Pertronix 1.5 vs 3.0 OHM coil and Tach

[Arne]

So unlike the poor, snowed-in folks on the East coast, I am able to drive my car today here in Western Oregon. And since this thread brought this back to mind, I spent some time experimenting with coils and such.

The good part is that - after doing a little work to maximize voltage (minimum voltage drop) to the Ignitor (the red wire), I have finally been able to make it run correctly with the 1.5Ω coil/resistor combination. So Zforce's thought about low voltage being a cause appears to be at least partially confirmed.

I consider this to be a good interim step. At least this way there's no concern about damaging various components such as the coil or the tach.

While doing this, I got to thinking about things like the resistor and the tach, and I'm not certain about a few things. It appears to me that the four wire tachs (which measure at an inductive loop) probably shouldn't have any problems when run w/o resistor. Indeed, my experience would seem to confirm that. I don't know enough about how the three wire tachs work to say whether the increased voltage running to them might hurt or not. Barring further info, I think I'd err on the side of caution if I had a car with a three wire tach and stay with the 1.5Ω coil and resistor.

After getting it working with the stock coil and resistor, I then re-installed my 3Ω Pertronix coil and bypassed the resistor. And even with the higher voltage to the Ignitor, this combo again failed to run under load at higher RPM. Acts like a rev-limiter at about 4000.

Now I will concede that it is possible that I have a bum coil. But this made me ask myself why we even use the resistors anyway? I've always heard that they were only there to improve the lifespan of the points. So - no points, no need for a resistor?

The instructions with the Pertronix coil say that the 3.0Ω coil can be used on 4 & 6 cylinder engines for "normal" use w/o a resistor. The 3.0Ω is not recommended for "racing" use on a 4 or 6 cylinder. The 1.5Ω coil should be used with resistor for "normal" use, and w/o for "racing".

The makes me think that neither the 3.0Ω alone, or the 1.5Ω w/resistor are going to perform well at high RPM. And unlike the pushrod British 6 cylinder engines that are a big part of their market, I'd submit that even a stock L24 is closer to a "racing" application than it is to "normal".

Considering all of this, both to ensure long-term component reliability as well as giving multiple options, if I do it again I think I'll buy a 1.5Ω coil and use it either with or without the resistor, as seems appropriate.

[jwgarvey]

Considering all of this, both to ensure long-term component reliability as well as giving multiple options, if I do it again I think I'll buy a 1.5Ω coil and use it either with or without the resistor, as seems appropriate.

It is always nice to get your input Arne. Along with your experience and the information recieved from multiple threads regarding this topic, I have decided to use the 1.5 coil w/o ballast.

[alhbln]

...

While doing this, I got to thinking about things like the resistor and the tach, and I'm not certain about a few things. It appears to me that the four wire tachs (which measure at an inductive loop) probably shouldn't have any problems when run w/o resistor. Indeed, my experience would seem to confirm that. I don't know enough about how the three wire tachs work to say whether the increased voltage running to them might hurt or not. Barring further info, I think I'd err on the side of caution if I had a car with a three wire tach and stay with the 1.5Ω coil and resistor.

...

Now I will concede that it is possible that I have a bum coil. But this made me ask myself why we even use the resistors anyway? I've always heard that they were only there to improve the lifespan of the points. So - no points, no need for a resistor?

The three wire tach measures the inductive kickback from the coil, which is around 100-200 volts, so a different resistor on the primary side should be no problem at all.

The resistor is required to limit the current going to the coil for creating the magnetic field. Depending on the coils primary wiring and resistance, an external resistor might be required to limit the current. Some coils (such as the Bosch blue coil) already contain the resistor as part of the coil circuit.

In a classic points setup, the resistor is bridged while starting, to still get the full voltage at the coil even when the starter is engaged. As soon as the starter is disengaged and the voltage supply is stable again, the coil is fed via the resistor.

A typical coil in a points setup has a maximum primary current of 2-4 Ampere. A higher current will saturate the coils windings, the additional current is then just turned into heat (overheating the coil in the mid to long run). The job of the resistor is to keep the current just below the saturation.

As a rule of thumb, if a coil is too hot to touch (e.g. >70C/150F), you are running too much current through the coil.

The condenser increases the lifespan of the points by eliminating the high voltage of the coil primary kickback, which would otherwise create a discharge arc between the points when the points open, quickly eroding the points contacts. In a transistorized setup such as with the Pertronix Ignitor, you do not need a condenser as there are no points to protect anymore.

Edited February 8, 2010 by alhbln

[akorna]

On the issue of low voltage to the coil - when I measured system voltages I found that with the engine off and ignition in run - there was about a 0.7v loss through the ignition switch to the resistor (black wire). Not a huge drop, but you could install a relay go direct from the battery - would also reduce current on the ignition switch - right?

[alhbln]

Correct, but with a four wire tacho setup ('72 and pre) you would lose the tach output, as the black wire also runs through the induction pickup of the tach. 0,7V is not so bad as a loss, though.

[akorna]

alhbln,

The heavy gauge black or black/white wire goes from the ignition to the resistor. Inserting a relay there would not influence the tach operation.

[alhbln]

Just had a look at the circuit, that would work indeed!