Determining proper HEI advance curve for your application

[ddsmith]

This is information derived from The Doctor's Guide to Automotive Ignition. A highly recommended read if you get a chance. Most people that I know are willing to put in a HEI distributor in their vehicles because of the benefits of less maintenance. The problem most of you are aware of is that the distributors came stock in many smog motors and the curves are designed to be used on that type of engine. This is why most people run the distributors at around 12-14 degree advanced for initial timing to get the performance they desire. Well there is more to it than just initial timing and total timing. The advance curve play a big role in the way the truck responds. Having the correct amount of advance at all RPM's is very important to getting the most from your combination. There are some design limits on distributors that may make putting the right curve in your distributor difficult if not impossible. They would include inability to run a large initial timing because of hi compression. One other limitation you may run into is physical limits on advance weights (the amount of mechanical advance available with that distributor). One thing we can determine is the correct advance curve even if we are limited because of the items above. Like we said the distributor has a certain initial timing, mechanical advance and vacuum advance. To accomplish the total advance curve setup and correct vacuum advance will require you to have an adjustable vacuum advance setup(I think that runs about $12), a dial back timing light or degreed harmonic balancer/vibration damper, a tachometer, and eventually an advance curve kit to try and duplicate to the best of your ability the perfect advance curve you will determine.
The process involves treating each 1000 RPM increment as a separate data point. The idea is to advance your distributor in 2 degree increments. Let me just run through one round. You advance the distributor 2 degrees. Disconnect and plug the vacuum advance. Heat up the engine to normal operating temperature. Take the car out onto a section of road that is clear and place it in second gear and cruise at 1000 RPM. Then floor it. Rev the engine to 4500 to 5000 RPM and listen for ping or detonation. If the engine pinged during this retard the timing 2 degrees and do it again. If the engine did not ping during the initial round then advance initial timing 2 degrees and do it again. The thing your trying to pinpoint is the timing at particular RPM the pinging just starts. Let's say for instance that the engine pinged at 1000 RPM. We would verify our results by retarding 2 degrees to verify the pinging no longer exist at 1000 RPM. If no pinging we would advance 2 degrees and reperform. To verify pinging returned. Then we would check what the mechanical advance was at 1000 rpm using the dial back timing light or a normal timing light and the tape. We make a graph of the RPM 1000 - 5000 one axis and the advance degrees on the other axis. We place a dot on the graph that is 1 degree below the reading attained for 1000 RPM. For instance if the timing is 17 degrees at 1000 rpm which is where pinging just starts we mark 16 degree point on the graph that crosses the 1000 rpm line. This gets reperformed for the next point which is 2000 rpm and so on until you've collected all the data needed. Be careful doing the high rpm stuff the purpose is not to blow the engine up. Most advance will probably be in long before 5000 rpm anyways. They say only do 5000 rpm if you can get a good ping doing 4000 rpm testing. So once you've got all your points on the graph plotted your job is to try and duplicate that advance curve in the distributor by different combinations of springs and weights. Hey, I didn't say it was simple, but if you can rip apart one of these trucks and put it back together this shouldn't be to bad. With the mechanical advance as close to perfect as we can get, we'll move on to the vacuum advance. Advance the initial timing in 2 degree increments until the engine is just pinging. With the engine at operating temp adjust the vacuum advance for the largest advance for the least vacuum. Drive the vehicle at normal stop and go traffic and all vacuum levels. The engine will ping worse at light or mid-range throttle and just ping at full throttle. Then make an adjustment to the vacuum advance. A little less advance at the vacuum level. Do the stop and go thing again. The object is to keep doing this until the engine pings equally at all throttle positions. Once that has been accomplished retard timing 3 degrees and lock down the distributor. That is your correct initial timing and vacuum advance for your current setup. Modifications will alter this correct timing and will require this be done again for the ultimate timing. Hope this information has been of use to all of you. It will certainly give you something to occupy your free time. There is also a road dynamometer test accurate to 2% in the book, determining intergear ratios in transmissions, and best shift points for your application. I think I've borrowed enough information from this book so I'll go. Best of luck.

Don