FBO's black HEI ignition box

Plug gaps: engine that runs perfectly from 850 rpm upto red line with plug gap x , the gap is probably correct
making the gap larger will not make more power, it just reduces the life of cap and leads and puts the coil under more stress.

you need just enough plug gap to suit cam, CR, chamber shape etc and of course the right heat range of plugs,
if you are still running what the book said was good for leaded 1960s petrol....is the heat range right for modern oxygenated ethanol blend in your ambient temperture, fuel blends differ with the seasons and temperature and you want your car to work in winter and summer, OR now that you have stuck in a wild cam and upped the CR you need different plugs.

engine that doesn't run cleanly from 850 to redline with plug gap x might benfit from a wider plug gap y, IF and only if the coil and leads will support it

if you do try plug gap y, and the engine runs cleanly all the way to red line there has to be somwhere in the torque and BHP curve where both are higher than they were previoulsy becasue the motor now runs properly, and therfore you make more power.

indexing the plugs to face the hot exhaust valve Might make more power becasue you are sparking into the hottest part of the mixture in the chamber. The heat has probably turned the mixture into a plasma under the exhaust valve, electrons already ripped off atoms and molecules, ready to carry current, so the spark travels much easier. as such the initial burn will be more of a homogenous thing, meaning it probably spreads across the piston crown more evenly. that has to produce a tiny increase in torque and hence more power somewhere in the rpm range.
indexing may make a plug fire that previously did not becasue the stuff between the electrodes is easier to spark across, thats the gist of it. face the cool inlet....no spark at a specific rpm, face the hot exhust valve....spark at that rpm and that RPM will be when cylinder filling is best...i.e on cam or worst on cruise.

the main point of much of the talk about igntion systems isn't about making more power its about getting the power you are owed, from your motor you built, becasue the igntion system you currently have could be marginal given the changes you made.
None of us have chrylser's engineerng department to design an ideal curve for our hotted up motor runnning modern petrol, so its always going to be best efforts or pay someone else.
and paying someon else is often fraught with danger, as igntion system components are advertised with scant regard for the truth or indeed the science.
don't be paying big $$$ for a coil which is red, and has a lightning bolt sticker on it, but is not really any better than an OEM piece. its an 80KV coil.. no its not... thats a lie, its a coil that has insulation in it that will not break down at 80KV. that 80KV sales headline, has noting to do with its real world performance.

that flat spot or hesitation you have been chaseing for years is just as likley to be timing and an inability to light off the mixture at a specific rpm and throttle opening level.... than it is jetting.

sort igntion first then mess with the carb.

its more likley to be a lack of inital timing and an advance curve which was designed for 1968 petrol not 2022 petrol.


Olds or pontiac told HEI equipped customers to set plug gaps at 80 or 90 thou that lasted for only a short period due to the warrnty issues, plugs, leads, caps. This would be on a low CR car motor of the 70s with a massively restrictive exhaust, they went back to the 45 thou specified orginally pretty quickly.
perfect for the coil cap and leads specified for the original hei spec. a perfect example of if more was good even more will be better and it was quickly proved not to be the case by real world customers.
but it does show how robust the coil was if it could manage a 90 thou gap and still spark enough of the time to keep the customer happy on the way home from the dealer.


Dave