View Full Version : Distributor - Dumb Question

07-13-2007, 12:14 PM
in converted ignition system (converted to computer controlled via crank trigger), you lock out the mechanical & vac advance, and let the computer do it..... but how does the rotor align then?

for instance, suppose you idle at 12* advance, and right there, you have the rotor pointed to the appropriate plug wire. with a physical advance system, the rotor actually advances and sparks when aligned still.

but... when it's locked out, it's still aligned at 12*, but sparks via computer at say 34*. quick geometry calc suggests that on a 2.5" diameter circle, the delta on the circumference for that 20* is nearly 1/2". does the spark just jump?

sorry i'm so thick.... :rolleyes:

07-13-2007, 02:16 PM
i guess what i am talking about is 'rotor phasing'. generally does not require any adjustment, except on computer controlled ignition, needs to be phased.

i called MSD support and struggled with my conversation. i explained that it would be crank triggered with advance controlled by megasquirt. he said that you set your crank to your advance, align the distributor and tighten up. his example was that if you want 40*, you set it there, and that's your advance. "no curve for RPM?" i asked. "no, 40*".... we were talking past each other. i thought i articulated myself well, so either i really don't get it, or he doesn't. hmm. hope it's me...

so i found an MSD tech bulletin on rotor phasing. it had a section on computer controlled advance. says that on a locked distributor, you essentially phase it so that the rotor gets phased so that you split the difference between your range around the center. the example they showed was a 16* difference. okay.. so that seems like a reasonable explanation.

but... what about if you have 20* of mechanical advance, and another 20* of vacuum. i just don't see how that much advance could be phased on a locked out distributor?

you lock out the distrib, and you set your two advance maps, one based on RPM, and the other based on MAP. how does potentially that much advance not jump to another cyl? or for that matter, how do you not start to lose quality when you are not in the center of alignment?

why even lock out the rotor? it seems that if you had mechanical advance and vacuum advance hooked up, and those curves were similar to your ECU maps, then you would be sparking near the center of alignment?

anyone following me?

07-13-2007, 03:34 PM
The one thing that your are asking is why lock-out the dist. The computer doesn't want or understand any advance other than what is mapped into it. I suppose you could get it to work with the distributor unlocked, but why? The advance maps can be programmed exactly as your setup needs and it will always be that way, no need for vacuum or weights.

07-13-2007, 06:57 PM
... I suppose you could get it to work with the distributor unlocked, but why? ...

because the rotor will not be aligned? the rotor is *fixed* with the crank when locked in. so when it gets locked my point is that doesn't the rotor get too far out of alignment with computer advance?

07-13-2007, 07:39 PM
That's what I think they meant about the 16 degree difference. As long as the rotor tip is anywhere around the correct lug in the dist cap for the plug being fired when the signal is sent to fire the coil, you would be o.k. I have a MSD boost retard system and it retards the spark without actually changing rotor phasing. I could see where if you have a big enough range of spark advance where it could be a problem though. The later HEI and other GM distributors all work basically locked out. In general it would be more important to keep the trigger wheel in exact time than the cap and rotor. I guess it would need to be checked. I'd say if you had a range of timing from around 10 to 40 (round numbers) you'd try to keep the rotor centered at 20, possibly a degree or two towards the range most used. If you dril a hole in the cap at number one you can check phasing with a timing light against the rotor. Basically when you change timing in a dist. car you are more worried about actually changing the trigger wheel to rotor, than cap to rotor. The stock phasing can't be exact anyway because you would be limited by the tooth count on the gear, or the spines on the dist. driveshaft . I hope this helps some, it made my head hurt!:rotfl:

07-14-2007, 03:25 PM
Don't forget, you're measuring crank degrees, since the distributor ( and cam ) turns at half that speed, your 20 degrees is now ten at the distributor. You mentioned that the diameter of the spark plug terminals on the distributor is 2 1/2", but most aftermarket use the large F*rd style cap, thats 5" in diameter., again, less likely to have problems of spark scatter.


07-14-2007, 06:58 PM
Yes... of course the distributor moves at half speed. forgot about that...

I had read something about aligning the rotor in center for the advance with the heaviest load. suppose i wanted 12 initial, 22 mechanical, 18 vacuum... i would think that centering it at initial plus mechanical would make sense, 34*. that way, 18 more in a high rpm low load or 22 fewer in a low rpm high load would be the extremes. and those degrees would be half on the distributor, so 11 degrees off center would be only 1/33 of the circumference in the distributor.

So, I can see how this would work fine (obviously, that's how everyone is doing it, it's just that now i get it).