Technical origins of "long tip" and "short tip" distributor rotors?

[Joe C.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

While the following data may not be exact for a specific engine, it does clearly show the trends that all engines follow. Further, remember that leaner mixtures require higher firing voltages than richer mixtures.

The calculated values are for air only and do not include fuel ratio effects. Nor do they include plug electrode materials, tip geometry, and accumulated deposits. Given a static compression ratio of 11.4 and a trapped (dynamic) compression ratio of 9.0, inlet temperature of 100F and inlet pressure of 1 Bar:

If the plug fires at -45 deg BTC the cylinder Temp will be 399F , Press will be 66psi
Required firing voltages at various gaps: .050" = 12.1kV, .080" = 18.1kV, .100" =22.1kV

If the plug fires at -35 deg BTC, the cylinder Temp will be 496F, Press will be 97psi
Required firing voltages at various gaps: .050" = 15.3kV, .080" = 23.2kV, .100" =28.3kV

If the plug fires at -25 deg BTC, the cylinder Temp will be 609F, Press will be 144psi
Required firing voltages at various gaps: .050" = 19.6kV, .080" = 29.8kV, .100" =36.5kV

As can be seen, larger gaps require higher firing voltages. But retarded ignition timing also requires higher firing voltages.
The worst scenario is a high compression engine, with large plug gaps and retarded ignition.

I had 2 problems throughout the years with my engine. First, was with standard tension points, which bounced beyond 5500 RPM, and this issue was solved by installing high tension points. Next was spark plug fouling and resultant high speed misfire. This was improved but not solved by installing an electronic triggering device (initially M & H, Breakerless SE, and then Pertronix II). These are not subject to dwell changes at high speed, thus maintain a constant open circuit voltage potential from the coil............................if needed, of course. During the period when I had the M&H installed, I fashioned my own version of a .020" gap rotor

http://img.photobucket.com/albums/v216/speedo1952/Joes Photos/IMG_0004-3.jpg

(before I bought the one that has been available for the last few years). I did not notice any difference with this rotor, which seems to tell me that (I am by NO means an electrical expert) the entire secondary circuit was in good condition.

What made the biggest difference of all was installing a Pertronix II trigger with 40 Kv (potential, if needed) coil, bypassed ballast and .045" plug gap. This ended all misfiring. Finally, the addition of a MSD 6AL (multispark capacitive discharge) module enables smooth firing up to 8000 RPM (as far as I need to go). I can run diesel fuel in a haboob and the thing will not misfire, even with old, dirty plugs. Of course, the wires are upgraded with 8.5 mm OD insulation which contains all that voltage. None of it can be seen beneath the stock ignition shielding.

[Dan D.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

I stole these off the 'net in part because I am off soon for the Road Tour and in part because my garage is a mess. My distributor machine has a plate for mounting 8 spark plugs and the SP wires to the distributor. I'll get a picture when I get back. We used to use it in school back when for practical demo of the ignition system operation. My SUN oscillo came from school too (larger than this one and on a cart) when they got rid of that "old Tech" stuff. The gen/alternator - voltage regulator bench went to Atlanta Georgia where it gets more use than it did in school; but that is another story.

[ATTACH=CONFIG]47258[/ATTACH][ATTACH=CONFIG]47259[/ATTACH]

I'll help you clean your garage out!

[Terry M.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

My wife has a person on speed dial, and I have made her promise the first call is to him. Sorry Dan, you are too late. On the other hand there may be hope. When she gets real POed at me she threatens to bury me with a tie on, so maybe that first call won't happen if I die in flagrante delicto.

[Terry M.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe, how did you measure cylinder temperature? BTW: excellent research with real data. Great job.

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe, i just had a problem at a race with the distributor rotating under load. It advanced and caused preignition ,burning away the electrode and melting the top edge of two pistons. it seems from the data that advancing would have done the opposite. i'm confused.

While the following data may not be exact for a specific engine, it does clearly show the trends that all engines follow. Further, remember that leaner mixtures require higher firing voltages than richer mixtures.

The calculated values are for air only and do not include fuel ratio effects. Nor do they include plug electrode materials, tip geometry, and accumulated deposits. Given a static compression ratio of 11.4 and a trapped (dynamic) compression ratio of 9.0, inlet temperature of 100F and inlet pressure of 1 Bar:

If the plug fires at -45 deg BTC the cylinder Temp will be 399F , Press will be 66psi
Required firing voltages at various gaps: .050" = 12.1kV, .080" = 18.1kV, .100" =22.1kV

If the plug fires at -35 deg BTC, the cylinder Temp will be 496F, Press will be 97psi
Required firing voltages at various gaps: .050" = 15.3kV, .080" = 23.2kV, .100" =28.3kV

If the plug fires at -25 deg BTC, the cylinder Temp will be 609F, Press will be 144psi
Required firing voltages at various gaps: .050" = 19.6kV, .080" = 29.8kV, .100" =36.5kV

As can be seen, larger gaps require higher firing voltages. But retarded ignition timing also requires higher firing voltages.
The worst scenario is a high compression engine, with large plug gaps and retarded ignition.

