back to ignition basics for me!

[twin plugged targa]

Thanks for your patience, I am still learning the internal workings of engines.
- background history, I am concerned that my ingintion system may not be setup correctly (detonation at high 4k+ revs). I will have some experts have a look at it, but before that, I wanted to get my head around it and perhaps get some more suggestions from the experts.
Butch Stokes of S Florida (fairly well respecte Porsche mechanic) said that my 964 twin dist. setup had the original internals?
Maybe I understood him wrong but I would have thought it would need the points and ignition curve from a 2.7 dist.???
My questions are:
-Is the ignition curve controlled/set by the springs and weights in the points or another part of the dist.
-Is it possible that my curve is wrong?
Next-
Is this a suitable setup and accurate enough ignition system for the car or should I be looking for a different setup to control the curve.
and PS wayne, my 101 book is in France with the bloody car

[img]

[Oldporsche]

You have a sort of nebulus type of question here.

I have a 964 distributor in the shop, and am about to perform an autopsy on it. I will know each and every detail of it before I'm done. I have been told that there are actually two models of this distributor. There are some differences in them also.

I see that you have an early air cleaner on your car. So it must have early MFI or Carbs. This makes a difference in optimal curve for your motor also.

I think the largest thing determining you optimal timing curve is the camshaft. When you start mixing induction systems with camshaft changes and throw in different combustion ratios you are looking at a little dyno time to determine the opitmal timing for your motor. As a base line I would start with the curve for the camshaft.

There are outside stand alone timing computers such as the MSD #8981 or MSD #8983.

I noticed that your distributor case has been modified for an adjustment to the hold down mechanism. I'm sure that it was taken apart before some heliarc work was done on the case. I don't know if there is room in there for a set of points, etc. I do know that my distributor has something in there but I've not taken taken it apart to see. I know that there are wires leading to the area under the rotor. I suspect that there is some sort of trigger mechanism in there.

I hope that this have been on some help.

Good luck,
David Duffield

[Lorenfb]

The 964 engine and its distributor are setup for a Motronic ignition system.
The Motronic system controls the ignition advance for the timing. There's no
points or any overall advance mechanism in that distributor. You may be
running at a fixed ignition advance. There's a very small (minor) spark
advance in the distributor, though, having little effect.

Question: What's being used to control the timing advance? If there is none,
or not properly designed, then that's why the detonation is occurring.

[emcon5]

The problem with buying someone else's custom job is trying to figure it out if things go wrong.

As you probably know, the 964 distributor doesn't have a trigger mechanism or advance in stock form. Spark timing and advance is handled by the DME. To get it to work in a non DME car, and advance mechanism needs to be installed, as well as some sort of trigger input to fire the CDI boxes.

There are CDIs in the picture, There are 3 likely ways of doing the trigger. Installing points, installing some sort of aftermarket trigger like Pertronix, or using an electronic trigger from an SC distributor.

The points and Pertronix methods will use the older style 3 pin CDI box, the SC type will use the later 6-pin CDI. The SC trigger installed in a 964 distributor looks like this:


In your picture, the green wire pluged into the side of the distributor looks exactly like the trigger wire from a 911SC. My guess is that if you pulled the connector from one of your Permatune boxes, you will find 6 pins.

I think your path is pretty clear:

1: Buy an inductive timing light, preferably with adjustable advance. This is a good one:
http://www.sears.com/sr/javasr/product.do?BV_UseBVCookie=Yes&vertical=TOOL&pid=00921023000

2: Verify that both CDI/Coils are working. Connect the timing light to any plug wire from the top cap and see if the strobe flashes when you pull the trigger. Repeat with any plug wire from the bottom cap. If you get a flash from one, but not the other, you may have a bad coil, or ignition box.

3: Connect the light to the #1 plug wire, start the engine and set your base timing. You really don't have any idea how much mechanical advance the distributor has, so I would probably start at 0°. Run the engine up to ~4000 RPM and a) make sure it does advance and b) note how much advance it has. If you start at 0° at idle, and at 4K it has 15°, you know that your distributor has 15° of mechanical advance. Subtract that number from 24°, and set that as your idle timing.

