HEI rotor marks off center - Chevelle Tech
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post #1 of 28 (permalink) Old Jun 10th, 19, 11:36 AM Thread Starter
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HEI rotor marks off center

I just changed the cap on the HEI of my 454. With the old cap upside down and looking into it, the rotor marks are to the far right edge of the contacts. I remember other distributor on other cars having the marks more or less centered. I just checked the new cap after a few hundred miles and the marks are off-centered to the right in the same way. Everything runs as normal so apparently it isn't a problem. Or is it? What would cause the rotor to be making contact off center?
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post #2 of 28 (permalink) Old Jun 10th, 19, 11:45 AM
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Re: HEI rotor marks off center

Are you sure the cap is sitting flat on the distributor.

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post #3 of 28 (permalink) Old Jun 10th, 19, 11:56 AM Thread Starter
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Re: HEI rotor marks off center

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Originally Posted by HKalin View Post
Are you sure the cap is sitting flat on the distributor.
Yes, it's flat. I see your reasoning, but the marks are horizontal and all at the same height on the contacts. I would think if the cap were tilted there would be a slight slant.
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post #4 of 28 (permalink) Old Jun 10th, 19, 12:05 PM
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Re: HEI rotor marks off center

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Originally Posted by 658Chevy View Post
Yes, it's flat. I see your reasoning, but the marks are horizontal and all at the same height on the contacts. I would think if the cap were tilted there would be a slight slant.
I was thinking the contact area would be how you said it would be high on one side and low on the other. The only other thing I can think of is the center bore for the shaft is not concentric with the outside of the housing where the cap sits on.

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post #5 of 28 (permalink) Old Jun 10th, 19, 12:55 PM Thread Starter
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Re: HEI rotor marks off center

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Originally Posted by HKalin View Post
I was thinking the contact area would be how you said it would be high on one side and low on the other. The only other thing I can think of is the center bore for the shaft is not concentric with the outside of the housing where the cap sits on.
Thanks, that makes sense.
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post #6 of 28 (permalink) Old Jun 10th, 19, 12:58 PM
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Re: HEI rotor marks off center

The rotor should never "make contact" with the cap. If you are referring to the spark marks then it probably has to do with where you have your timing set. As you move the distributor to set the timing the alignment of the rotor to the cap changes.
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post #7 of 28 (permalink) Old Jun 10th, 19, 1:40 PM Thread Starter
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Re: HEI rotor marks off center

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Originally Posted by BillK View Post
The rotor should never "make contact" with the cap. If you are referring to the spark marks then it probably has to do with where you have your timing set. As you move the distributor to set the timing the alignment of the rotor to the cap changes.
I understand they don't make actual physical contact. It was a figure of speech.

I speculated that the marks may be aligned with the timing, but according to that I seem to be at the outer edge of one parameter or the other. Since looking down into the cap is upside-down, the marks would actually be to the far left with the cap on. Would that signify retarded timing or advanced?

My initial timing is 12*, mechanical 19* (total of 31*), and cruise timing with the vacuum advance at 51*. Those setting are middle of the road and I would expect the marks to the more-or-less center on the contacts. Most on this board set their total timing at 34-40*. I know that many GM crate motors come from the factory with initial timing at 5-8*, which is of course at the other end of the spectrum. If the rotor marks align with the timing settings, then either of those two should be at their respective edges, and mine should be in the middle.

Please clarify.
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post #8 of 28 (permalink) Old Jun 10th, 19, 2:51 PM
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Re: HEI rotor marks off center

It sounds like the rotor is at the very start of the contact as it is rotating. It really does not signify advanced or retarded timing. Simply that the rotor is not in the optimum position for your exact setup. There really is not much you can do about it without modifying the distributor. If it gets too far off it can cause crossfiring.

Google "distributor rotor phasing" and there is plenty to learn

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post #9 of 28 (permalink) Old Jun 10th, 19, 3:30 PM Thread Starter
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Re: HEI rotor marks off center

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Originally Posted by BillK View Post
It sounds like the rotor is at the very start of the contact as it is rotating. It really does not signify advanced or retarded timing. Simply that the rotor is not in the optimum position for your exact setup. There really is not much you can do about it without modifying the distributor. If it gets too far off it can cause crossfiring.

