Ignition rotor to distributor cap contact phasing.
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Re: Ignition rotor to distributor cap contact phasing.
The Chevrolet Power Manuals from the seventies state that the best power total WOT advance is in the range of 36-40 degrees for both small and big blocks. I usually shoot for 38-39, and as long as there is no detonation, it's good to go. As a general rule, if you know the exact best power total advance for a given engine configuration, retarding it three degrees will drop peak power about one percent.
The rotor tip's relative location to the distributor terminals is fixed by the gears and is not affected by either centrifugal, vacuum, or initial advance. The centrifugal advance advances the cam, but it can be confusing because with the engine stopped you can rotate the rotor, so it appears that it moves relative to the cap terminals, but it doesn't because with the engine running it is the cam that rotates, not the rotor.
The vacuum advance advances the breaker plate, and initial timing rotates the distributor base, so, again the rotor doesn't move relative to the cap terminals with variation of vacuum advance and initial timing.
Duke- Top
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Re: Ignition rotor to distributor cap contact phasing.
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The rotor tip's relative location to the distributor terminals is fixed by the gears and is not affected by either centrifugal, vacuum, or initial advance. The centrifugal advance advances the cam, but it can be confusing because with the engine stopped you can rotate the rotor, so it appears that it moves relative to the cap terminals, but it doesn't because with the engine running it is the cam that rotates, not the rotor............
Duke- Top
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Re: Ignition rotor to distributor cap contact phasing.
With the 7.5 degree movement the position of the spark will move approximately 3/16” of position relative to the distributor contacts. At some point there is a sweet spot to have the actual spark in a particular position on the contacts to maximize spark. At a given position the spark time is at a different time as the rotor passes the distributor cap contacts. This can alter the spark transfer point as rotor passes cap contacts.
Understand dwell time goes down as rpm goes up.Last edited by Gene M.; December 5, 2018, 11:33 PM.- Top
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Re: Ignition rotor to distributor cap contact phasing.
My thoughts are the position of the points plate at rest sets the point where the rotor fires, when the vacuum pulls the points plate the rotor fires before the tip and cap (advanced). When the centrifugal weights move the weight base the alignment between the tip and cap terminal come back together because the weight base rotates clockwise.
You gotta think about it because it gets confusing, I think you can change the rotor phasing by moving the at rest position of the points plate. In a test you could cut a hole in the cap at #1 and see where the rotor is at 10BTDC then pump down the V/A can and check then unhook the springs and see where the rotor is when the weight base is at the other end of the centrifugal slot. This would confirm the check but the V/A can at times move things a little.
Or do I need to think about this a little harder.. :-)- Top
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Re: Ignition rotor to distributor cap contact phasing.
Yeah, I was wrong. It's been too long since I thought about this... goes back to the seventies when I had my dist. on a Sun machine and figured it out... or at least I thought I did.
The rotor is attached to the cam assy., not the main shaft, so when the cam advances, the rotor advances, too. Vacuum advance advances the breaker plate and has no effect on the rotor phasing. Initial timing varies the phasing of the dist. cap, but not rotor phasing. So we can ignore centrifugal advance and just think about the effect initial timing and vacuum advance on rotor/cap terminal alignment.
Consider the 365/375 HP 327s with 24 centrifugal, 14 true initial, and 16 vacuum, so the rotor will be displaced 8 degrees at max vacuum advance and the cap terminal spacing is 45 degrees. OE initial timing recommendations vary from 4 to 16 on non-emission controlled C2 small blocks depending on engine and the recommended range. Somewhere in that range the rotor should be perfectly aligned with the cap terminal. Let's assume it's halfway, which is 10, so initial timing in the 4-16 range will add or subtract 6/2 =3 degrees to rotor phasing for a total misalignment of 5 to 11 degrees at maximum vacuum advance with initial timing in the 4-16 range. Apparently the terminals are wide enough that the spark can still jump the gap without significant loss of energy within this range and the 45 degree cap terminal spacing is enough to prevent cross firing.
Observing terminal erosion on a used cap and rotor can tell you the center point of the spark. HEI caps have wider cap terminals because of their larger diameter. I've also seen some rotors that have wide tips to make up for rotor-cap terminal misalignment, but it appears GM never thought that was necessary or I expect they would have implemented a similar design.
Duke- Top
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Re: Ignition rotor to distributor cap contact phasing.
Good point you guys bring up about a wider rotor tip. Being that spark likes to jump from sharp points a sharp edged wide tip should provide a bit more latitude if one see the inside cap contacts are favoring one side. I do notice the points cam is retarded to the rotor to cap contacts by about 2 to 3 degrees. Not a exact measurement but definitely not centered with the distributor contact when the rotor is in position that the points cam puts it.
i might just get one of the MSD adjustable rotors and play around with it to see any effects.- Top
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Re: Ignition rotor to distributor cap contact phasing.
Outside of installing an adjustable rotor, I'm not sure there is much that can be done or benefit realized in reference to phasing in older points driven ignition systems, or for that matter, how you would go about it. Electricity will always follow the path with the least resistance. As long as the rotor is not excessively far away from the cap terminals, it would seem that the most resistance in the circuit will still be at the plug. I believe the coil output potential on older stock coils is somewhere in the neighborhood of 15-20K volts. Resistance at the plug if I remember correctly will typically be overcome at somewhere around 9-12k volts with plugs gapped at around .035. Actual coil output is determined only by maximum resistance in the circuit, and I would question whether that is at the rotor/cap terminal in older systems. It would seem to me that phasing might be more beneficial in a system that can think for itself as opposed to the standard point driven ignition systems from the sixties.
Steve- Top
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Re: Ignition rotor to distributor cap contact phasing.
There is more madness to my agenda. I’m working on a small diameter GM distributor with magnetic pick up and HEI module using a “E” type coil. With everything modified so it can be put back as original. The mechanical advance piece that has the points cam was modified to accept a reluctor wheel. This is another spare piece since it is destroyed from an originality perspective by turning down the points cam. Everything else is returnable to stock.
There is a mounting 1/4” thick heat sink for the HEI module that uses the thu holes for the centrifical advance just threaded (8-32 flat head screws) all the way thru the exhisting holes. The breaker plate has two 8-32 threaded holes added for the magnetic pick up. But does not inhibit reuse of points.
I have a long tip rotor for best spark to contacts relationship. I started out with the rotor tip to distributor contact a couple degrees (about 1/16” at contacts) retarded at rest for the reluctor position. I’ll go from there using an adjustable MSD rotor to optonize spark position. Once I’m happy with that I can reposition the reluctor and use a standard GM rotor (old style long tip). Testing will be on my 65 327 pretty much stock driver.
Blue rotor in pictures is a long tip rotor I modified myself the tip is secured with a screw, threaded the plastic for it. Originally the tip was held in with a rivet type drive pin. It came loose and was the whole reason I pulled the distributor after the car wasn’t running well. Found the problem and the whole thing just grew........ Gotta have a winter project.
The weather was good for two days and allowed for test run the ‘65 with the modified HEI distributor. Once I set timing and adjusted the idle down the engine ran so smooth and felt stronger thru all the gears. The supply of 14.2 volts vs the old 8 volts with the ballast resistor and points sure is felt in the seat of the pants. The coil was also replaced with the “E” type HEI stand alone coil. I fitted a stainless tube to the bottom of the coil which enabled mounting in the original coil mount. I replaced the feed wires with 16 or 14 gauge.
Understand this is not NCRS correct by any means, but does add to the driving experience. And it does give the guy something to do that just can’t leave well enough alone.- Top
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