Distributor rotor phasing

[tubman]

I was talking to a friend of mine who is an electronics expert. Several years ago, he was involved with an outfit that was building engines for ARCA racing. He was telling me about a problem they had with distributor rotor phasing with some highly modified engines. Essentially, the distributor had been modified enough that the rotor was not pointing directly at the terminal in the cap, causing high-speed misfires. They solved the problem by slightly re-positioning the drive gear on the input shaft.

This leads me to wonder if this might be a problem with SBC Chevrolet distributors converted for 8BA use. Perhaps I am overthinking this, but it seems that, unless some care is taken, the rotor might just end up pointing between two terminals in the cap, causing problems at different portions of the advance curve.

Bubba, have you ever encountered this?

[Kerk]

Many years ago when I was involved in a race car we advanced the cam timing by enlaging the holes in the cam gear and using an offset piece.Ran the engine between 5000 & 7500 RPM !! It worked for us.

[PeteVS]

I would imagine that this would entail the rotor being lined up with a cap terminal at the time the points would cause the spark to fire. I'm wondering if there's a delay in the firing taking place so that at high speeds the rotor would be "leaving" the cap terminal by the time the spark got there? (I never really did get a good understanding just how that all works.)

[BUBBAS IGNITION]

Rotor phasing is needed when the cam lobe ( distributor cam lobe) has changed relationship to the points open position ( or electronic signal target point in electronics). Its important that the spark stays on the rotor thru the full movement,
We check this phase only when rpm is really high 8-10,000 rpm or some trick machine work has been done. Example installing a pick up coil in a older distributor. We would verify the firing point on a machine where the distributor cap would have holes in it and a timing light is used to show where the spark is at in relation ship to cap terminal. Use it quite a bit with the older mallorys that are sent in as the distributor cam its self can be removed seperately. No too hard to do with the distributor machine firing lights etc.
Only needed with mechanical advance distributors.

The drive gear would have no effect on this service!!

Your friend has forgotten how this actually works!

https://www.youtube.com/watch?v=aWMlNwGW0tM

[tubman]

Bubba,

I see what your saying. I know he said they moved something; I probably heard it wrong.

If I understand what you're saying, as long as the distributor cam, points location, and cap terminal location stays the same (as it would in an SBC conversion), this is a non-factor.

[JWL]

Sponsored Links (Register now to hide all advertisements)

Bubba is correct in that changing the drive gear location would not alter a rotor to cap phasing problem.

I have wasted a number of hours examining and perfecting rotor phase with zero performance enhancements resulting. This includes engines which were operated above 10,000 RPM. MSD even offers(or offered in the past) adjustable rotors to make phasing easier to accomplish.

For peace-of-mind it is another thing to examine when there is a miss-fire condition. But there are other, more likely, areas to look first.

[tubman]

It looks to me like this started out and ended up as a theoretical discussion with no practical implications.

[Bored&Stroked]

Bubba, JWL and others are correct as far as it not being an issue when the relationship between the points, point cam, cap and rotor have not been changed. However, there are some modified distributors where this does have to be accounted for (think Lincoln V-12 dual-coil distributors - converted for V8 usage). On these distributor conversions there were typically two methods used to get the rotor tips (two of them) in phase with the cap --> by soldering special brass strips/wires to the rotor or to the side caps. The reason was that the rotor would not be clocked in the correct place once the conversion was done, so either the rotor or the cap needed 'extended contacts' to get them back in alignment.

Where it is super important (think MSD instructions) is related to when you use something like a crank trigger to fire the ignition. (It is up to you to phase it correctly). If you don't correctly phase the rotor, you can have a cross-fire situation or you may be jumping the spark a huge distance and have performance issues, or you might burn up the cap/rotor from all the heat generated.

At Bonneville in 2013 we had a rotor phasing issue (my best guess) on the FlatCad on my 175 mph return run. The car was staged at the starting line, me in it . . . ready to go make a backup run. We'd set a very nice record the day before. Well, the car started and then died - we could not get it to fire again. So much for that record run! I'd phased the rotor before, but we'd monkeyed with the crank trigger and timing - and I forgot to rephase it.

When we did a post-mortem, found the end of the rotor was gone - as well as the brass tang that picks up the spark form the coil. Burned right off. This was a very high-output ignition and we learned a very big lesson . . . the hard way as usual!

[BUBBAS IGNITION]

Originally Posted by tubman

Bubba,

I see what your saying. I know he said they moved something; I probably heard it wrong.

If I understand what you're saying, as long as the distributor cam, points location, and cap terminal location stays the same (as it would in an SBC conversion), this is a non-factor.

Yep .

[Bruce Lancaster]

Early Fords, '32-48, had solid shaft from rotor right back to the camshaft. Advance change involved distributor cam rotating on shaft without affecting rotor. Phasing stayed right where Marion Mallory told it to stay in 1932.
'49-53 also had locked in phasing, with advance handled at the plate.
One less thing to worry about.

