Quote:
Originally Posted by psalt
Can anyone verify the 72 911T, E & S timing spec?
38 BTDC, vacuum disconnected, max advance. Sunoco 260 was 39 cents a gallon.
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Wow, that is a lot of timing. They were only 7.5 and 8/1 CR motors if I remember correctly. They also had higher over lap valves that lowered the effective compression. They also had a shorter flame path. I guess that all added up to a very slow burn.
I could see -38 with vac advance and more like -32 disconnected. Might be worth verifying as Porsche did have a test w the VacAdv hooked up as well as not. I am probably wrong on this.
A change in timing up top of two deg effects timing at idle the same amount. That is because the mechanical advance is a fixed amount. Vac advance and or retard is accumulated with this.
More ideal advance at cruse is going to improve fuel economy.
More ideal advance under load is going to improve power.
Less than ideal advance at idle allows for easier starting and improves emission at the cost of response and economy. It also create heat.
Risk of detonation is usually highest at TQ peak. Thus, with a weights and springs mechanical advance modified by a vac or retard pot is going to be at less that ideal advance much of the time even if set at the limit at the most sensitive point.
Risk of detonation changes w AFR, air density, temp. If we set it at its ideal under one set of conditions, it will be at less than ideal at other times.
Thus, a programmable electronic ignition w the right sensors can be more accurately set to run at the ideals more of the time.
MSD makes a programmable unit. This could be something to look at. With its multiple sparks at low rpm it might also help throttle response as CIS seems to have a lean surge with first acceleration and a lean mix is harder to fire.
I am more of a 930 guy. With the 930's timing was set to about -26 at hight rpm or about -16 on boost. When Porsche added electronic ignition to the 91 turbo they seem to have set it up for up to -40 at cruse and closer to -20 on boost. They could do this because there was compensation for temps bullt in and a programable sustem could do things the mechinical system could not.
Thus, there is margin built into the stock system for reasion of safety, emmissions, and fuel quality vartions. If one is carfull these margins can be tested for an increase in both economy and power.