3.2 distributor mechanical advance, why???
 

hi All,
something i have been wondering about for a while, why does the 3.2 carrera still have a mechanical advance weight system in the distributor, when it clearly cant have any affect on the timing as there are no electronic parts and no wiring, hence the only reason the distributor is still in place is to distribute the spark to each individual spark plug. If this is the case, why wasnt the shaft locked up at the factory and the weight and spring system removed ?
Please if anyone knows, i would be very interested, and if i am wrong, be gentle with me...:)

Anthony.

Read post #5
http://forums.pelicanparts.com/porsche-911-technical-forum/619982-911-owners-who-has-looked-mechanical-advance-their-distributor.html

The plain and simple of it:
The advance mechanism makes it so the rotor is still aimed at the distributor contact when the ignition advance kicks in.

Rotor phasing. Even though the timing is controlled by the ECU, the rotor still has to be pointed at the terminal on the cap roughly speaking for the spark to jump across. If the factory were to lock the distributor, you would end up with possible misfires at the extreme ends of the advance range (which is a wide range).

hi,
Thanks for the reply, i did think about this, but having worked on many diffrent types of cars over the years especialy from this era, porsche seem to be amongst the extreme minority that felt this was necessary, as none of the other engines from major manufacturers i have worked on from this era felt it necessary to still incorporate the mechanical advance when they went over to the early ECU systems.
I just wonder how much diffrence it realy makes...
Anthony.

Let's see here. If the rotor tip points directly at the appropriate connector in the dist. cap at rest. Why would it have an easier time if the rotor advances, e.g. 30º further away at 5000 rpm (2500 dist. rpm)? Is there such a delay in spark creation that the separation provides enough time for the spark to jump at the most opportune time?

Just wondering.
Sherwood

I wondered the same thing when i first looked at my 3.2 distributor. I think the advance is needed or at least helpful because of the relatively small diameter cap (terminals close together) and large advance numbers especially at light throttle.

When BMW went to Motronic they used a fixed rotor on the camshaft but a very large diameter cap.

Interesting...
A...

Just curious, what other cars of the period have no advance mechanism? Not doubting you, just trying to remember.

Porsche even used rotor phasing/advance on the 993.

Only Porsche and possibly BMW was still using distributors (antiquated technology) then, while everyone else was on coilpacks or coil on plug ignition.

Correction of my earlier post #6.

Since the distributor only rotates at 1/2 crank speed, if the crank timing advances 30º, then the distributor rotor only rotates 15º from its at rest position. Still doesn't explain why the rotor advances away from the plug wire contact instead of staying aligned with it.

Sherwood

Hi Sherwood,
we seem to be in a minority here who feel that this subject needs a proper explanation, maybe we think too deeply...:D
Anthony.

You're thinking about this inverted.

The rotor isn't advancing away from the contact, it's advancing toward it at the time of ignition.

The ECU controls spark timing. Under full throttle and high RPM, it's going to fire the coil 35 degrees ahead of TDC. With a locked distributor, the rotor points at the contact only when the cylinder is at TDC. (Or wherever you lock it) Which means that the coil would be firing when the rotor is 17.5 degrees ahead of the contact. The rotor phasing simply moves the rotor closer to the contact in proportion to high RPM.

Is this perfect? No, because under part throttle conditions and light load, the Motronic box can fire with pretty large advance values e.g. 40+ degrees. That increases the distance the spark has to jump inside the cap which affects ignition performance-- but under those conditions, it's not as critical as it is at high RPM and full throttle.

A fun visualization of the DME's advance values here: http://www.911chips.com/ignition.htm

I wonder if you can use optronic ignition with the standard 3,2 set up?
The optronic ignition is a propeller(6 blades,one for each cylinder and a optical eye)

Any knows if this can be used together with the original DME system?

Why go backwards? The DME already uses highly precise speed an reference sensors on the flywheel for ignition calculations and timing.

Hi 304065,
[Strange name:D]
Interesting read, as you say, its not a perfect system, and when worn, or not functioning as it was when new, its operation is nowhere near as critical as it was when the engine relied heavily on the acuracy of the weight and spring system for optimum engine ignition timing.
Anthony.

But I am certain that the wizards of Bosch have calculated the rotor phasing at rpm much better than we could have imagined. I seriously doubt they put this out without some thought or laboratory testing.

Of course, with our limited knowledge, we can always think we know better.

"When BMW went to Motronic they used a fixed rotor on the camshaft but a very large diameter cap"

Right, and a larger rotor as did Mercedes on the 103/104 in-line 6 cylinders,
which eliminates the rotor movement requirement.

Bottom line: As explained by others on this thread, use of a mechanical system
(distributor) and an electronic ignition timing (DME ECM) requires synchronization,
thus a moving rotor or a larger one.

Hi loren,
First of all, thanks for confirming my initial statement that there were indeed other cars of that era that used fixed rotors, [thought i was going mad for a moment there] also, thanks for taking the time [like others here] to give your understanding of why this was done.:)
Anthony.

Realize the discussion here is purely theoretical. Obviously, the system works as is, but I find it interesting on that level.

Given that during static timing, the rotor tip coincides with the spark plug contact on the cap; the rotor rotates independently of the dist. shaft due to the centrifugal weights and springs at a set rate and finite distance....

DME advances the spark timing according to predetermined parameters; thus as rpm increases. However the rotor remains independent as in previous points/coil systems and simultaneously rotates in an advanced direction, away from the spark plug contact, not toward the referenced position at idle (TDC). The only time the rotor might move toward the contact is when the static timing is set ATDC. However, thereafter, the spark (and rotor) move away in an "advanced" position.

Obviously, the max. distance the rotor moves isn't sufficient to affect the ignition system.

Did the engineers plan this to increase the rotor-to-distributor contact gap to increase the secondary voltage as engine speed increases? Perhaps so. However, increased secondary voltage is usually needed at low speed rather than high speed. This can verified by observing the secondary voltage requirements on an ignition analyzer while the engine is under load, especially on a dynamometer.

In order to increase the secondary voltage, one can merely increase the spark plug gap instead of working backwards, i.e. increasing rotor tip gap via centrifugal advance weights. Eventually, modern engine management systems would contain all ignition control via software commands and thus an independently rotatable rotor (if one is used at all) is no longer needed.

I'm probably missing something.

Sherwood