Pertronix flamethrower coils: spec's ?

S

SAXBY2

Senior Member
2 cars with the following:
1. 6 volt 1.5 volt epoxy, stock head, 3076 plugs (proper reach), 12 Ga stranded copper wires from coil terminals, stock ignition.
2. 12 volt 3 ohm epoxy, h.c head. W18 plugs (proper reach), 14 Ga. stranded wires from coil terminals, stock ignition.
Secondary wire from coil to distributor is copper with soldered ends, etc.
What should be the recommended points gap and spark plug gaps for each configuration
based on experience and data based info. ?
 
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The only difference that matters for plug gaps between these two setups is the compression ratio. The point gap is a function of the mechanical limits of the breaker point system, it's not going to shift because of the changes you list here. Stock gap of .020-.022" is fine.

For a strong coil, .035" plug gap should work fine for both cars. Most likely .040" would work in car #1, but if you're not in the mood to experiment then stick with .035.
 
The Pertronix Flamethrower coils are good ones. You can use a larger gap because of the higher output voltage. I concur with what AlexIskai said above.

The only caveat is that there are fake high performance coils on the internet. So be careful and do a thorough research when buying online.
 
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If my points gap closes down below .017 it runs bad . If the points gap widens up ( yes it can widen if the pinch screw is loose ) more than .023 it runs bad . I tried experimenting like with your HC head set up on spark plug gap , I really couldn’t tell any difference from the stock .035 .
 
Big hammer said:
If my points gap closes down below .017 it runs bad . If the points gap widens up ( yes it can widen if the pinch screw is loose ) more than .023 it runs bad . I tried experimenting like with your HC head set up on spark plug gap , I really couldn’t tell any difference from the stock .035 .
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Yeah, people don't really think about the point gap being too wide, but it can happen. There are three main issues with a too-wide point gap.

1. If the gap has widened from what you set it at, the timing has been advanced from what you think it is.

2. A wider gap means the coil circuit is broken earlier, so the coil has less time to saturate, i.e., less dwell. On a Model A this isn't usually a big deal because the system has, if anything, excessive dwell, but at some point you do run out of dwell.

3. A wider gap means the cam follower spends more time being pushed up the lobe of the cam, so the cam is imparting more force pushing it away. At high speeds, the force from the cam can be greater than the opposing force exerted by the flat spring that holds the arm onto the cam. This causes the point arm to bounce, briefly losing contact with the cam and then snapping back onto it. This makes spark timing unpredictable.
 
Pertronix suggests increasing plug gap by .003 +/- due to 40,000 v. Others suggest reducing plug gap to .030 to .032 for hi compression (5.9, 6.1 .etc ) heads. So perhaps .037 is fine for the stock head, .035 for the h.c head to account for higher pressures.
 
SAXBY2 said:
Pertronix suggests increasing plug gap by .003 +/- due to 40,000 v. Others suggest reducing plug gap to .030 to .032 for hi compression (5.9, 6.1 .etc ) heads. So perhaps .037 is fine for the stock head, .035 for the h.c head to account for higher pressures.
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If you end up setting a plug gap that's too wide, you'll just get misfires under load. Doesn't hurt the engine, it'll be obvious what's happening, and you just pull the plugs and regap them a little smaller. Not a big deal.
 
Interesting; why the topic started, some prior topics suggest that a wider plug gap is needed to accommodate the Pertronics higher voltage output for complete combustion though haven't seen empirical data..Idling seems to be an issue with engine misses but not so much high speeds unless some backfiring when on compression. Another side issue is that oil filled coils have failed due to poor quality control, terminals pointing down, etc.
 
SAXBY2 said:
Interesting; why the topic started, some prior topics suggest that a wider plug gap is needed to accommodate the Pertronics higher voltage output for complete combustion though haven't seen empirical data..Idling seems to be an issue with engine misses but not so much high speeds unless some backfiring when on compression. Another side issue is that oil filled coils have failed due to poor quality control, terminals pointing down, etc.
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It is true that a narrower plug gap can lead to incomplete combustion, but as someone else pointed out, the performance gains plateau once you actually get to complete combustion. The only catch is that the coil has to be able to generate enough voltage to jump the air gap. So you want a gap that is large enough to get full combustion and a coil that's powerful enough to jump the gap. Widening the gap from that point gives you no gains, and increasing the coil power from that point also produces no gains.

There's a lot of marketing myth around coils, and the Pertronix ones in particular. I've tested Flamethrower coils against high-quality coils from the '60s. They're fine. They're not stronger. They may be stronger than whatever NAPA sells these days, but fundamentally they work the same as coils always have. Treat them like normal coils and set your gaps normally.
 
Burtz head with stock 6 volt ignition system and I have never had any problems with .035 gap on old stock plugs. Dale
 
Another widely misunderstood fact is that the coil does not continuously produce its advertised output. A coil will produce only the amount of voltage necessary to jump the gap(s) in the secondary circuit. (rotor/cap,wires,plug gaps) and no more. Most of the time the engine runs on a fraction of a coils rated output.
 
nkaminar said:
So if you had a coil that put out 400,000 volts, you could run a plug gap of 3/8 inch?
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Somebody recently was saying that drag racers use absurdly powerful coils and it basically destroys the plugs after one race. Melts the electrodes.
 
The drag racing that's melting the electrodes is top fuel it's because they use quite high amp magnetos it's almost like a dam welder going off when they fire!!!
 
Yes apparently top fuel (nitromethane) burns so slowly that the 60 kV spark has to last through 26° of the piston travel. I can see why the electrodes don’t last long.
 
Turn ratio of the primary to the secondary coils generally sets the high tension output voltage of the coil. Most are around 20K to 20 or 100:1 but it's also affected very much by the input current and the primary voltage spike when the field collapses. The capacitor or condenser capacitance rating is dependent on these factors since the ratio design can be increased or decreased depending on the needed results.

The old Mallory design coils that Ford used on the early V8 vehicles even went so far as to add a ballast resistor even though it was only a 6-volt system. The capacitance of the condenser was a bit higher that what the model A ignition used but they wanted it to give a good spark over a long period of service for best reliability. The coil could only draw 4 amps but it worked very well.
 
When I bought and installed a FS distributor years ago, I also installed a Flame Thrower coil. It failed within a few hundred miles. The Chinese replacement I bought over the counter in some out of the way outback town is still going.
That's my experience.
 
Synchro909 said:
When I bought and installed a FS distributor years ago, I also installed a Flame Thrower coil. It failed within a few hundred miles. The Chinese replacement I bought over the counter in some out of the way outback town is still going.
That's my experience.
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Sorry to hear that. I installed my Pertronix Epoxy filled Flamethrower ~14 years ago, no issues.

Was yours Epoxy or Oil filled.
 
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