One of the teams winning with...
One of the teams winning with the idea of achieving optimum cylindertiming is Dale Earnhardt Inc. (DEI). The Zero-Cross Pro BilletDistributor on this engine is ready to face the racetrack challenge.
Essentially, a multi-cylinder engine is a collection of individualengines operating at different combustion and mechanical efficiencies.If we can optimize the power output from each one, the overall totalpower will increase.
When the typical multi-cylinder engine's ignition timing is tunedthrough a distributor, it is being set at a compromised value that isthe most total ignition timing allowed by the weakest (defined by itsengine knock limit) of the individual cylinders. For example, if thenumber two cylinder of a Chevy V-8 only tolerates 29 degrees BTDC(before top dead center) of total ignition timing before damaging engineknock in that cylinder begins, we have to set a conventional ignitiondistributor to that overall value of ignition advance for all of theother cylinders. We could be giving up power gains from those othercylinders that could tolerate more total ignition advance, say up to 32degrees BTDC.
From its CNC machined billet...
From its CNC machined billet housing to oversized cap, this appears tobe a typical MSD Pro Billet Chevy distributor. Inside, you can adjusttiming by cylinder. It's ignition timing management without a laptop.
Staggered Timing
Back in 1992, General Motors Racing, Richard Childress Racing, andHendrick Motorsports experimented on the engine dyno, retarding theignition timing of cylinders 1, 2, 7, and 8, and advancing it in theremaining cylinders of a restrictor plate Chevy V-8 Winston Cup engine.(They also experimented then with building engines with different staticcompression ratios in individual cylinders, but that's a topic foranother time.) By using real time combustion pressure analysis of eachcylinder (which was advanced test measuring at the time, but nowregularly done at top-line Cup shops), they determined this "staggeredignition timing" resulted in net torque gains and reduced the averageengine stress in knocking cylinders by 25 percent and peak stress by 19percent. Taking this positive power result off the dyno and onto thetrack would seem straightforward--mechanically modify the ignitiondistributor triggering to advance or retard the timing in each cylinderby 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 theirimpressive CNC machining centers to cut custom-designed magnetic pickuptriggering reluctors (also called "star wheels" or "paddle wheels") fortheir racing ignition distributors. They were machined with a fewdegrees of advance or retard built into each reluctor arm. Thismachining practice is much more common today.
The reluctor assembly of the...
The reluctor assembly of the Zero-Cross distributor features eightindividual magnet tabs, one for each cylinder. Each one of these tabshas magnets that trigger the distributor's primary and secondarypickups. Position of the tabs can be adjusted up to six degrees with afeeler gauge.
The reluctor arms are angled a few degrees (typically between two andfour) off their normal axis (every 45 degrees in a V-8) to advance orretard the ignition timing trigger signal per cylinder. Some racerswithout such precise machining abilities have approximated the sameresult by gingerly bending the trigger arms or filing their leadingedges. These custom-modified reluctors are installed and engine outputis dyno measured; eventually, the best staggered timing is achieved fora specific engine combination.
Staggered ignition timing makes for interesting reading from a timinglight: cylinder 1 might indicate 29 degrees BTDC, but cylinder 7 mightshow 33 degrees BTDC, to use a broad example. Plus, if a distributor hadbeen modified for staggered timing, you want to make sure it is clearlymarked as such, so that it doesn't get installed, or timed in an enginenot built for it. Broken parts can result.
Precision Without Compromise
The pickups of the Zero-Cross...
The pickups of the Zero-Cross distributor produce a different triggersignal that must be converted into a signal that the MSD ignition boxcan recognize. This is done in the Ignition Module. If teams run aredundant ignition, they will need one for each pickup.
Aftermarket electronic ignitions have progressed quite a bit since 1992.You can now buy retrofit digital ignition systems that permit ignitiontiming curve programming via a laptop. Unfortunately, most stock carracing series still don't allow such digital manipulation of ignitiontiming. However, you can now find a mechanical equivalent as MSD isoffering the Zero-Cross Pro Billet distributor.
This racing ignition distributor allows you to adjust the ignitiontiming for separate cylinders by incorporating a reluctor assembly witheight individual magnetic trigger arms that can be advanced or retardedas needed. They can be adjusted 0-6 degrees of crankshaft timing. Nolaptop is needed, just a feeler gauge.
Cleverly enough, this individual cylinder timing control is packaged inthe race-proven billet housing and uses a large cap and rotor assembly,so it's not directly obvious that you're running a distributor that hassuch adjustable timing precision inside. You don't have to set a fixed,compromised ignition timing for all the cylinders, but can individuallymanage timing in each cylinder for best output before engine knockoccurs.