To pick up the thread again, its Saturday and I had a chance to test some of your suggestions. The most promising seems to be the theory about low voltage between ballast resister and coil but I don't know if I did this right. I took the meter and ran one wire to the plus side of the coil and one to the top far side of the 4 prong Ballast resister (any other arrangement gave me negative voltage) . This gave me about a 6.5 volt reading at high idle. I'm thinking I should have 12-13 volts (the battery has about 14 volts across the terminals). Is that the right way to test it?
If we're driving with low voltage to coil then we are not getting the hot spark we need at the plugs under load (thinking out loud). If I hot wire from the plus on battery to plus on coil I should have full voltage. When I ran the meter from battery to coil directly I got 5 volts! This makes me think I am losing a lot of voltage through the harness. Comments?
KJSheridan and son

Correct.

The harness, the bulkhead connectors, etc.... will cause some resistance and it all builds up by the time you get to the coil. When you have the key in start position you get a full 12 volts, but once it is running it drops back down.

With a 3 ohm coil you can bypass the resistor and use the full 12 volts instead.
The GM HEI is also a full 12 volts as well as the Pertronix Ignition systems, Ignitor I, II and III.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

You are supposed to drop a lot of voltage across the ballast resistor. One of the main features of the stock ignition system is that it bypasses the ballast resistor during cranking to give full voltage to the coil while the battery voltage is being pulled down by the starter. It's better system the old cars which used full battery voltage at all times and suffered low ignition voltage during cranking. HEI is a better system yet because it actively controls the coil dwell instead of passively controlling the coil current.

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Joshua

So then should I bypass the ballast resister altogether? I have done that in the past on other cars when it broke and I had to start them but I always thought that that was bad for other components on the car not to have the resister when running. It seems you have bypassed and put another resister in its place.
KJSheridan

Only if you purchase a 3 ohm coil. Pertronix sells a 3 ohm stock style canister coil in black or chrome and the HEI version like I used.

If you remove the resistor and are still using a stock coil, it will burn out the coil.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Thanks Ted, I am starting to get it! If I bypass the resister and use that Petronix 3 ohm coil then thats all I have to change and I should get significantly higher voltage through the coil without damaging the coil. That seems like an easy thing to try. I appreciate the help!
KJ and son

P.S. It won't do anything to the electronic ignition, right?

Nope..........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Correct, your ignition will be fine.

I have been running this setup with a Orange MOPAR ECU for a few years, over 50,000 miles on my 74 Dart. I have run both types of 3 ohm coils (stock canister type and HEI type on the Dart and my wife's 72 Imperial) which get a full 12 volts or more. I also like running NGK platinum plugs gaped at .045 instead of .035. It runs very smooth with this setup. It can idle all day with out missing a beat. I often get caught in rush hour traffic on my commutes and may be creeping along or stopped for 2.5 hours or more.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

You anticipated my next question which was how big to go on the plug gap. I will try.045 too.
Thanks and now to work on it!
KJSheridan

To give an update on the 74 Duster Gas Mileage project, we installed the Pertronix Flame Thrower Coil this weekend and there was a noticeable change in the car. It was smooth before but seems even smoother now and the idle seems lower. I was expecting maybe the idle to go up but the opposite occurred. We ran out of sunlight before we could open up the plug gaps but that is next. Someone should have reminded me how hard it is to get the coil out. They install the coil first and then assemble the car around it! Also the Pertronix is just slightly fatter so the stock clamp won't work with the standard tightening bolt. We made it work but I was surprised that the clamp had so little room for adjustment.
Much thanks for all your advice so far. Time to check the mileage!
KJ Sheridan

Yes, that is a bugger of a place to mount and cook the coil.
Mounting it up on the fender allows it to cool much better and will help it to last longer.

When running the stock style canister type coil I ordered the Pertronix mounting bracket. You can also use other after market chrome brackets.
I used one for quite awhile then switched to the HEI style 60,000 volt coil.
My SL6 runs super smooth never missing a beat especially at a idle for hours at a time in my evening commutes.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG