HEI (high energy ignition) distributor dwell settings
I’ve been following the problems and solutions regarding the skipping 350in the ‘55 Chevrolet. I have a similar problem with my ’80 Pontiac equipped with a Pontiac engine. I’ve checked and changed everything under the hood except the windshield washer fluid, including the distributor once and the module twice. I’m afraid it is a bad cam lobe since the compression numbers are good but the skip isn’t consistent.
The latest item in the September edition got me to thinking about how to read the dwell in an HEI distributor and what should the numbers be. After all, the components of an HEI distributor do the same job as those in a points distributor but they do it electronically. Why shouldn’t there be dwell time to charge the coil? My Pontiac shop manuals don’t specify the dwell.
I discussed the question with a friend who, as usual, thought I was crazy but it prompted him to do some research. He located an article on the Internet concerning GM HEI distributors. The article states the dwell at idle should be10 to 15 degrees. (Mine is 5 degrees if the meter is correct.) Dwell should increase as the rpm increases, so there is 30 to 35degrees @ 2500 rpm. (The dwell for my Pontiac increases but I’ve got to compare it to the numbers provided.) The article mentions several things that affect dwell with the module being center stage. The article states aftermarket modules sometimes don’t provide adequate dwell time at idle or at high rpm.
I checked the dwell of the GM HEI in my Caballero equipped with a 305engine and a rebuilt distributor. It too read 5 degrees of dwell. Now, do I have a bad dwell meter or two bad modules? Dwell meters are hard to find but I guess I’ll have to find one and I need one with a higher rpm scale.
One of the module’s functions is electronic control of the dwell circuit. The reason for adjusting the gap in the old-style points was to define the dwell time, which was the length of time that the points were closed, measured indegrees of distributor rotation. Most of the time the setting was 30 to 35 degrees all in.
Dwell is the amount of time the primary circuit is complete, which was originally the time the points were closed on the old breaker point systems. Longer dwell times are used with inductive ignitions to fully saturate the coil. This is important at higher engine speeds because there is less time for the coil to charge.
It’s hard to pinpoint exactly what might be wrong, though I would not suspect your dwell meter in this instance. But here are some possibilities: Some aftermarket replacement modules do not provide the proper range of dwell. They often have an excessive amount at idle and provide too little at increased rpm. This can result in misfiring and poor performance at the top end because coil saturation is not adequate. On the other hand, if the dwell becomes faulty on the module, the engine will have a rough idle and misfire, hesitate under load, and cause poor performance. The only recourse if the dwell is incorrect is to replace the module. But I would first consult the manufacturer and ask them which module the distributor should have.
Another possible problem is the screws holding the rotor in place. They should be plastic because metal ones can arc to ground through the distributor shaft. This causes misfiring and can eventually damage the centrifugal advance system.
Heat buildup in the module is another possibility for your problem. The white grease that you smear on the bottom of the module is special and is designed to enhance the transfer of heat to the distributor body. Do not use ordinary dielectric grease for this purpose because it will act as an insulator and shorten the life of the module.
As for your Caballero, it may be that the dwell settings are adjusted to work with more modern fuel delivery systems that are designed to run leaner. One of the main reasons the manufacturers went to HEI ignition was to allow them to run their engines leaner in order to meet smog and fuel consumption mandates. Doing so requires a hotter spark, and can also mean that the HEI system isn’t effective above 5000 rpm. There are aftermarket systems intended for hot rodders that overcome this problem.