Secret of Speed Article about distributor rotors "UPDATE"

[40 Deluxe]

Originally Posted by CHARLIE YAPP

I don't know who Delux 40 is but I would listen to him (her?).
He (her) is correct. Why most people's identities hidden in this site/thing??
You want HP and stability?
There are better ways to spend your time than playing with your rotor.
Joop asked me about this. . .guess I wasn't clear enough. Was in the throughs of publishing deadlines for "Secrets" . . . 29 years and still going!! Send your questions into Secrets Magazine and let a few thousand more people from around the world take a crack at the answer. Four-Titude to you and yours.

Hi Charlie, I'm a 75 yr. old retiree; grew up on a farm watching Dad and uncles doing their own repairs and building various one of a kind machines as most farmers do/did). Most of my working life (37+ years) was spent as a mechanic; 35 years as an ASE Certified Master Auto Technician. Along the way I added Master Heavy Truck Technician, Advanced Gas Engine Diagnostics, Advanced Diesel Engine Diagnostics and Natural Gas Engines. Passing these tests required an understanding of electrical theory as it relates to starting and charging systems, various ignition systems and modern electronics. I'm not trying to boast, but you asked who I am. The incentive to acquire these certifications, besides the knowledge gained, was an extra $1,000 a month on my paycheck.

[40 Deluxe]

Originally Posted by 30 Closed Cab PU

I thought one of the reasons all rotor gaps to each other should match and all Plug gaps should match each other is for best idling. If not consistent between cylinders, the amount of time for spark to occur when referenced to the points opening would vary causing even idlibng to suffer.


Please correct me if this is wrong.

Well, you're not giving electricity the credit it deserves! Spark generation occurs so rapidly that any variation in rotor gap is totally meaningless. Any rough idling would have other causes, like worn distributor bushings, uneven wear on the point cam, worn distributor drive gears, or worn timing gears (plus carb and valve issues).
If you can, get your mechanic to let you watch ignition traces on his scope. Or check out an evening class at a community college. Notice the time it takes for each ignition event.

[30 Closed Cab PU]

Originally Posted by 40 Deluxe

Well, you're not giving electricity the credit it deserves! Spark generation occurs so rapidly that any variation in rotor gap is totally meaningless. Any rough idling would have other causes, like worn distributor bushings, uneven wear on the point cam, worn distributor drive gears, or worn timing gears (plus carb and valve issues).
If you can, get your mechanic to let you watch ignition traces on his scope. Or check out an evening class at a community college. Notice the time it takes for each ignition event.

Thanks for the info.

[Terry, NJ]

40 Deluxe, Can you explain to me exactly how the points work. For years, I believed that the spark was generated when the points closed and contacted. For the last few years, I've rethought this, and believed that the spark occurred when the points opened. There has to be buildup of voltage in the coil and the capacitor This is what the dwell is for, to give it a millisecond to build up the voltage. Am I correct?
Terry

[30 Closed Cab PU]

40 Deluxe - After re-reading these posts and looking elsewhere, one question. why is this gap checked/adjusted?


A larger gap creates a higher voltage before spark occurs, just do not understand how the gaps (Plug + rotor) affects combustion. Am a little OCD on details.


Thanks

[40 Deluxe]

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Originally Posted by Terry, NJ

40 Deluxe, Can you explain to me exactly how the points work. For years, I believed that the spark was generated when the points closed and contacted. For the last few years, I've rethought this, and believed that the spark occurred when the points opened. There has to be buildup of voltage in the coil and the capacitor This is what the dwell is for, to give it a millisecond to build up the voltage. Am I correct?
Terry

You're right, the spark occurs when the points open. That's why you set the timing so the points are just starting to open. With the points closed, current is flowing through the primary windings on the coil, which builds up a magnetic field. When the points open, current flow stops and the magnetic field collapses, which induces high voltage in the secondary windings. When this secondary voltage builds up high enough to jump the plug gap, the spark occurs. The capacitor/condenser doesn't build voltage until the points open. It is kind of a buffer that absorbs current flow just as the points start to open so a spark doesn't jump across the points.
Dwell is the amount of time, expressed in degrees of rotation, that the points are closed.

