What are the limits of an HEI that is setup with the correct amount of mechanical and vacuum advance? When, if ever, is it necessary to step up to aftermarket equipment? I normally just run the HEIs, but recently picked up two cars with fairly radical big blocks, one 454 with a MSD 6A system and a 427 with a 6AL. Both have very radical idles and no vacuum advance. Past experience has shown me that vac advance can help tame the idle a bit, and I was wondering if I could yank all this red stuff and toss an HEI in there. Is cylinder pressure the deciding factor if a system makes enough spark to fire the mixture? Just when do you really need the aftermarket systems?

Drop,
The HEI will probably work fine. The other option would be to use the HEI distributor to trigger the MSD...that gives you the advantage of the wide cap spacing of the HEI plus the rev limitier and higher output of the MSD box.

I run MSD stuff because it is easy to tune and if the book says you will have x amount of advance at y rpm with a given spring and limiter thats what you have. Vacuum advance is to make up for the lack of initial timing you can run on a street car. Without vacuum advance you need to run 18 or so degrees initial but this is hard to do on a street car so 8 degrees initial with a 12 degree vacuum advance would do the trick. If you could buy more springs and advance cams for an HEI, they would be hard to beat by any aftermarket company.

In a sbc build-up article from a Chevy HP book the Mallory Unilite added 1 HP and 1 ft lb of torque over the stock HEI. This was a smog motor, a 350, that got different heads, cam changes, and a blower. They were looking for 400HP.

I've read several articles that claim aftermarket ignitions are usually not needed and a waste of money. I don't know when you need them, but there were plenty of high CR engines before the HEI came out. Of course the cams could have reduced the cylinder pressures.

Dave, Dave Ray told me , cylinder pressure and revs is wherer hte MSD helps. And yes, I felt a noticeable difference up in the R's ( 5800+ ) over the stock Ignitor system. Might even be less of a difference with HEI.

Sounds like the HEI may work then. The 427 is radical enough that its almost a PITA when idling and at low revs, so I think I will setup an HEI with vac advance and give it a try. Cant hurt smile.gif Thanks guys

The only problem with many HEI's is that they were evidently used from the factory with port vacuum actuated vacuum advance and only have 14-16 degrees of mech advance. That means you have to put in 20-22 degrees of initial advance just to get 36 total (before vac adv). On those, lighter springs, which are probably needed anyway to get the max mech adv rpm where you want it, and some grinding on the weight cam will get more mech adv so you can lighten up on the initial. I like about 24 deg mech adv in the dist so you can start out with 12 initial and still be able to go up a little. On HEI's I use a new vacuum adv can for a '75 350 Chevy with a fabricated limiter to limit vacuum adv to 10-12 deg.

Our L78 runs a recurved HEI and works just fine to 6500. Of course I have done the same thing with a standard distributor with heavy duty Delco D112P HD points. Advertising sells. You may need something better in a high compression, high rpm application.