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

[Joe R.]

I have seen several references to a "long tip" distributor rotor that was reportedly used by GM up until around 1970, and a "short tip" version that was used after that. Does anyone know why GM changed the length of the tip, and which version is preferable for a C2 Corvette?

I made some measurements on a "long tip" rotor that I bought and compared it to a (presumed) short-tip replacement rotor. The gap between the rotor tip and the distributor terminal measures about .050 with the long-tip rotor and about .090 with the presumed short-tip rotor. I have seen two "explanations" for the change that GM made:

1) The short tip reduced radio interference
2) The short tip helped with emissions performance

Niether of these reasons are compelling for my C2, so it seems the long-tip version might be preferable. Intuitively, it would seem that a smaller gap in the distributor would provide better firing of the spark plug, but I realize that the actual physics involved are complex.

Is there a good technical explanation regarding which rotor style is preferable for a C2?

[Terry M.]

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

Bill Clupper, a frequent poster here, wrote an article addressing those very issues in The Corvette Restorer not so long ago -- perhaps even within the last year or certainly two. I know the Internet is full of good, and not so good information; but occasionally the printed word has what you want.

[Joe R.]

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

Hi Terry:

Thanks for the pointer. I found Bill's article in the Summer 2009 Restorer (yup, time flies). In the article Bill says the change to the shorter rotor was emissions-related, and, counter-intuitively, was supposedly made so that the spark would be hotter. He also says that the new, shorter rotor impairs performance at high RPM.

I realize that the physics involved are probably complex, but I'm interested if anyone knows more about what the rationale was for the change. I'd also like to know if the consensus view is that the long-rotor version is preferred for a C2 running stock-style carbon plug wires and resistor plugs (one source I found suggests that the rotor was changed to work with resistor plugs).

[Dennis O.]

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

I have a '67 L79, and it would not run to the "Orange" line (between the yellow line and red line; I don't push my luck) without breaking up. I purchased one of the "long tip" rotors, and problem solved. It runs fine as far as I want to rev it. BTW, I am running repop LL plug wires.

[Bill M.]

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

Hi Terry:

Thanks for the pointer. I found Bill's article in the Summer 2009 Restorer (yup, time flies). In the article Bill says the change to the shorter rotor was emissions-related, and, counter-intuitively, was supposedly made so that the spark would be hotter. He also says that the new, shorter rotor impairs performance at high RPM.

I realize that the physics involved are probably complex, but I'm interested if anyone knows more about what the rationale was for the change. .

Maybe. I worked for the GM Engineering Staff in the summer of '68 on advanced emissions. (My co-worker was working on catalytic converters. This is 6 years before catalytic converters were introduced.) At that time, and up until catalytic converters, even one mis-fire was a disaster for an emissions test; unburned hydrocarbons would spike and the test would fail miserably.

I don't know for a fact that the resistor plugs and shorter rotor were introduced to make a mis-fire less likely, but I do remember my father showing me how to clear a fouled plug by pulling the plug wire from the cap and making the spark jump the gap. That would cause the plug to fire and the combustion heat would clean the plug.

[Terry M.]

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

Joe, time does indeed fly. I would have thought that article was far more recent than 2009.

Bill could do this much better than I. Wires and ignition were/are his life's work. But I do it once in a while for school.

I think the idea that the short tip is emissions related stems from two things. The OEM distributor caps with the short tip (at least when first introduced) were stamped with an "E" on the tip.

The voltage on the secondary (high voltage) side of the coil is dictated by the level needed to jump the gap from the spark plug center electrode to the ground (moveable) tip. If the spark plug gap is opened up slightly the voltage required to jump that gap will increase. Obviously there is a limit that the coil will produce and at that point the spark fails to jump the gap and we have a misfire, but up to that maximum the voltage will rise as necessary to jump the gap.

Adding a gap into the system (the short tip) makes the secondary voltage go higher before it even begins to reach the plug -- so the voltage will be higher at each call for a plug to fire. The higher voltage produces a "hotter" spark which causes more of the air/fuel mixture to burn producing more power and fewer emissions – up to the high RPM point at which the coil cannot keep up with the need for the voltage to rise and fall for each cylinder (called coil saturaion) and again we have misfire. Of course these misfires do just awful things to both power and emissions, but since far in excess of 90% of vehicles ever reach the high RPM range of misfire, the short tip is for the good of the majority. Or so the theory goes.

One can see all this with an oscilloscope -- the famous "SUN" machine, among other brands, that was once so popular in auto shops. One can still see these traces on what is now days called a "lab scope" that basically looks like a notebook computer. We still teach some of this in the electrical portion of auto shop.

Edit add: This is probably a simplification of what takes place as the spark plugs fire -- perhaps even an over simplification -- but it is the best I can do in plain language. If it gets to more obtuse language, I am out of it too.

[Dan D.]

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

[QUOTE=Terry McManmon (3966);670037]

[Gene M.]

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

Dan,
Do I understand this right? If the coil saturation time is marginal as speed increases (time decreases) then a lesser distributor gap is help full. I guess I do not understand how a wider distributor gap can attain higher voltage and produces a "hotter" spark. Using this theory a set of plug wires with breaks will do the same multiple times. But as we know this is not true, in-fact it runs real bad.

