Rev limiting vs non-rev limiting rotors

[Pazuzu]

The PO had put a rev limiting rotor in my 1980 SC, which was unneeded. I got a new non-limiting rotor and cap. My question is...

The Rev limiting rotor seems to be slightly offset where the probe electrode is, vs the non-limiting one. Therefore, if I put my cap back on the same way, the rotor will be slightly rotated from where the old setup was.

Does the distributor now need to be clocked for timing? Does the factory CDI unit fire a long enough spark that it will hit the electrodes in the cap anyways? Can I simply rotate the cap slightly, or do I need to actually rotate the distributor body? Engine starts and runs fine after the install, no stumbling or coughing, so I assume the timing is fine.

[RoninLB]

never thought of that. I would say it's same timing.

just check your timing and come back for a report.

timing does change when installing a Pertronix points replacement.

[Pazuzu]

Originally Posted by RoninLB

just check your timing and come back for a report.

Don't hold your breath I *think* I still have a timing light, but it hasn't been used in many years...hhhmmm...

I might swap the plugs tomorrow morning, if the timing light still works, I'll see what I can get from it.

[frankc]

Well, rotating your distributor body will certainly change your timing, which is determined by when the points/pickup in the distributor fire the CD box, rather than when the rotor strikes the cap plug electrode. I guess as long as the rotor striker is wide enough to be in contact with the appropriate plug wire when the current comes through (taking into account any timing advance), there should be no issue. But I assume that's what you're trying to find out.

Frank

[Pazuzu]

Bumping this for the weekday crew. I'm sure I'm not the first one to replace the different rotors like this...

Dropped new spark plugs in (there's a story, I'll post tonight with some pics). Buttoned everything up and went out to warm up the engine so I could do a proper timing. Well, decided from that 10 minute drive that if the timing goes off from these different rotors, it's so small that I couldn't feel it, 6500rpm came up as quickly as ever, and the engine ran slightly better when cold (less coughing for that first block or 2). I decided to not bother with the timing for now.

[SpeedracerIndy]

I just dug this old post up while doing a search.

I think I can confirm that the rev limiting rotor does need to be timed different than the US spec rotor. When I bought my car many years ago, it had a us spec rotor installed from the previous owner. I didn't know I needed a rev limiting one until I planted my foot to the floor and missed a shift. To my surprise, the tach spun well above the 6800 limit. After much research I purchased and installed the correct rev limiting rotor. Ever since, I have noticed that my timing was advanced a few degrees too much. I suspected the rotor, but never made the connection. I have yet to adjust the timing to the correct specs, but need to get around to it, as I have a bucking under load when the engine is very hot. Does this confirm that the rev limiting rotor is clocked slightly different than the US spec?

Just thought I'd put in my $0.02.

[ossiblue]

This is just my take on it from reading the thread. All rotors have a "wide" tip, not a point, and it is this span that must be in the range of a wire terminal for the spark to bridge the gap. I agree with Frank in the above thread that timing is determined by the opening of the points, not the micro-positioning of the rotor tip. As long as the span of the rotor tip is in the range of the wire terminal when the points open, timing will be correct.

I will change my understanding of this if experts contradict the conclusion from their own experiences. (Hey, we're all here to learn)

[Pazuzu]

Originally Posted by ossiblue

This is just my take on it from reading the thread. All rotors have a "wide" tip, not a point, and it is this span that must be in the range of a wire terminal for the spark to bridge the gap. I agree with Frank in the above thread that timing is determined by the opening of the points, not the micro-positioning of the rotor tip. As long as the span of the rotor tip is in the range of the wire terminal when the points open, timing will be correct.

I will change my understanding of this if experts contradict the conclusion from their own experiences. (Hey, we're all here to learn)

I'm still running the non-rev rotor with no timing change, and it's still fine. Based on that, you can swap back and forth.

However, the contact patch for the rev limiting vs. non-rev limiting rotors seem to barely, if at all, overlap. Is it possible that the PO had a rev-limiting rotor put in accidentally, they never adjusted the timing, and so when I put the correct one in, everything was perfect? Who knows...

Someone with more time, money and care than me should check this out! Good weekend project...

[wwest]

"80 SC with a rev-limiting distributor rotor..

WHY....?

The rev-limiting is done electronically.

CDI ignition shuts down at rev-limit and "trigger" signal goes to the rev-limiter which "switches-off" the fuel pump if/as the engine reaches over-rev.

BANG...!! if you reach the rev limit rapidly, under full thrust, feels as if something broke, something SERIOUS...!

Otherwise, slow RPM rise to over-rev limit kick-in, just prevents the engine from over-reving due to being under power.

If there is such a thing as a rev-limiting rotor then it will have a MATCHING, "paired", distributor cap as it would be EXTREMELY impractical, unwise, to change the base timing.

[Steve@Rennsport]

Originally Posted by wwest

"80 SC with a rev-limiting distributor rotor..

WHY....?

The rev-limiting is done electronically.

CDI ignition shuts down at rev-limit and "trigger" signal goes to the rev-limiter which "switches-off" the fuel pump if/as the engine reaches over-rev.

BANG...!! if you reach the rev limit rapidly, under full thrust, feels as if something broke, something SERIOUS...!

Otherwise, slow RPM rise to over-rev limit kick-in, just prevents the engine from over-reving due to being under power.

If there is such a thing as a rev-limiting rotor then it will have a MATCHING, "paired", distributor cap as it would be EXTREMELY impractical, unwise, to change the base timing.

Pre-'78 cars; 2.7, 2.4 and 2.2 engines did use rev-limiting rotors and generally speaking, functioned very well. They quit doing that when catalytic converters began being installed since a fuel cut-off type rev limiter became necessary.

One should never assume that the timing remains unchanged between the two types of rotors, given the consequences of too much timing in these engines. Rotor phasing is a concern so timing should be checked and adjusted as necessary.

[wwest]

I will still stick with a rev-limiting rotor requiring a MATCHING distributor cap.

Changing the timing, rotating the entire distributor, to accomodate a rev-limiting rotor is not "german".

[911pcars]

Ignition timing is a function of the ignition contact points or other triggering device, not the rotor. However, the rotor position, relative to it's current path contact in the distributor, may make a difference, especially during ignition advance - the gap increases as timing advances.

If the rotor tip is too far from the contact, this may cause intermittent misfiring (the system isn't able to produce sufficient voltage to overcome the excessive gap). On CDI or other high-energy systems, excessive gap can over-stress the electronics and components.

Sherwood