[40 Deluxe]

Originally Posted by 30 Closed Cab PU

40 Deluxe - After re-reading these posts and looking elsewhere, one question. why is this gap checked/adjusted?


A larger gap creates a higher voltage before spark occurs, just do not understand how the gaps (Plug + rotor) affects combustion. Am a little OCD on details.


Thanks

Rotor gap is one of those things considered a maintenance item. High voltage slowly erodes the rotor tip and corresponding contact in the cap. So periodically we replace the rotor and/or cap. If the gap gets excessively large, firing voltage may rise to the extent that the spark finds an easier path to ground than through the wires and cap, or the coil fails. The plug gap is far more the critical of the two. The amount of gap is always a compromise, especially in cars like an A or V8. Compression pressure raises the voltage required to jump the gap and fire the mixture. If the gap is too large for the coil capacity, you get a misfire. (Or in rare cases, the rotor tip has been allowed to burn away and this causes a misfire). So plugs were kept around .025" to .035".
When emissions standards came about, fuel mixtures were leaned down considerably. But it took a larger gap to fire this lean mixture (fewer fuel molecules within the plug gap, thus a larger gap was needed, like .045"-.060"). The larger gap required higher secondary voltage so better coils were needed. This lead to thicker plug wires, bigger caps, etc. The higher current is harder on points, plus the ignition system had to last longer between 'tune-ups'. Thus the changeover to electronic ignitions.

[30 Closed Cab PU]

Originally Posted by 40 Deluxe

Rotor gap is one of those things considered a maintenance item. High voltage slowly erodes the rotor tip and corresponding contact in the cap. So periodically we replace the rotor and/or cap. If the gap gets excessively large, firing voltage may rise to the extent that the spark finds an easier path to ground than through the wires and cap, or the coil fails. The plug gap is far more the critical of the two. The amount of gap is always a compromise, especially in cars like an A or V8. Compression pressure raises the voltage required to jump the gap and fire the mixture. If the gap is too large for the coil capacity, you get a misfire. (Or in rare cases, the rotor tip has been allowed to burn away and this causes a misfire). So plugs were kept around .025" to .035".
When emissions standards came about, fuel mixtures were leaned down considerably. But it took a larger gap to fire this lean mixture (fewer fuel molecules within the plug gap, thus a larger gap was needed, like .045"-.060"). The larger gap required higher secondary voltage so better coils were needed. This lead to thicker plug wires, bigger caps, etc. The higher current is harder on points, plus the ignition system had to last longer between 'tune-ups'. Thus the changeover to electronic ignitions.

Thanks for the taking the time to provide the details. I do have an electronics background, but not automotive theory. Knowing why I am doing something instead of blindly following a procedure reinforces the info, and helps me remember the correct way.

[40 Deluxe]

Originally Posted by 30 Closed Cab PU

Thanks for the taking the time to provide the details. I do have an electronics background, but not automotive theory. Knowing why I am doing something instead of blindly following a procedure reinforces the info, and helps me remember the correct way.

With your electronics background, you might like searching for "automotive ignition scope patterns". One sample pattern shows spark duration to be one millisecond. The rotor in an idling Model A engine won't move very far in one millisecond, so I don't think rotor gap would affect ignition timing, nor would even a big variation in rotor gap affect timing to the slightest degree. To illustrate how quickly ignition events take place, I drag race a Mercury Cougar with a 289 engine, single coil ignition. I shift around 7500 RPM, which calculates out to 500 sparks needed per second, or 5 sparks every 1/100 sec.!

[Terry, NJ]

Thanks For taking the time to explain it! It is much appreciated.
Terry

Originally Posted by 40 Deluxe

You're right, the spark occurs when the points open. That's why you set the timing so the points are just starting to open. With the points closed, current is flowing through the primary windings on the coil, which builds up a magnetic field. When the points open, current flow stops and the magnetic field collapses, which induces high voltage in the secondary windings. When this secondary voltage builds up high enough to jump the plug gap, the spark occurs. The capacitor/condenser doesn't build voltage until the points open. It is kind of a buffer that absorbs current flow just as the points start to open so a spark doesn't jump across the points.
Dwell is the amount of time, expressed in degrees of rotation, that the points are closed.

[old31]

Thank you 40 for your time and expertise.

[Joop]

Thanks for all the comments and information on this
I learned a lot. Joop