Around 1975, General Motors made a significant advancement in stock passenger ignition technology: the High Energy Ignition (HEI). One of its major features was installation simplicity. The coil was contained within the distributor cap and it required connection of only one 12-volt wire. Significantly better than the standard at the time, the points distributor and the HEI went on just about everything GM produced until the mid-'80s, when electronics began running the engine controls. This ignition was designed for low-revving street vehicles.
Racers running a stock HEI often develop a high engine-speed miss around 5,500 rpm that essentially acts as a rev limiter. The HEI can't develop enough ignition energy to properly fire the spark plugs beyond that rpm.
Because the stock HEI was never designed for racing, better and more dependable racing ignition systems have been developed. You won't find an HEI on a race car in the higher divisions, but many Street Stock and other entry-level racers are required to run OEM-type ignitions. Because the majority of this population races the Chevrolet 350, they're faced with competing using the bone-stock HEI.
The good news is that there are options available for hopping up an HEI. Depending on your rules, options could be anything from simply locking out the advance to using the HEI to trigger a full-blown, high energy external ignition control capable of running a V-8 up to 14,000 rpm.
"Stock" Tactics The first option is to purchase a new HEI and perform a few basic modifications to maximize its capabilities. Because energy is required for a spark to jump an air gap, minimize your plug gaps. Start with a gap between .032 and .035 inch, which should be a good amount to make a spark and give the fuel/air mixture enough space to get in there and light up. While you're at it, install a new set of spiral core wires designed for racing.
With that done, block off the vacuum advance. It isn't necessary for racing applications because the engine normally operates at an rpm range-even during cautions- too high for it to work. Do the same with the mechanical-advance mechanism under the cap. It is possible to find an advantage with a properly tuned, dependable mechanical advance, but those are rarely found stock because they're prone to sticking over time when the hinge points on the throws become worn or the springs break.
The springs are often so weak they go to full advance too early. One stable solution is to lock it up (you can tack-weld the weights in the closed position), set your engine's advance for the racing rpm range, and forget about it. Most ignition manufacturers offer an inexpensive device to lock out the vacuum advance, and some even offer a mechanical advance lockout.
"Modified" Stock These steps are normally allowable even under the most restrictive rules. How far to go beyond this is up to you, your wallet, and your tech inspector. As with everything else on the race car, different inspectors have different parameters, even when the rules stipulate "stock." You might want to do a little asking around to see exactly what is legal and how much you can get away with.
If you want to continue down the HEI improvement path, look at tuning your own advance curves. Ask your engine builder when he recommends your engine be at full advance. You can order more durable mechanical-advance kits that come with several springs in different weights, and with a little experimentation and a lot of patience, you can tune your mechanical advance to your needs. This is helpful because, while locking the engine at full advance is a viable option, it can also make it difficult to restart when it's hot and also cause the engine to bog when accelerating from lower rpm (read, when coming out of the turns).