Magneti Marelli S135 Distributor

[rjplenter]

With special thanks to Steve Cecchele I now have a Marelli S135 dizzy for the new engine build.

It's in very nice condition so just a quick spruce up is all it needed. I figured while I had it apart I would perform some measurements and calculations to exactly ascertain it's specifications.

The S135 is a centrifugal advance only unit, i.e. no vacuum advance or retard. Since I will be installing a capacitive discharge ignition system with an optical trigger I want to be sure it is set up correctly, I won't be able to rely on the distributer shaft cam and points to set the timing correctly. The optical trigger can be a variable and must be installed just right.

First I measured the advance:



As you can see at the dizzy the advance is 14 degrees, which is 28 crank degrees (the Haynes joke book states 28 +/- 2 degrees). Therefore, a static initial advance of 10 degrees BTDC will result in a total advance of 38 degrees.

Next I measured the angle of the rotor blade and found it to be 19 degrees (38 crank) and the posts in the cap are 12 degrees each. That means that the rotor blade will be passing each post in the cap for a total of 31 dizzy degrees (62 crank degrees). That allows for some variation which is nice, but it would be best to set the optical trigger to fire as the rotor blade is passing the meat of the cap posts. It also allows for increasing the centrifugal advance if required. For now I will leave the centrifugal advance mechanism as it is.

One unexpected measurement did reveal itself though. The backlash from the auxiliary shaft to the distributor shaft is 2 degrees, or 4 crank degrees. I wasn't expecting that and I'm not sure if it's acceptable or desirable or if it can be reduced. My auxiliary shaft gear and dist. drive gear appear to have minimal (if any) wear and I can't find any specs for backlash in my workshop manuals. Does anyone out there have experience with that?

Cheers,

Rob

 

[fiatfactory]

If you look closely at the slotted advance plate there will be a "14" stamped on there, so yes 28 degrees of mechanical advance.

A bit of lash is usual, but once the engine is running the lash is generally taken up and timing variance disappears. What you need to watch for is 'spark scatter'from the drive gear lash and the oil pump loading / unloading as pressure holes are covered / uncovered ... any wear in the splines (internal and external) will allow the timing to 'float' back and forth ... not ideal at all.

That's why crank triggered ignitions are the bomb...

But... for your level of build (under 100hp/litre/8k rpm) the S135 distributor with an optical trigger will be just fine... just make sure you've got a good coil and plug leads to maximise the spark intensity, the stock ignition curve should be more than adequate too.

and the 'before' pictures would be these ones...





SteveC

 

[rjplenter]

14

Thanks Steve,

I did see a 14 stamping somewhere, so that makes sense. But you know me, always measuring so I know just what specs I have, especially when the Haynes quotes a 4 degree variance. I'm hoping in operation the lash I measured won't be too much of an issue. It hurts my head to think I will be trying static advance between 5-10 degrees BTDC but may have 4 degrees of float! :doh:

What I should do is send my dizzy to TonyN so he can run it up on his newly acquired Sun machine and measure the rpm/advance curve for me.

I have the coil recommended for my ignition module and good leads etc. I'll be setting my plug gaps in the 0.038" to 0.040" range.

I will post after pics, but they won't be Yves-worthy. Internally I'm shooting more for reliable service than bling. And I'm not foolish enough to try to compete with Yves!!!

Thanks for your help!

Cheers,

Rob

 

[rjplenter]

Setting up the CD ignition optical trigger

To get the most out of my capacitive discharge multi-spark ignition system the optical trigger needs to be set up correctly.



The black marks on the circumference of the distributor body correspond to the cap posts in both position and size.
Although there is an indicator line on the optical trigger a dab of bright paint makes alignment much easier.

First, position the leading edge of the rotor blade with the leading edge of the cap post remembering that the rotor moves clockwise when viewed from the top.

Then position the trigger at the leading edge of the gap in the chopper. This ensures that the spark starts as the rotor blade passes the cap post. It also helps with multi-spark systems so that multiple pulses can be relayed to the spark plug. Careful positioning also reduces the risk of cross firing to another cap post, which probably happens more often that we would like to think. Those errant sparks rob voltage from the intended plug and power from the engine.

One aspect of the Crane system that I don't particularly like is the mounting bracket for the infra red head. If you do it the way that is suggested in the installation instructions...

...you get something like this...


The yellow arrow indicates the head mounting to the two piece bracket. The green arrow indicates where the two parts of the bracket are screwed together, and the red arrow shows where the lower bracket fastens to the plate in the distributor. I don't like it because the brackets are a little lightweight and the distance from the plate mounting to the IR head may be subject to vibrations.
Instead I prefer to mount the brackets like this...

This shortens the brackets and attachment points. And if at all possible I also like to incorporate some form of stabilization, but haven't got to that point yet.

Perhaps the biggest advantage with a system like this compared to the original points, besides reduced servicing and having higher and more reliable voltages, is the timing components can be synchronized quite precisely to maximize the spark potential. As specifications of a non-standard engine build deviate further from the norm, i.e. higher compression etc., so must our spark work harder. To produce a bigger spark to get the high compression mix burning the spark plug gap must be widened which means we also need a higher voltage to bridge that gap. Bigger voltages can result in cross-firing and precise positioning of the components can aid in reducing that likelihood. Multi spark systems also help, although these are most effective at lower RPMs, as RPMs rise there is less time available for multiple sparks to be generated.

More to come...

Cheers,

Rob