Dual point dists did not provide a secondary spark. There were a couple of types but the basic operation was to extend dwell [ length of time coil received current ] because with an 8 cyl engine, plus high rpm, there was insufficient time to fully saturate the coil with single points; result was misfire.
Hei vs points.
- Thread starter dartfreak75
- Start date
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SS Lancer
Well-Known Member
Nothing wrong with points system they are simple and basically require a little maintenance periodically thats all !
I can appreciate your attempt at keeping car original versus the norm of dropping a Chinese Billet MSD in like most do .
A well done video showing how to screw up the timing advance so the car goes slower in the quarter mile and make the engine ping on a long trip.
Lets look at what we know.
(All timing and rpm values in this post will be in terms of crank degrees and rpms.)
Example he starts with is a smallblock magnetic pickup distributor. So its smog era.
Its intended for an early Barracuda. So no smog and possibly hot rodded engine.
On the distributor machine, the advance maxed out with 24* at 4000 rpm.
But he only took that one measurement.
We don't know even what rpm it starts advancing.
Finally he shows a slot plate stamped 15, indicating 30*.
After his modifications, which seem to include installing the 18 degree slot plate.
On the distributor machine, the advance maxed out with 12* at 1500 rpm.
Finally he tells us his goal is to install for 32* maximum, and 20* at idle.
So lets plot the before and after info on a graph using 32* as the reference.
Because this distributor started with its original springs we can sketch in a guestimate timing curve.
We can do that because idle speeds around 750 rpm were common in the early 70s and that typically LA engine combustion efficiency builds above 1400 rpm.
Likewise with just one sping stretched, and assuming the plate with shorter slots was installed, the new timing curve must actually look something like the red dashed line.
Would that work?
Yes.
except with vac advance its pretty much guarenteed to ping at part to mid throttle between 1800 and 3000 rpm
and
if you dont mind losing timing at high rpm.
and
if you're OK with timing at idle that changes with small changes in engine rpm.
(All timing and rpm values in this post will be in terms of crank degrees and rpms.)
Example he starts with is a smallblock magnetic pickup distributor. So its smog era.
Its intended for an early Barracuda. So no smog and possibly hot rodded engine.
On the distributor machine, the advance maxed out with 24* at 4000 rpm.
But he only took that one measurement.
We don't know even what rpm it starts advancing.
Finally he shows a slot plate stamped 15, indicating 30*.
After his modifications, which seem to include installing the 18 degree slot plate.
On the distributor machine, the advance maxed out with 12* at 1500 rpm.
Finally he tells us his goal is to install for 32* maximum, and 20* at idle.
So lets plot the before and after info on a graph using 32* as the reference.
Because this distributor started with its original springs we can sketch in a guestimate timing curve.
We can do that because idle speeds around 750 rpm were common in the early 70s and that typically LA engine combustion efficiency builds above 1400 rpm.
Likewise with just one sping stretched, and assuming the plate with shorter slots was installed, the new timing curve must actually look something like the red dashed line.
Would that work?
Yes.
except with vac advance its pretty much guarenteed to ping at part to mid throttle between 1800 and 3000 rpm
and
if you dont mind losing timing at high rpm.
and
if you're OK with timing at idle that changes with small changes in engine rpm.
Last edited:
BillGrissom
Well-Known Member
I recall a youtube which does more exacting tests, of points, HEI 4-pin, and GM LS coils, I recall. He videos in the dark and such. The LS coil sparks hardest. The advantage is you aren't sharing a coil between cylinders, which definitely helps at high rpm so more time to charge-up the coil. As shown, it is easy to jump a spark in the air (inches), and even easier in a vacuum (yards), and not so easy at high cylinder pressures, which is worst at WOT with a turbocharger. I doubt that with points you could set your spark gap at 60 mil, like in a modern engine, without misfiring when trying to accelerate on an on-ramp.
goldduster318
Overzealous Car Modifier
Yeah, that's true, it doesn't even have to be a system as new as LS coils. My sister had a 1994 Oldsmobile Cutlass Supreme with a 3.1L, this engine calls for 0.060" plug gap when new. 115k on the original spark plugs (don't ask why it went that long), I noticed it had a slight misfire at idle, but it seemed to drive fine, so I go to change the plugs (still kept the original wires!) and they were at 0.110" gap. This is a car with a crank trigger and 3 coils where it sparks two cylinders at a time. I also pulled an even worse plug out of a 2003 Honda Accord and that only had very slight misfiring.
When I did my EFI swap, I did install the LS round coils, the spark is certainly excellent. I don't know how much of the extra idle smoothness is sequential injection but I'm sure the ignition helped out a lot.
BillGrissom
Well-Known Member
My 1996 Plymouth 2.4L has the same distributor-less coil-pack system, termed "wasted-spark" since 2 cyl share a coil. The factory spec was 60 mil spark gap, but after 100K miles it started misfiring bad when going high-throttle at low speed, like on an on-ramp. Could be the PCM was damaged slightly after a front-end side hit which knocked it loose (ignition circuit is in PCM). I lowered the gap to 50 mil, based on a TSB for the same engine w/ turbo-charger, and no more misfires. A turbo boosts the cylinder pressure even higher, making sparking harder.
goldduster318
Overzealous Car Modifier
Yeah there are kind of a few variations, the cars like your 2.4 and a lot of Chrysler cars have the ignitor in the PCM, the older GM DIS/EDIS cars like the Cutlass, the ignitor is in the base plate that the coil bank mounts to, and then the stuff like the LS coils, each coil has a built in ignitor so the computer just uses low current logic to turn the coils on and off, because it has a separate power and ground for the high current side.
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