Going from Petronix back to points - capacitor?

[Quickstep192]

I’m switching back to points on my ‘73 Targa with Bosch 0231.169.XXX distributor and CDI.

I can’t remember what all I removed when I installed the Pertronix.

Would there have been a capacitor on the distributor?

Thanks for any help

[rwest]

No, the points act more like an on off switch, so the tiny round silver tube on old distributors is not used.

[HarryD]

You need a capacitor with points if your do not have a CDI to help reduce the chance you will rapidly burn your points. Pertronix does not require a capacitor.

[rwest]

Originally Posted by HarryD

You need a capacitor with points to help reduce the chance you will rapidly burn your points. Pertronix does not require a capacitor.

Harry,

He has a CDI, does it still need a capacitor? My car didn’t have one.

[HarryD]

I stand corrected.

[mysocal911]

Originally Posted by HarryD

You need a capacitor with points if your do not have a CDI to help reduce the chance you will rapidly burn your points. Pertronix does not require a capacitor.

That's a small part of the reason. The real reason is that without a capacitor, the rise time of the coil primary voltage is too short when the points open,
resulting in a poor spark at the secondary winding to the distributor.

[HarryD]

Originally Posted by mysocal911

That's a small part of the reason. The real reason is that without a capacitor, the rise time of the coil primary voltage is too short when the points open,
resulting in a poor spark at the secondary winding to the distributor.

We are really saying the same thing as the consequence of not getting the energy to the coil, is an arc across the points.

The capacitor(condenser) absorbs the excess energy then releases this energy to the coil.

[Quickstep192]

It doesn’t take a lot, but I’m confused.

No capacitor as long as I have CDI, correct?

[HarryD]

Originally Posted by Quickstep192

It doesn’t take a lot, but I’m confused.

No capacitor as long as I have CDI, correct?

yep

[mysocal911]

Originally Posted by HarryD

We are really saying the same thing as the consequence of not getting the energy to the coil, is an arc across the points.

The capacitor(condenser) absorbs the excess energy then releases this energy to the coil.

Technically, it's controls (defines) the dv/dt (rate of change of the primary voltage) based on the primary inductance.

[mysocal911]

Originally Posted by Quickstep192

It doesn’t take a lot, but I’m confused.

No capacitor as long as I have CDI, correct?

It doesn't matter!

[HarryD]

Originally Posted by mysocal911

Technically, it's controls (defines) the dv/dt (rate of change of the primary voltage) based on the primary inductance.

Errrr. Ummmm. Let’s not bring calculus into this!

[PeteKz]

Re the purpose of the condenser (capacitor): it's a moot argument. Back when Charles Kettering first invented the "Kettering ignition system" he found that the points were burning too quickly, due to the arcing across the gap as the points opened. Knowing a few things about electricity, he added a capacitor, sized to absorb the current across the points until they opened far enough so that the spark would no longer jump and burn the points. dv/dt had nothing to do with this innovation. It was a practical adaptation to stop points from burning.

It also serendipitoulsy turned out that adding the capacitor created a resonant RCL circuit that increased the peak voltage of the coil. Win-win. Then comes the post hoc explanation of dv/dt.

Anyway, if you have the CDI box, the points don't carry significant current, so you don't need the capacitor. They only signal the CDI box.

[mysocal911]

Originally Posted by PeteKz

Re the purpose of the condenser (capacitor): it's a moot argument. Back when Charles Kettering first invented the "Kettering ignition system" he found that the points were burning too quickly, due to the arcing across the gap as the points opened. Knowing a few things about electricity, he added a capacitor, sized to absorb the current across the points until they opened far enough so that the spark would no longer jump and burn the points. dv/dt had nothing to do with this innovation. It was a practical adaptation to stop points from burning.

It also serendipitoulsy turned out that adding the capacitor created a resonant RCL circuit that increased the peak voltage of the coil. Win-win. Then comes the post hoc explanation of dv/dt.

Anyway, if you have the CDI box, the points don't carry significant current, so you don't need the capacitor. They only signal the CDI box.

You obviously don't understand the simple electronics circuit analysis of what happens when power is removed, e.g. points open,
from an inductor (ignition coil) without a capacitor or a voltage limiting device. The change in voltage versus time (dv/dt) becomes basically uncontrolled
and very high, potentially damaging the coil. That's the key purpose of the capacitor in the distributor of a simple points ignition system.
And as a result of the controlled dv/dt, it also reduces the burning of the points (arcing).

It's that simple!

[PeteKz]

You've got the actual development backwards.

[yelcab1]

@quickstep192, why are you going back to points away from Pertronix? Is it originality or something else?

[Quickstep192]

Originally Posted by yelcab1

@quickstep192, why are you going back to points away from Pertronix? Is it originality or something else?

Mostly just tinkering and trying to go back to an original base point. I’m chasing a mid-RPM miss.

I originally put in the Pertronix before I learned that the points in a CDI system are just a trigger.


I’ll see how it does with points and then maybe switch back.