Megasquirt timing when using a distributor?

[Jon98064]

I have MSD Billet distributor with MSD 6A 6200 and planning Megasquirt II ignition control
I am aware of http://www.megamanual.com/ms2/msd6a.htm

But I cannot see how this could work in principal?

Once the mechanical and vacuum advance are locked out of operation in the distributor,
and the initial timing has been set and the distributor housing has been locked down,
then AT ALL TIMES,
the relative angular position of the rotor button with respect to the crank will remain fixed and
the relative angular position of the rotor button with respect to the distributor cap will remain fixed

Therefore the timing cannot be changed if the plug leads remain in the distributor cap.

So how does it work?

[Jon98064]

I finally figured it out for myself and then discovered this is called Rotor Phasing.
My opinion is that this is buried way too deep in the Megasquirt manual .
All that is required is to state right up front that an "ECU controls ignition timing by varying the rotor phasing".

I am planning on using an almost new MSD Pro-BIllet distributor already in the car.
But now I see that this has a fairly narrow rotor tip but my estimate is that the tip can deal with a range of 15 deg distributor degrees which corresponds to 30 deg crank angle.
However getting the full range will require a special distributor cap sold by MSD where the phase of the tip can be adjusted relative to the spark lead locations in the cap

Without this special distributor cap the rotor phasing will go from zero in only one direction giving only around 8 degree distributor or 16 degree of crank angle.
NOT GOOD, even with initial 10 degrees advance.
With the special cap the initial rotor phasing can be adjusted to be one side of center so that the ECU will pull the tip through the center and out the other side of the spark lead connection

I have not been able find ANYTHING at all about this anywhere?
Nothing about it here? http://www.DIYAutoTune.com/tech_article ... butors.htm

Understanding this problem explains another reason why crank timing is better but more difficult
Again, attempting to figure this out for myself,
Better: Because the rotor phasing of the distributor can be adjusted independently by simply rotating the distributor housing.
which allows the range of rotor phasing with a standard cap to operate +/- the center position?
More difficult: How to set the initial phase?
May be accurate enough to set the phase statically?
Cut a hole in a spare cap and set the phase angle by watching with strobe?

Does this make sense?
Am I missing something?
Is there some rotor with a wider tip that can be used with MSD Pro-BIllet distributors?
Should I just throw away the new distributor and get something else?
Any advice anyone?

[Jon98064]

Here is the MSD link for kit to adjust Rotor Phasing
http://www.msdignition.com/WorkArea/Dow ... 5032386563

[Bernard Fife]

All that is required is to state right up front that an "ECU controls ignition timing by varying the rotor phasing".

Jon,

Your statement is not correct.

Rotor phasing is an interesting topic, but it does not determine the spark timing (it only determines if the spark produced by the coil will make it to the plug or not).

The spark timing is set by the when the primary current to the coil is cut. Primary current is what the ECU controls: the time the current is started, and the time it is cut. The total time between these is the dwell. This is also what 'old school' points controlled - dwell and timing by setting the start and stop time for the coil's primary current.

As long as the rotor tip (and associated plug terminal inside the cap) is wide enough, the spark will be sent to the right plug as long as the current timing advance retard is 'reasonable'. Rotor phasing becomes important mostly when components can't be made to line up properly because of custom components and/or extreme timing values.

Lance.

[trakkies]

It's usually easy enough to do on a four cylinder - but can be more tricky on an 8. One way to check is to cut a hole in a spare cap and use a strobe (timing light) to freeze the rotor position.

[Jon98064]

I understand that rotor phasing does not determine the spark timing
But rotor phasing IS EXTREMELY CRITICAL to ensure the engine will EVEN RUN
i.e. that the spark produced by the coil makes it to the plug.

AND I did not find this out by reading the Megasquirt manual which I suggest is lacking in this area.
For example, rotor phase is NOT EVEN MENTIONED in the section on using MSD Pro-BIllet distributors?

I think it will be best to use a crank trigger because of the narrow rotor tip MSD Pro-BIllet distributor

A crank trigger will allow adjustment of the rotor phase, so that it ranges from negative to positive angles away from dead center alignment of the rotor tip with the plug lead socket.
Another alternative for this would be to adjust the MSD distributor phase using the special MSD cap for this purpose.
But the crank trigger will be better from so many other points of view.

Without the crank trigger or without the special MSD cap to adjust rotor phase,
the rotor phase range will start from zero, thereby halving the available range, which will then be borderline, at least in my opinion.

[trakkies]

Using either a crank trigger or an internal dizzy trigger, you still have to set the rotor phasing. With a crank trigger, you'd do this by simply turning the dizzy body. With an internal trigger, the mounting plate for it has to be locked in the right place.

But if you're going as far as a crank trigger, you've done the difficult part of converting to dizzy less ignition like wasted spark. Which is a better solution as dizzy caps and rotors ain't ideal.

IMHO, covering mods to a dizzy when using mapped ignition is not an MS specific thing, and some general knowledge of such things is assumed.

[Jon98064]

But wasted spark on a V8 will require 4 coils?

Nice, but the line has to be drawn somewhere.

Else it would make sense to keep going and swap the classic small block Chevrolet for an LS1,
in consideration of all the other benefits too, such as native EFI, lightweight etc

[trakkies]

You could simply use the MS fuel only - then upgrade to a decent ignition system later. If a dizzy with mechanical advance isn't good enough, best to go the whole way and get rid of it completely. Used EDIS coils are cheap.

[Vermillion06]

I understand that rotor phasing does not determine the spark timing
But rotor phasing IS EXTREMELY CRITICAL to ensure the engine will EVEN RUN
i.e. that the spark produced by the coil makes it to the plug.

Isn't rotor phasing just your base timing, without any timing advance added to it?

[Matt Cramer]

Rotor phasing is not the timing itself, but the adjustment of what portion of the crank rotation that the rotor is in contact with the distributor cap and able to deliver a spark.

[trakkies]

If you must use a dizzy - perhaps for use on a older car to retain the looks - it's easier to leave the mechanical advance still working, and trigger from the crank. That way, the rotor arm will say (closer) in phase with the cap contacts. I experimented with a locked Lucas dizzy on an 8 cylinder engine, and never quite got the rotor arm phasing perfect. It will obviously depend on the individual design, but if you increase the size of the rotor arm contact to allow for a wide range of advance, the danger of cross firing increases too.

Using a crank trigger gives much more accurate and consistent timing.