My non-scientific-wild-ass guess is that only one of your Permatunes is working, and you are only firing on one plug.

Tom

[twin plugged targa]

Thanks for the come back,
The car has series 40 webers, not sure what venturis.
It has custom crane cams which I have no info on.

Tom- what you said makes sense to me and I think the dist. may have an SC trigger- this has been suggested to me in the past.

Does this mean that it also has SC points?.

Are these points what sets the 'ignition curve'.

I have saved your info on the timing light, and will get one.

Are points like this the way to go? or would I be better to invest in a more tuneable ignition system where I can maybe adjust the advance/retard with more accuracy and obtain peak performance with least risk of detonation?

Which system would mate with my current setup with best 'bang for the buck?'

PS I did check for sparks on both the upper and lower plugs and both were present although I cannot vouch for the strength of the sparks. - My 1st thought when I had detonation was that the belt for the 2nd dist had broken.

Thanks again guys for the help.
rgds Ben

[Oldporsche]

I've been following this thread to find out all I can before I open the distributor.

Emcon's comment about the custom jobs is very true. Very often you have to perform an autopsy to find out how all the parts work.

From Emcon's photo, it shows a set of advance weights. I take it that this is where the advance could come from. From Loren's comment he indicates that the distributor has some sort of advance built in.



Thus you could be getting the advance from this mechanism.

The 3.0 SC and 76-79 turbo distributors rotated in a counter clockwise manner. Those weights in the picture would appear to work in a clock wise rotation. The 964 distributor initially was supposed to rotate in a clockwise rotation. The same as your 2.7L (and 2.0-2.4l). There is a difference in the drive gear on the 964 distributor from the early drive gears. I have it somewhere. The gears look very similar. If these advance weights are in your distributor as the photo shows, you could be getting too much advance. The way to check this is by putting your distributor on a distributor machine and patterning the advance curve.

I think we will find the reason why you are experiencing detonation at the higher rpms. I don't think you wil have to purchase anything. Just get all the components "act together".

BTW, does ths detonation act like the motor is misfiring? (High resistance spark plug wires?)

Enquiring minds want to know!
Thanks,
David Duffield

[Bill Verburg]

This thread may be of interest re. the 964 internals.

There is an advance mechanism in the 964 distributors. But it is only used for phasing the rotors.

The stock Motronic engine management usually used w/ theses distributors contains ignition maps on the chip. These maps control Motronic ignition curves.

Usually for this kind of twin plug setup an SCs ignition advance mechanism is modified to fit in the 964 distributor. Steve Weiner is the expert on this sort of thing, I believe that Thom(Emcon) has this set up on his.

In a pre electronic world advance was controlled by weights constrained by springs and vacuum signals from the intake manifold, centrifugal forces push the weights out levering the trigger plate into the direction of rotation, advancing the spark advance, the advance weights are constrained and returned by springs retarding the spark. Additionally a vacuum signal was used to further modify the spark advance curve, one modification was the neeed for additional advance under light throttle/ lean mixture. In this latter case the high part throttle vacuum is used to further advance the spark.

Points are merely one type of trigger for the spark, Points were last used on a 911 in 1977. For the SC and latter models various magnetic triggers have been used.

It is most likely that a mod done by or similar to Steve Weiner is being used. As has been suggested get a good timing light and see whats happenung to the advance as the engine is revved.

You may also wish to check that the dist belt is still intact and nominal.

[304065]

Interesting thread. Can somebody explain to me this: is the contact on the distributor rotor sufficiently wide that it touches the contact on the distributor cap for a fairly long time, and the ignition trigger, whether points, magnetic trigger or DME, fires the coil during that contact period? In other words, if you retard or advance the spark, how can you be sure the rotor is touching the cap when the coil fires?

Is that what the flyweights accomplish, "phasing" of the rotor?

[Bill Verburg]

Is that what the flyweights accomplish, "phasing" of the rotor?

