say up to 32 degrees BTDC.
One of the teams winning with...
One of the teams winning with the idea of achieving optimum cylinder timing is Dale Earnhardt Inc. (DEI). The Zero-Cross Pro Billet Distributor on this engine is ready to face the racetrack challenge.
Staggered Timing back in 1992, General Motors Racing, Richard Childress Racing, and Hendrick Motorsports experimented on the engine dyno, retarding the ignition timing of cylinders 1, 2, 7, and 8, and advancing it in the remaining cylinders of a restrictor plate Chevy V-8 Winston Cup engine. (They also experimented then with building engines with different static compression ratios in individual cylinders, but that's a topic for another time.) By using real time combustion pressure analysis of each cylinder (which was advanced test measuring at the time, but now regularly done at top-line Cup shops), they determined this "staggered ignition timing" resulted in net torque gains and reduced the average engine stress in knocking cylinders by 25 percent and peak stress by 19 percent. Taking this positive power result off the dyno and onto the track would seem straightforward--mechanically modify the ignition distributor triggering to advance or retard the timing in each cylinder by just a few degrees.
Remember, no digital ignition control of ignition timing was (or is) allowed in many racing series. Top Winston Cup teams began to use their impressive CNC machining centers to cut custom-designed magnetic pickup triggering reluctors (also called "star wheels" or "paddle wheels") for their racing ignition distributors. They were machined with a few degrees of advance or retard built into each reluctor arm. This machining practice is much more common today.
The reluctor arms are angled a few degrees (typically between two and four) off their normal axis (every 45 degrees in a V-8) to advance or retard the ignition timing trigger signal per cylinder. Some racers without such precise machining abilities have approximated the same result by gingerly bending the trigger arms or filing their leading edges. These custom-modified reluctors are installed and engine output is dyno measured; eventually, the best staggered timing is achieved for a specific engine combination.
Staggered ignition timing makes for interesting reading from a timing light: cylinder 1 might indicate 29 degrees BTDC, but cylinder 7 might show 33 degrees BTDC, to use a broad example. Plus, if a distributor had been modified for staggered timing, you want to make sure it is clearly marked as such, so that it doesn't get installed, or timed in an engine not built for it. Broken parts can result.
 From its CNC machined billet...  From its CNC machined billet housing to oversized cap, this appears to be a typical MSD Pro Billet Chevy distributor. Inside, you can adjust timing by cylinder. It's ignition timing management without a laptop. |  The reluctor assembly of the...  The reluctor assembly of the Zero-Cross distributor features eight individual magnet tabs, one for each cylinder. Each one of these tabs has magnets that trigger the distributor's primary and secondary pickups. Position of the tabs can be adjusted up to six degrees with a feeler gauge. |  The pickups of the Zero-Cross...  The pickups of the Zero-Cross distributor produce a different trigger signal that must be converted into a signal that the MSD ignition box can recognize. This is done in the Ignition Module. If teams run a redundant ignition, they will need one for each pickup. |