How do you lockup the Advance in a SC Distributor

[slow car]

What is the best way to lock up a 82 SC dizzy when you go to a programable system?
Thanks
john

[Grady Clay]

John,

I assume your programmable system uses a crank pulley or flywheel sensor.

You normally want the distributor to continue to advance.
This allows the rotor to meet the particular spark plug terminal in the cap.
Since you are no longer using the distributor to control spark timing, there is no downside to having the mechanical advance control the rotor advance.

Typically you have about 30º (crankshaft) of advance. This is 15º in the distributor.
The end of the rotor is wide enough to accommodate some mis-match but 15º can start to push the limit.
If a serious mis-match occures, you get 'cross-fire' where the wrong sparkplug fires occasionally.
If it is the sparkplug ‘just after’ in the firing order, there is a simple misfire.
If it is the sparkplug ‘just before’ in the firing order, the cylinder is just starting the compression stroke and fires (maybe) an additional 120º ahead of intended.
This is hard on the engine.

Best,
Grady

[slow car]

Hi The MSD 6530 is only programable to advance timing so there is no need to have mechanical advance. so i need to lock up the distributor some way?
John

[jpnovak]

John,

There is no simple solution to this...

The solution to your problem requires that the trigger signal come from a fixed point, distributor shaft or crank, while allowing the rotor to mechanically advance independent of the trigger signal. This is the only way to ensure that the rotor lines up with the cap at the point of ignition fire.

If you measure the width of the rotor contact, the width of the cap contact and then the radius of these two points you can calculate the effective dwell time where the distributor rotor and cap are in alignment (phase). If you lock the rotor relative to the shaft it will not have a long enough dwell in crank degrees for them to fire correctly once you consider the total advance required for your SC.

If, during your total retard you fire the spark and the rotor contact is mostly past the cap from an alignment point of view the charge will dwell until it gets to the next plug contact and then there will be a partial mis-fire since it neither fired N1 or N2 in the sequence.

There is a thread many years ago related to this topic to use the locked distributor to trigger an EFI system with integral spark. The poster (Tony - tbitz) ran into mis-fire problems with the locked distributor and abandoned the trigger idea.

The system would likely work for a twin-plug setup since the total advance requirement is much less than a single plug solution.

FYI, I have thought about this problem quite a bit and to reliably trigger without having a misfire is beyond the system capability. It will work but it is outside my comfort zone. I agree there are significant gains to be made by modifying one's advance curve.

you would do better to install a 3.2 flywheel and use a VR sensor or add a crank pulley with toothed wheel or magnetic hall effect trigger in addition to your distributor.

If you really want to try this the only way is to disassemble your distributor, remove the mechanical advance post and epoxy it to the shaft. Of course, you could also weld it up. Then you have to reassemble the magnetic coil and reluctor onto the top shaft.

Look up Gunter's post about disassembling the distributor on the bench for more information about the internals.

[sc_rufctr]

This is worth a read.

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Support/frm28392_tech_bulletin_rotor_phasing.pdf

[sc_rufctr]

Some questions that may help the OP.

Does the 3.2 distributor have a mechanical advance?
Does the Motronic computer in the 3.2 adjust the timing of the spark?

[stevie 77 930]

Have a look at this thread on the 930 side....This may help explain what you want to do

MSD Success

[Steve@Rennsport]

Originally Posted by sc_rufctr

Some questions that may help the OP.

Does the 3.2 distributor have a mechanical advance?
Does the Motronic computer in the 3.2 adjust the timing of the spark?

If I can,....

1) The weights & springs inside the 3.2 distributor are only for rotor phasing and camshaft position reference.

2) All of the timing functions on Motronic cars is done in software.

[EarlySport]

If you lock the distributor in the full advance position and leave the distributor position the same the only rotor phasing issues you 'might' have are at low speed. At high speed (when full mechanical advance is in) the distributor and phasing are exactly the same whether locked or not.

So I guess it depends on whether or not some misfiring at low speed is of importance to you.

I have run a locked distributor for a few years now with MSD electronic timing with no mis-firing.

Cheers,

[slow car]

Craig,
What MSD unit have you got?
How did you go about locking the Dizzy?
What curve are you running?
Cheers
John

[racing97]

The Motec guys used to tack weld the counter weights together, Just a light tack.

regards

[Gunter]

To be clear:

The question is for an '82 distributor which has a mechanical advance plus vacuum advance/retard.

Is the engine still stock? With CIS and all Lambda components in place and functioning?

If any changes were made, what are they?

What is the actual set-up re: Intake, CR, cams etc.?

What do you expect to gain with a locked distributor and MSD?

[brads911sc]

Why dont you call Barry Hershon, tell him what you are trying to do... Perhaps he can help you or at least offer some advice. Also, if your 82 Dizzy has never been rebuilt, it probably needs it anyway...

[EarlySport]

Originally Posted by slow car

Craig,
What MSD unit have you got?
How did you go about locking the Dizzy?
What curve are you running?
Cheers
John

The dizzy is tack welded in the full advance position
I am using an older MSD timing computer with a 6AL. The timing is set via pots. It's a pretty simple advance curve from just after idle to all in at 5K from memory. I intend to change to a 6AL-2 for my next engine build.

Cheers,

[PrimeMvr]

I locked mine in full advance, both the mechanical and vacuum/boost, but I also indexed it back one tooth on the drive gear. This gets me 26* when the dizzy is halfway in the slide. The MSD takes care of backing that advance out at lower RPM and on boost (930).

I have put a timing light on the 5 and 6 wires and there is no crossfire. If there was, I'd see them fire when the cyl 1 timing mark is lined up.

I don't know the inards of a 82 SC dizzy, but once you get it open it should be pretty clear what needs to be done. My method of shimming the rotor mechanism and tapping a screw into the stator (magneto? not sure what it's called) that is shown in my MSD Success thread stevie referenced above isn't the most elegant, but it works and fits my very limited machine work skills.

EDIT: I use the stock magnetic trigger in the dizzy. The 6AL-2 can use a hall effect sensor or ECU trigger as well.

Ripping your $$$ dizzy apart isn't for the faint of heart, but it isn't difficult either. Good luck!

Steve