Gil, it stands for High Energy Ignition; GM brought it out in the 1970's when reduced compression ratios, exhaust gas recirculation (EGR), and leaner mixtures made it more difficult to light the mixture off. So they (and everybody else) came up with electrinic ignitions which hit the coil with a higher primary voltage having a square wave pattern rather than the slow to build and slow to end sine wave pattern typical of points ignitions. The higher voltage and square wave form really hits the coil hard and the coil responds with a much higher secondary (spark) voltage. The resulting much hotter spark across the plugs electrodes does a much better job of lighting the fire.

The other benefit of these electronic ignitions is the elimination of points. This gives better wear qualities as there's no mechanical switch to wear out requiring adjustment and replacement. Since mechanical wear, point bounce and manufacturing/assembly tolerence issues are eliminated the timing is stabilized and accurate over a very long period of time (years to decades instead of weeks to months).

Spark plugs are useable longer and provide a more stable spark as they age because the high voltage/current with the square wave form will jump the electrode gap even when it's widened from wear or when it has conductive deposits on the electrodes. The old fashion point type ignition's sine wave volt/current form would allow the slowly rising energy to bleed away on these "short circuits" then there wouldn't be enough energy left to jump the electrode gap and the engine would missfire.

The factory HEI is designed 2 ways with the coil in the cap and the more conventional external coil, both work the same way. There's advantages and disadvantages to each design. The coil in cap takes more space in one location making it harder to use where the engine backs up tight to the firewall, say a SBC to S-10/15 conversion where you need the engine back as far as you can get it. But the coils close proximity to the trigger system and the rotor/cap reduces the chances of that high voltage getting out somewhere other than the spark plug. The seperate distributor/coil design is easier to put in tight places, but keeping the voltages both input and out put within their respective wires is a greater problem and this system also generates more radio frequency noise which forces better design of the radio/stereo and other electronics like the fuel injection computer to keep the ignition's Radio Frequency Interference (RFI) from messing with the circuits of these things.

Factory HEI is designed around typical passenger/truck use and is usually only good to 5000 RPM or so. The aftermarket supplies "Hot Rod" parts from modules and coils to complete distributors for those who need reliable sparks beyond 5000 revs.

Bogie 

what considerations do i need to make before going to HEI.  or is this simply a plug n play part?  

If this is a pre computer vehicle, it's just plug and play.

Bogie

I'm going to refer you to the Jegs, Scroggn ####, and Summit on line catalogs, there are many others as well who are equally good.

http://www.jegs.com/

http://www.summitracing.com/

fficeffice" />> >http://www.sdpc2000.com/

I'm guessing that this isn't a high performance application so in the neighborhood of 100 to 250 dollars you should find a good selection from Accel, GM Performance Parts, Pertronix, Proform, and certainly some catalog houses feature their own inhouse brands. You don't need a billet distributor, the machined housings look nice but otherwise provide no benefit, they're just expensive. You need to stay away from the computer years, so basically 1984 or older. If your not running a roller cam, no special gear is needed. If this truck has a mild cam and is mostly street driven you do want vacuum advance, as this helps considerably with part throttle low to moderate speed fuel economy.

If you want to listen to AM radio in the truck you need resistance ignition wires; MSD Heli-Core is good at about 50 bucks for a kit, Taylor makes Spiro-Pro which is similar to the MSD in design, performance and cost. There are the older carbon core wires that provide excellent RFI protection such as Accel Super Stock for about $25 a set. While the carbon core wires work well they are a bit delicate so if you get these care must be taken to always remove them by grasping the boot, not that I recommending you ever remove wires by pulling on them, it's just the carbon core type is much less tolerant of physical abuse.

Bogie 

thanks for the recommendations - the engine will be slightly tricked out - i have a mild cam with a nice slow lope - edelbrock performer rpm 65cc heads, 600 cft holley, with a edelbrock duel plane manifold.  i'm hoping to have to have a little giddy-up, after all, the heads are rated for 420hp out of the box with the setup i have.   

Ahh, you sly dog, you; a little cam, a little carb, a set of custom heads. Strap it all on a 350 and toss it into an old truck and who's the wiser.

Enjoy

Bogie

I would suggest going to local salvage yard and purchase used hei from '75-'80 model chevy car or truck with 305-350-400 ,should be avail for $50 or so,then you can spend remainder on msd or accel upgrades to match performance with rest of set-up.GM wires are good bet also(correct fit if order for make/model)and have great noise suppression characteristics,may not look as "trick" as some aftermarket but are well designed  to withstand oil,heat,etc under hood.   

on most classic cars (originally points type) Hei is not plug and play. Some will have a ballast resistor and others a resistor wire. the points aren't designed to get a full 12 volts. while an hei distributor will still work with the resistor in place you'll be losing a few volts on your primary side which equals to a more significant amount on your secondary. It's usually more advisable to remove this wire and replace it with a 14 or even 12 guage wire.  

Your right, that's one of those things I do so automatically, I forget to tell folks about it.

Actually it's been a long time since I've built anything with points, I mean at least a decade, maybe longer.

Bogie