I had 2 problems throughout the years with my engine. First, was with standard tension points, which bounced beyond 5500 RPM, and this issue was solved by installing high tension points. Next was spark plug fouling and resultant high speed misfire. This was improved but not solved by installing an electronic triggering device (initially M & H, Breakerless SE, and then Pertronix II). These are not subject to dwell changes at high speed, thus maintain a constant open circuit voltage potential from the coil............................if needed, of course. During the period when I had the M&H installed, I fashioned my own version of a .020" gap rotor

http://img.photobucket.com/albums/v216/speedo1952/Joes Photos/IMG_0004-3.jpg

(before I bought the one that has been available for the last few years). I did not notice any difference with this rotor, which seems to tell me that (I am by NO means an electrical expert) the entire secondary circuit was in good condition.

What made the biggest difference of all was installing a Pertronix II trigger with 40 Kv (potential, if needed) coil, bypassed ballast and .045" plug gap. This ended all misfiring. Finally, the addition of a MSD 6AL (multispark capacitive discharge) module enables smooth firing up to 8000 RPM (as far as I need to go). I can run diesel fuel in a haboob and the thing will not misfire, even with old, dirty plugs. Of course, the wires are upgraded with 8.5 mm OD insulation which contains all that voltage. None of it can be seen beneath the stock ignition shielding.

[Joe C.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe, how did you measure cylinder temperature? BTW: excellent research with real data. Great job.

Terry,
That's published data that I found a few years ago while gathering info for an engine build. Cylinder temperature is most likely measured with a thermocouple. When timing for maximum torque, the goal is to have max cylinder pressure occur between 12 and 25 degrees ATDC depending on rod length and crank throw.

[Joe C.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe, i just had a problem at a race with the distributor rotating under load. It advanced and caused preignition ,burning away the electrode and melting the top edge of two pistons. it seems from the data that advancing would have done the opposite. i'm confused.

Jerry,
Those cylinder temps are obtained BEFORE the spark plug fires, and are strictly based on the temperature rise caused by engine compression alone. Once the plug fires, cylinder pressures continue to rise until max pressure is reached, ideally at some point between 12 and 25 degrees ATDC. Those temps serve to illustrate how ambient temp and pressure affect firing voltages (required) for a fixed plug gap.

Too much spark advance causes detonation, which is the initiation of a secondary flame front, AFTER the spark plug fires. The effects of this are what you describe...................melted electrodes, broken/melted pistons and cracked rings.

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

That makes sense. my compression ratio is 14:1 and the clip holding the distributor just could not hold the distributor so the advance went to over 50 degrees, nasty. went to very heavy duty clip. (MSD)

Jerry,
Those cylinder temps are obtained BEFORE the spark plug fires, and are strictly based on the temperature rise caused by engine compression alone. Once the plug fires, cylinder pressures continue to rise until max pressure is reached, ideally at some point between 12 and 25 degrees ATDC. Those temps serve to illustrate how ambient temp and pressure affect firing voltages (required) for a fixed plug gap.

Too much spark advance causes detonation, which is the initiation of a secondary flame front, AFTER the spark plug fires. The effects of this are what you describe...................melted electrodes, broken/melted pistons and cracked rings.

[Joe C.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

That makes sense. my compression ratio is 14:1 and the clip holding the distributor just could not hold the distributor so the advance went to over 50 degrees, nasty. went to very heavy duty clip. (MSD)

14:1...........what are you using for a cam?
Strange that the timing went advanced, since the rotation of the shaft should drag it retarded if the hold-down clip came loose.

[Michael H.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Strange that the timing went advanced, since the rotation of the shaft should drag it retarded if the hold-down clip came loose.

That's what I was wondering too.

[Joe R.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hi Michael:

You may be onto something. I think I saw a reference somewhere that implied the R spark plugs were part of GM's change to the rotor. GM may have had a "system level" solution that involved both the rotor and plugs.

I always thought that R plugs were "resistor plugs," but when I put an ohm meter on an AC R45 plug is shows essentially zero resistance in the center conductor. Does anyone know what characteristic of the plug is different in the R versions?

