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i use to rotor phase on drag car MSD flying mag. systems---how critical is this for the ti for gm cars? "heard" that it is? or could? be a 5K+ rpm problem on gm factory ti ignition systems---heard there is a fix for the problem---cap shift kit or ?
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Re: LET'S TALK ABOUT ROTOR PHASING ON T.I.
Phil,
The problem that you describe is somewhat common in Delco TI distributors. The rotor tip to dist cap terminal index can be corrected in a Corvette distributor without major rework. There are two ways to do this. The easiest is to remove the vacuum advance unit and file the attaching holes until they are slots. Also, the locating lug has to be removed. The slots will now allow the unit to move several degrees in either direction. Hopefully, this will be enough to achieve proper rotor/terminal alignment.
If more adjustment is needed, you can remove the stationary pole piece, open up the hole for the vacuum advance rod and braze a flat washer on top of the plate in a new position.
To check rotor tip/terminal alignment, drill a hole in the side of an old distributor cap just below one spark plug terminal and on center with the terminal.
Mark the exact center of the outer portion of the rotor that aligns with the rotor tip. You can now use a timing light to check the index while the engine is running.
Engines that use vacuum advance units do not fire with the rotor tip pointing directly at the distributor cap terminal at zero vacuum. They also don't fire with the rotor tip pointing directly at the terminal at high vacuum. Ultimately, the tip and terminal should be in alignment when the vacuum advance has pulled the plate through HALF of it's rotation/travel. This allows a range of different vacuum advance applied point plate positions while still positioning the rotor tip near the terminal. The problem is, different vac adv units sometimes index the plate in a position that is off by several degrees. This problem is worse by several degrees with TI ignition because the point of ignition is when the internal/external points of the pole piece align. In an identical distributor that has points instead of TI, this occurs roughly 7* earlier than it does in a TI unit.
Something to think about when your trying to fall asleep tonight. Beats counting sheep. -
Re: LET'S TALK ABOUT ROTOR PHASING ON T.I.
Michael,
Now that's a technical response....with limited experience I think I only understood paragraphs 1 and 3. Can you check the index without running the engine....thanks.Comment
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i just aligned the points on the pole pieces
and than checked where the rotor was pointing thru a large hole cut in a old cap. since i was not using a working vacuum advance,race car engines, i just fabed a metal piece to hold the advance plate in a fixed location.i allowed for the mechanical advance movement by making sure the rotor tip was equal distance on either side of the cap pole piece at full advance and full retired.Comment
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Re: i just aligned the points on the pole pieces
Clem,
You'll just have to come up here and show me....LOL.Comment
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Re: LET'S TALK ABOUT ROTOR PHASING ON T.I.
Hi Rick,
Pretty confusing stuff. This would make a great article for the Restorer. Pictures would eliminate much of the confusion.
We used the timing light method decades ago but it can easily be done without it. Basically, all we're trying to do is make sure the rotor tip is aligned with the terminal in the dist cap when spark occurs. With conventional point ignition, spark occurs the instant the points open. At that moment, the rotor and terminal should be in near alignment.
If you have the distributor out of the car, mount it in a fixture or vice. Using a section of coat hanger for a pointer, secure one end of the pointer to something on the vise or bench and bend it so the other end points exactly to the center of the side of any distributor cap wire terminal tower. Now remove the distributor cap without disturbing the pointer. Slowly turn the shaft/gear so the rotor turns CW and stop at the exact spot that the points JUST begin to open and ignition occurs. (you can use a test light for more accuracy if desired) At this point, the C/L of the rotor tip should be approaching the C/L of the cap terminal, (our pointer) but not exactly aligned. It will probably be roughly 5* away from aligning.
Now, using your hand held vacuum pump, apply full vacuum to the advance unit and note the position/alignment of the rotor tip and the pointer. The rotor tip should again be in near alignment with the terminal but off center the same amount as in the first test, but in the opposite direction.
The only time the rotor tip and cap terminal will be in exact alignment is when the vacuum advance is pulled down to half it's travel. At zero or max vacuum, the alignment will be off by as much as several degrees in one direction or the other, but the two dimensions should be roughly equal. At either full vacuum or zero vacuum, the alignment of the rotor/terminal will be within an acceptable range.
Typically, on most GM distributors, these dimensions are within an acceptable range. However, GM transistor distributors are usually not. If the entire curve is off center enough, it will cause problems at either zero or full vacuum, depending on which direction the misalignment is off.
Some new LT-1 cars in about 1971 had serious problems with engine idle quality. Distributors, amplifiers and wire harnesses were being replaced under warranty at an alarming rate. It turned out to be the exact problem that we're discussing.
Also, aftermarket vacuum advance units often change the alignment by quite a bit.
The easiest way to understand all of this is to study the operation of an old distributor. Hope this helps. I'm sure others may be able to explain this more clearly.Comment
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Re: i just aligned the points on the pole pieces
Clem,
Rotor alignment isn't affected by the mechanical advance. The point cam and rotor are one piece so when the cam rotates to advance, the rotor follows and alignment is maintained. The vacuum advance, however, does not automatically compensate for this alignment. VERY confusing stuff.Comment
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correct the rotor and the rotating pole piece are
fixed.i guess it has been a while since i have done this. the idea is still the same line up the pole piece points and check the rotor tip location to the cap terminal. how ever you do it you this you should use your timing light thru a hole in the cap to check you work. also you should use the "long tip" rotor not the one with the "E" stamped on it which stands for emissions which has the shorter rotor tip. if you can find any aftermarket rotors like accel they have the long tip. the HEI caps have a long contact area which lessen the need for phasing but it it still should be checked. the HEI cap contacts should be bent inward to get them closer to the rotor tip. i had a fixture which fit into the brush hole on the cap which showed how far to bend in the cap contacts inward.Comment
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Re: correct the rotor and the rotating pole piece
Thanks Clem, I didn't know about bending the HEI cap contacts in. I suppose that gap was/is for the same reason that the "E" gap rotors had the wide gap?Comment
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i would guess that is the reason *NM*
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