[911st]

Modifying the stock system:

Because of the design of the stock system it is not a good idea to just set the timing for more static advance. This is because the on boost timing will move with it. Thus, on cruse advance is going to mostly have to be lived with at the expense of MPG. Again, this dose not have any solid impact on performance.

On Boost: Setting the stock US dist at -26 deg yealds about -16 to -18 deg of advance on boost. I suspect we really need to be near the lower end of this for safety. Bruce Anderson in his book on modifying Porsches suggests setting timing at -25 if running .9 bar boost on a modified 930. We do not want to advance this any furter.

Off idle: It looks like -15 with very quick advancement offers better throttle response. To get this the best way will probably be to shorten the mechanical advance feature. Further, we see full advance by -3000. Some readings seemed to indicate that we might benefit from seeing full advance by 2500rpm. How do we modify for this. One is to “re-curve” the distributor by putting softer springs in it so the weights advanced faster. The other is to shorten the mechanical advance. This can be done by limiting how far the weights are allowed to extend.

How much we limit this is open for debate. I am leaning toward killing the Vac-Ret function totally. This would advance the timing to about -8. If my goal is -15 at idle that means we need to take about 7 deg out of the mechanical advance. 7 deg of 18 is about 40% of total advance. Thus, we would need to reduce the weights ability to advance about 40%. This will effect when we get total advance and it might make total come about 25% faster so this might be all we have to do.

If however one wants to keep the Vac-Retard function to maintain the increase in rpm that is available during the cold start function (and maybe A/C tuning on but not sure), we might target something like -10 at idle with every thing hooked up. That would mean taking about reducing mechanical advance about 11 of the total 18 available. This is about a 55% reduction in weight travel.

This, we should be able to increase our off throttle response by shortening the mechanical advance part of the total advance available to us.

Note: I suspect the small solenoid that is hooked up to the low pressure side of the distributor (Vac-Ret) has an important function in achieving boost-retard. If this solenoid id not function, boost will enter both sides of diaphragm on the distributor and in effect equalizing pressure on both aides keeping it from retarding. One resource indicated that with his test, applying boost to both sides of the retard-can still retarded boost, however I would not want to count on that.