Electrical is not my suit. In fact I struggle with it.

[Ken A.]

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

Sort of, remember these smog motors were not designed to run very good above 3500 RPM. The "spark" always seeks the nearest ground, so your wires, with breaks, short out to the closest ground-not to another section of wire. Always see GM's thinking "What can we do, that's really inexpensive, to meet emission standards?" Answer-make a hotter spark to fire worn out plugs and not worry about performance. Solution, widen SP & rotor gaps. Also remember-higher RPM & a wide gap equals greater chance of crossfire or worse, firing late AND burn thru on SP wires of the day. The HEI system fixed most of the problems that pre HEI systems had in meeting the Emission standards of the day BUT was a huge investment for GM, who had lobbied hard to get the standards reduced. Look at what it takes today to meet standards, coil packs, computers, sensors everywhere, & special SP wires.

[Dan D.]

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

Hi Gene,

What happens is the plug will fire as soon as the voltage is high enough to bridge the gap(s). This happens before the coil reaches its maximum voltage. But as the speed increases the coil produces less and less voltage due to the reduced charge time - the time the points are closed. Eventually, as the speed increases, this voltage drops to the point that is no longer sufficient to bridge all the gaps (rotor to cap, and plug), and you get the dreaded mis-fire condition. With bad wires, when the breaks are more than a few thousandths wide, the condition occurs at a much lower speed.

I did some rough calculations a while back and determined, using the inductance of a standard 091 coil as a gauge, that with an 8 cylinder engine and conventional point/coil ignition, I believe this system will run out of bandwidth between 7000 and 7500 RPMs. In other words, above this RPM the coil voltage will not be sufficient to fire a standard .035 plug gap, and the gap would have to be reduced in order to prevent mis-fire. But i have have not tested this hypothesis, so I have not published anything (I would need your Sun machine to test it).

Tell ya what. I will trade some on my electrical knowledge for some of your mechanical knowledge. Sound like a deal? -Dan-

[William C.]

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

Well summarized Terry. My experience was from a more practical standpoint, as all my cars were 11-1 compression in the day, and they would not pull to redline after a week with a new "short" rotor, and would go 15,000 miles and beyond with a long rotor.

[Terry M.]

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

Gene,
The problem with gaps in the spark plug wires is that the distance of the gap(s) is not well controlled and most often it is too much for the spark to jump thus causing the dreaded miss-fire. Or alternately the gap is such that the spark finds it easier to jump to ground (all electricity, even in your home, wants to find ground) than to jump the gap in the wires. Electricity always takes the easiest path to ground, or as my professor used to say “electricity follows the path of least resistance.”

Please note in the above referenced story by Bill Clupper the gap in the distributor cap is on the order of 0.020-inch. (I haven’t reviewed that story, and I am guessing a little at that distance. Someone with the story in hand can post the more specific dimension.)

I knew there would be someone among all our members (and more importantly that someone has to be on this board) who could help with this. I thought it might be Bill Clupper, but Dan is doing just fine. As I said I simplified it - and it can get very complex. We haven't even scratched the surface. Maybe it helps that I deal with 12Kv (more or less) all day long. The conventional coil (not HEI) voltage can rise to 20Kv on an open circuit; HEI can double that. That is why with conventional coil spark plugs are gapped in the 0.035-inch range and HEI plugs are gapped at more like 0.060-inches. That increased voltage of the HEI will jump that larger gap with more force and ignite a poorer air/fuel mixture as well as that larger gap exposes more of the air/fuel mixture to the spark. Thus the HEI burns the air/fuel mixture more efficiently adding power and reducing emissions. Modern coil on plug systems minimize saturation of the coil by having one coil per every cylinder or every other cylinder. Firing those coils electronically via the computer eliminates the distributor and the long spark plug wires for more long term efficiency – but we are getting away from the original question.

Dan, since my SUN machine is as tall as I am and two or three times as wide the chances of getting it to you are not too good. Sadly one would have to find a really "old school" shop to find one of these now days. In the days of a separate distributer they were an extremely useful diagnostic tool.

[Michael H.]

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

I think the short tip E gap rotor was first used for the 69 model year?? The same year that R spark plugs were first used?

I've used both a short tip and long tip rotors on several different cars and I've never noticed any difference.

[Dan D.]

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

Oh Terry, you guys with Sun machines and growlers: I would have (almost) killed for those things back in the 50s & 60s, but ya know, we had no money.

My reason for saying "Sun" is Gene has a Sun machine, and he lives only about 8 miles from me. It would just be an interesting experiment to do. I have an oscilloscope, but I need the Sun to spin the distributor. We could map several things - single and dual point distributors, HEIs, various coils and resistors, plug wires. plug gap, etc. We could generate reams of data! -Dan-

[Terry M.]

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

Oh Terry, you guys with Sun machines and growlers: I would have (almost) killed for those things back in the 50s & 60s, but ya know, we had no money.

My reason for saying "Sun" is Gene has a Sun machine, and he lives only about 8 miles from me. It would just be an interesting experiment to do. I have an oscilloscope, but I need the Sun to spin the distributor. We could map several things - single and dual point distributors, HEIs, various coils and resistors, plug wires. plug gap, etc. We could generate reams of data! -Dan-

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.

SUN distributor machine.jpgSUN oscilloscope.jpg