Dual-point vs. Single-point distributors - pros and cons?

nice documnet on CIS/HEI as used on Jag/Triumph has section explaining why HEI is pretty good and why it does have some retard charactersistics as the rpm increases

obvioulsy this is just one mans view BUT if you check out the rest of his website link in the documnet, he does appear to be a cleaver chap. His spark energey measuring device is quite cool... and his CDI verse magentic coil igntion comparison is also good..... even if all you take away is an appreciation of the Bulls4it spouted in aftermarket igntion advertising its all a good read.

http://nebula.wsimg.com/5956cdbfbf7f...&alloworigin=1

ah link didn't work try this

HEI VERSUS STANDARD IGNITION & CUSTOMISING A RELUCTOR-HEI SYSTEM TO A TR4A

off target for the OP but HEI came up again..

Jags used a mopar LIKE pickup in a small cap distributor and a GM HEI module implemented with a nod towards the orginal Motorla application paper for the chip they made for the GM Delco Module

well i found it interesting


is there any advantage of HEI over single pont distributor?. Yes 50 years of technical advances....

chrysler removed the mechanical switch but did't increas current handling with a modern coil
GM changed the coil...... and that is where the benefit comes from.

HEI has faster switching time, becasue there is no arcing across the switch and no condencer suckig up some charge, at coil switch off.
HEI uses a coil of 0.5 ohm primary, and runs at 12 -14 volts and at higher current . It does its damndest to maintain 6 amps into the coil at mid to high rpm.
a points coil runs at 8 volts and the coil resistance and ballast resistor limit the current into the coil to 3 amps so that the points don't burn out.

the spark energy available with HEI dicates how wide you can go on plug gap, HEI specified a 45 thou gap for a big cap set up.
on a small cap set up run 35 thou for anything upt to 12:1 static 8:1 dynamic CR

more current = more intense magnetic feild in and around the coil. the coil is an laminate core e coil so the field leakage "around the coil" is lower and it runs cooler due to eddy current supression characteristics of the core design.

Faster switching means faster collapse of field which means more spark energy and higher voltage spark.
wider plug gap increases your ability to igntite a marginal mixture so a lean flat spot with points is no longer as flat with HEI. chaseing that progression flat spot by dicking with your carb is no longer necessary bcause its no longer there.....

your igntion spark energy is dicated by the coil and its primary resistance
the coils ability to create a high energy spark is dicated by the winding ratio and the current that flows in the primary
the effectiveness of sparking is dicated by how quickly you can get the magentic field to collapse and that is dicated by how abruptly you can switch off the coil

limits are therfore dicated by coil and the switch you use. provided all else like plug wires and cap are good.
the switch being points limits you to 3 amps or less and the current decreases as the rpm increases.
the switch being HEI limits current at low rpm to avoid coil over heating and burn out. and then does it best to mainatin 6 amps into the coil up to an rpm limit dictated by the module and how many cylinders you have. worst case with a modern module and a v8 is somewhere close to 6000 rpm and many do better higher for a 6 cylinder, and higher for a Special more expensive module when matched to a coil from the same manufacturer

HEI best bang for buck for a street driven -- quick squirt down the track at the weekend--- car.

lack of any other facilities, rev limit, start retard, etc means its not really a race set up.
but it kicks a certain amount of *** when compared with CDI. in fact there is little difference in spark energy, they both achive the same kind of output in different ways

when stand alone HEI E coil can be had for $25. a module can be had for £25 and a standard mopar electronic dizzy can trigger it.... you can't really go wrong... but its not for everyone



dave