Google "distributor rotor phasing" and there is plenty to learn
Thanks, Bill!
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post #10 of 28 (permalink) Old Jun 10th, 19, 3:51 PM
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Re: HEI rotor marks off center

Don't happen to have a crank trigger, do 'ya ??
Easy to phase the rotor then.
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post #11 of 28 (permalink) Old Jun 10th, 19, 5:37 PM
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Re: HEI rotor marks off center

Everybody calm down. If that HEI has a vacuum advance on it, the pattern is totally normal. When the mechanical advance moves, the firing tip on the rotor stays where it is, NO movement. The weights and scroll plate on the mechanical advance move the timing, but not the rotor tip.

BUT, with a vacuum advance, the V/A moves THE MAGNETIC PICKUP IN AN ARC, from one side of center at rest, and to the other side when fully vacuum'd up.

This is why when some dyno hero rips the vacuum advance hose, or, the advance itself off a distributor, and puts a fixed stop pin in its place, the rotor firing tip aims to one side of the center of the cap terminal, and doesn't move. So, MSD makes an "adjustable" rotor, so the rotor tip can be centered on the wire terminals inside the cap if some moron disables the vacuum advance

So, when the rotor moves, it makes a long pattern on the plug wire terminal in the cap, normal, no problem. The reason the wire terminals inside the cap are so wide is that there are more than 30 vacuum advance curves in large, coil in cap HEI's, so, some move the rotor tip a small amount if travel, others move the rotor a great deal. Instead of making 30 different caps, why not just lengthen the wire terminal in the cap? That is just what we did, and the idea came from the point distributor cap for the odd fire Buick V6, which fired not 120, 120, 120, 120, but 90, 150, 90, 150, with a set of long wire terminals in its points distributor caps.
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post #12 of 28 (permalink) Old Jun 11th, 19, 2:09 AM Thread Starter
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Re: HEI rotor marks off center

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Originally Posted by Dave Ray View Post
Everybody calm down. If that HEI has a vacuum advance on it, the pattern is totally normal. When the mechanical advance moves, the firing tip on the rotor stays where it is, NO movement. The weights and scroll plate on the mechanical advance move the timing, but not the rotor tip.

BUT, with a vacuum advance, the V/A moves THE MAGNETIC PICKUP IN AN ARC, from one side of center at rest, and to the other side when fully vacuum'd up.

This is why when some dyno hero rips the vacuum advance hose, or, the advance itself off a distributor, and puts a fixed stop pin in its place, the rotor firing tip aims to one side of the center of the cap terminal, and doesn't move. So, MSD makes an "adjustable" rotor, so the rotor tip can be centered on the wire terminals inside the cap if some moron disables the vacuum advance

So, when the rotor moves, it makes a long pattern on the plug wire terminal in the cap, normal, no problem. The reason the wire terminals inside the cap are so wide is that there are more than 30 vacuum advance curves in large, coil in cap HEI's, so, some move the rotor tip a small amount if travel, others move the rotor a great deal. Instead of making 30 different caps, why not just lengthen the wire terminal in the cap? That is just what we did, and the idea came from the point distributor cap for the odd fire Buick V6, which fired not 120, 120, 120, 120, but 90, 150, 90, 150, with a set of long wire terminals in its points distributor caps.
Thanks, Dave. Yes, there is vacuum advance in this HEI. I really appreciate your post. You set my brain free from a wild goose chase. Now on to the next wild goose chase on my list...
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post #13 of 28 (permalink) Old Jun 11th, 19, 8:03 AM
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Re: HEI rotor marks off center

MSD has adjustable rotors, but I think they're for small cap distributors.

And Dave is absolutely right as per usual. the timing does not depend on the rotor position. on an HEI the timing is determined by the relationship of the "points" or "tits" of the reluctor wheel and the pickup coil which can be considerable different from when the rotor tip is aligned with one of the contacts in the cap.

Ever see the cap contacts in a large cap V-6 HEI. they've got like tabs 3/4" long to help with alignment with the wacky odd-fire or semi-odd fire setup.

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post #14 of 28 (permalink) Old Jun 11th, 19, 8:05 AM
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Re: HEI rotor marks off center

here's one for the MechE guys. if a spark has a duration of say 3 milliseconds how many crankshaft degrees of rotation of the rotor does that cover?

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post #15 of 28 (permalink) Old Jun 11th, 19, 9:06 AM
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Re: HEI rotor marks off center

Quote:
Originally Posted by Tom Mobley View Post
here's one for the MechE guys. if a spark has a duration of say 3 milliseconds how many crankshaft degrees of rotation of the rotor does that cover?
I'll say a little bit. That's my engineering answer.

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