using 964 distr on a twin plug 3,5 engine

[greyes59]

I would like to know how to mount a 964 distributor on my 3,5 build . I am using the original bosch computer hooked up to a J and S knock sytem for knock control and to dial in retard for the twin plug system

[spuggy]

Originally Posted by greyes59

I would like to know how to mount a 964 distributor on my 3,5 build . I am using the original bosch computer hooked up to a J and S knock sytem for knock control and to dial in retard for the twin plug system

Don't know much about the 964 dizzy (traditional wisdom is that they don't fit under intercoolers, so aren't useful for 930's), but I think you should get the Motronic spark map re-programmed for twin plug ignition (which needs less advance than single plug). There must be a bunch of folks who can do that for you.

Disclaimer: I don't do this for a living. However, I would SWAG that there isn't a single value (offset) you could use which would provide the optimum timing for dual-plug on a single-plug ignition map.


Although the J&S Safeguard is useful as, well, as a Safeguard, it is quite often used to let you get more aggressive on timing than you otherwise could; pulling excess timing as necessary. On a single plug system - especially with a distributor, where you can't "tweak" the map - it'll pull timing in the "pockets" of the envelope (load, RPM, temperature, mech/vacuum advance etc) where you're not only at risk, but detonation is actually happening. And still let you enjoy the effects of more advance (better response, more power) when the conditions don't cause detonation, rather than having to go with the compromise timing that "mostly works OK".

With twin-plug, the "correct" setting is to run less advance anyway (charge takes less time to burn, so doesn't need to be lit as soon). Way I figure it, losing an ignition side (plug, coil, dizzy belt snaps etc.) leaves you with the retarded (over stock) timing - so you should merely experience a loss of throttle response/power, rather than harmful detonation.

In your scenario, the J&S would listening for detonation, but I think the thing most likely to cause that with the correct ignition map would be bad fuel/clogged injector - or a very hot day.

[JP911]

When you say "original bosch computer," I assume this 3.5 is built on a 3.2 Carrera motor and that you're planning to keep the Motronic computer? If so, there are a few things you need in order to use the 964 dizzy, which include a signal splitter, custom chip, second coil, and a wiring harness. AFAIK, the chip manages advance on a 3.2. If you search the forums you'll find information from others that have made this modification. I highly recommend calling Steve Weiner at Rennsport Systems, who guided me (and provided parts and services) through the use of a 964 dizzy on my motor.

[greyes59]

thanks for your advices.
I wanted to see if i could get away from reprogramming the 3.2 chip by dialing in a fixed retard using the J and S safeguard. the J and S safeguard can also act as a signal splitter.

[RWebb]

depending on year, the spin directions may differ