On a 964, yes. On an SC no.

[Oldporsche]

This is why Steve Weiner says that the 964 distributor with twin MSD's are a good set up?

David Duffield

[emcon5]

I do have Steve's 964 conversion on mine, the photo I posted is my engine. Steve installs a centrifigal advance with the SC trigger. It is a good solution, with readily available tune up parts. They are also a bargain compared to what a Bosch RSR 12pt cap and rotor costs, if you can find one.

It is less flexible than a electronic coil-pack type igniton, but it is also much simpler, and for a low-tech induction system like CIS or carbs, that extra flexibility doesn't get you much.

The down side is that the drive gear on the crank needs to be changed on 3.0 L engines, and it is difficult to set the timing. Mine does not have a slot in the mount flange like a standard distributer, or Ben's does. To set the timing, you need to loosen a set screw and rotate the trigger plate under the distributor cap. Luckily you don't have to do it very often.

David, the early turbos the distributor rotates clockwise. I have one for smog checks, and the car runs fine on it.

Tom

[wot]

Hi

I see in your first picture that your distributor don’t have ventilation, are you sure the distributor belt isn’t broken?

[twin plugged targa]

eric/bill , the belt is not broken as I am getting sparks on both sets of plugs but you have put an idea in my mind to check the condition of the belt as maybe it is worn and slipping under high revs/load?
if this is possible?
I believe the dist's do not have the vent kit as I was told that there was not room for the kit in this application, I do not know if this is true though?.
rgds Ben

[Oldporsche]

Tom (emcon5),

I tried to look up the rotation of the distributor for the early turbos in my literature.

If the early turbos distributors were clock wise rotation, then the pick up units should be the ones used in a 964 conversion for a 2.7 or earlier?

This would be another clue as to the advance curve that could be in Ben's distributor.

Thanks,
David Duffield

[Steve@Rennsport]

Hi Ben:

That drive belt does not slip,........ Its either functional or broken.

We install the vent kit in ALL 3.6 distributors as its the only way to get acceptable belt life. I recommend belt replacement at 50K-60K miles, as well.


David:

The 76-77 Turbo dizzy is a clockwise rotator, not CCW like the SC. Same goes for the 75-77 Carrera 3.0.

Its the ONLY CW rotating breakerless distributor.

Lastly, the 3.6 dizzy must be used with a 3.6 crank gear as the gear pitch is different and you'll get a bunch of backlash (and wear).

[emcon5]

Originally posted by old_porsche
Tom (emcon5),
If the early turbos distributors were clock wise rotation, then the pick up units should be the ones used in a 964 conversion for a 2.7 or earlier?

The trigger doesn't care which way it spins, but the advance mechanism does.

Ben, the vent kit should fit fine, your setup isn't much different than mine, and it fits with no problem.

I think you need to find out what the exact advance curve is of that distributor.

Tom

[twin plugged targa]

Thanks Tom, i will fit the vent kit when next with the car, i guess I can get it from Pelican Parts.
If I acnnot get the history of the internals of the distributor then how can i find out the advance curve?. Or would it just be a matter of swapping for another?
I will save this thread and repost when I resolve this.
thanks again all, for the advice/info.
rgds Ben

[emcon5]

Originally posted by twin plugged targa
If I acnnot get the history of the internals of the distributor then how can i find out the advance curve?.

See #3 above, in my first post.

Let us know what you find out.

Tom

[Randy Webb]

IS there anyone near you that can evaluate the thing to see what's going on?

Or you could pull the dist. and send it to Steve.

It might be simplest to swap the CD boxes and see if the problem can be isotated to a particular set of plugs (and also swaps with the different CD boxes) -- or find a buddy with a known good CD box and swap that in.

BTW, you do not have points in that setup -- it uses a reluctor.

[Diesel_mp5]

Is anyone selling these dizzys already done? If not, Second I need a 964 dizzy and a SC dizzy, On the early motors 2.0 and such do I need to worry about the geer on the crank?
Thanks Diesel8@cox.net