[Joe R.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hi Dan:

If all you need to do is spin the distributor to get the data you want, I can show you how to build a fixture for less than $100. In fact, the resulting fixture, combined with a dial-back timing light, duplicates the function of a Sun distributor machine. I have a Sun machine like Gene's, but unlike Gene's, mine doesn't work right yet. I rebuilt the mechanical portion and got that working fine, but the guage readouts are not correct, and I have not had time yet to go through the electronics.

While pondering how to test the calibration of my Sun machine, I realized that if I could simply spin a distributor with a variable speed control, the addition of a dial-back timing light (with tachometer readout) would give me the equivalent of a Sun machine. I actually used this fixture just last week to set up my distributor for the upcoming Road Tour.

Eventually I hope to fully describe this fixture in a Restorer article, but the basics are fairly simple. I used a 12V heater motor and a flexible coupling, powered by a 12V power supply and a PWM speed controller. Unlike the Sun machine, the distributor has a cap and coil installed so that it can directly trigger the timing light.

For on-car diagnostics, I have considered doing as you suggest, using a standard oscilloscope to duplicate the function of the Sun Ignition Scope. I have a modern 4-channel digital scope that I suspect could be set up to provide a similar waveform diagram. I haven't really thought that through, though.

The main point to keep in mind is that those old diagnostic machines were not "magic." If you look carefully at what they were doing, it should be possible to adapt ordinary, modern measurement devices to make the same type of measurements that those old machines made.

[Michael H.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hi Michael:

You may be onto something. I think I saw a reference somewhere that implied the R spark plugs were part of GM's change to the rotor. GM may have had a "system level" solution that involved both the rotor and plugs.

I always thought that R plugs were "resistor plugs," but when I put an ohm meter on an AC R45 plug is shows essentially zero resistance in the center conductor. Does anyone know what characteristic of the plug is different in the R versions?

Joe,

I'm far from being an electronics guy but I'll try to pass along the explanation the way it was explained to me in 1969.

There isn't actually a gap in the center electrode. Instead, a small resistor, or section of high resistance material (I don't remember what) is added to a portion of the electrode.

The purpose of the high resistance section is to cut off, or eliminate much of the random misc low level spark discharges at the beginning and end of each discharge.
Contrary to popular opinion, each spark/discharge consists of multiple sparks. (MSD didn't invent this) The group of discharges starts out with low level spikes, gaining energy to peak level and then falling off again to low level spikes. Remaining would be the most powerful spikes.

There are two reasons why this is desirable. The small low level end spikes, or peaks do nothing for actual ignition but they do interfere with the radio and other electrical equipment. The other reason is to increase spark plug life. (every spark removes a small bit of electrode so less sparks, longer life)

I'm pretty sure a real electronics guy will stop in and correct all of my terms and descriptions.

Edit: Just remembered, there are, or were spark plugs with what was called a "booster gap". (Champion?) There plugs actually did have a gap in the middle of the center electrode. The gap increased spark energy, probably similar to the E gap rotor.
If that's the case, the ohmmeter test would show no continuity.

[Dan D.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hi Joe,

Funny that you built a test fixture to duplicate the Sun. I was thinking along the same lines, but did not mention it as I have not thoroughly though it out yet. And besides, Gene is close by.

I would think that a heater motor would not have enough power to turn the distributor. Especially since you would have to gear it up to turn at least 3500RPMs. I saw the motor and speed control as a stumbling block for the design. For that matter, I don't know how fast a Sun machine can spin.

If you have 4 channel digital scope that should be ideal for this experiment. Plus it would be simple to store data and make hard copy prints. I have an older Tek 2 channel model 454 scope that I could make work, but would be lacking the data storage and hard copy capability.

Another problem I see, and have not really thought about yet, is how to calibrate the high voltage. It would have to be an inductive (or capacitive) pickup, and I'm not sure how to calibrate this to an actual voltage.

Anyhow, this thread started out very innocently, but has migrated to what could become an interesting experiment and learning experience. Definitely could become a Restorer article, I'm sure. -Dan-

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

This is heart attack serious Joe. I am in a rebuild of the motor right now getting ready for Monterey in three weeks. What i know is that i set the total advance at 38 degrees at 4000 RPM and went out for a race and came back and it was now at 52 degrees. This happened three times before i got smart enough to put the car on the trailer and go home. Any ideas what could be happening here. The distributor is a GM L-88 magnetic trigger that drives an MSD 6 system. Never had this problem with the FI distributor which as you know is pinned at the base. I haven't solved the cable breaking problem on the FI so I am going carburetor until i figure this out.