[BUBBAS IGNITION]

Lets discuus this action a little more :

Lets say on a V8 engine that the distributor cam lobes are 45 degrees apart. Then lets take the 36 degrees of dwell per point open and close. The points would be open 9 closed for 36 and then open for 9 degrees .
Added to the circle the points would be open for 18 ( two sides of the 9 on a lobe) closed for 36, then open for 18 again.
18 deg for spark and 36 for coil saturation.
So our spark duration in this example would be 18 degrees distributor ( 36 degrees engine as distributor degrees are x 2 for engine).
So in a good factory design the rotor could advance spark approx 18 degrees with out being out of phase a couple degrees allowing spark to be timed with rotor tip. Too far and it will cross fire or miss ...

[Ol' Ron]

Having a distributor machine and a cap with a hole in it,is what you need. I use this when converting the SBC dist.

[revkev6]

Originally Posted by Ol' Ron

Having a distributor machine and a cap with a hole in it,is what you need. I use this when converting the SBC dist.

or in my case a drill to spin the distributor.. .you guys and your fancy pants machines

[scicala]

I know this has nothing to do with the issue, but when I was 18 I had a '55 mercury with a 292 Y-Block and a Mallory dual point (4 lobe cam). I ran into an issue and an old time mechanic where I worked showed me that on Mallory distributors (not all I think), you can grab the rotor and rotate it without the shaft rotating to reposition the rotor to phase it with the wire terminals on the distributor cap. It was pretty stiff, so won't move on it's own.

Sal

[flatheadmurre]

Isn´t issue just that you got a limited amount of advance to use.
So when you need a lot of advance and a wide rotor tip you get arcover...
Solution bigger is better...atleast when it comes to rotorcaps...a degree becomes a longer distance for the spark to travel inside the cap.
A dualplane rotorsetup would be one way to overcomplicate things

[Richard in Florida]

Originally Posted by BUBBAS IGNITION

Rotor phasing is needed when the cam lobe ( distributor cam lobe) has changed relationship to the points open position ( or electronic signal target point in electronics). Its important that the spark stays on the rotor thru the full movement.

In the SBChevy conversion, rotor phasing, the relation of the rotor blade to the plug post inside the cap, is determined by the position of the "breaker plate" inside the distributor. If you're running only mechanical advance, you can tweak the position of the plate by "slotting" the hold down screw holes so you can turn the plate a bit. This works great unless your rotor blade is WAY off; then you have a different problem.

Vacuum advance advance works by rotating the breaker plate, so the best you can do is end up with a compromise relation between the rotor blade and the plug post. By elongating the mounting holes on the vacuum canister, you can reposition it and the breaker plate with it. With my street engine, that worked just fine.

And yes... I miss my flathead.

[BUBBAS IGNITION]

Originally Posted by scicala

I know this has nothing to do with the issue, but when I was 18 I had a '55 mercury with a 292 Y-Block and a Mallory dual point (4 lobe cam). I ran into an issue and an old time mechanic where I worked showed me that on Mallory distributors (not all I think), you can grab the rotor and rotate it without the shaft rotating to reposition the rotor to phase it with the wire terminals on the distributor cap. It was pretty stiff, so won't move on it's own.

Sal

Actually thats a major problem with the Mallorys the distributor cam is held down with a crows foot clamp and a single screw. This allows proper rotor phasing if needed etc. Problem is after many years of running and a swap meet or two it can get out of phase.
Incorrect phase often wont even run as the rotor gap is too large etc or in correct.
In our case we do a lot of this Mallory and i put a paint mark on every case and the check the rotor phase on a distributor machine. Maybe 3-4 out of ten needs to be readjusted......

[revkev6]

Originally Posted by BUBBAS IGNITION

Actually thats a major problem with the Mallorys the distributor cam is held down with a crows foot clamp and a single screw. This allows proper rotor phasing if needed etc. Problem is after many years of running and a swap meet or two it can get out of phase.
Incorrect phase often wont even run as the rotor gap is too large etc or in correct.
In our case we do a lot of this Mallory and i put a paint mark on every case and the check the rotor phase on a distributor machine. Maybe 3-4 out of ten needs to be readjusted......

or some idiot like me sees that crows foot and thinks that's how you are supposed to take them apart

[BUBBAS IGNITION]

Originally Posted by revkev6

or some idiot like me sees that crows foot and thinks that's how you are supposed to take them apart

Yep i get them with the cam in one box and the distributor in the other box etc. Ya know someone has had it apart .
I now know to just take a minute to check them. Next one if get it i will do a post on it and show how we check it and what screw NOT to loosen when working on it......

[scicala]

Thanks for the added info Bubba.

Sal