I studied the shop manual some more, and I think that the distributor information there is not clearly labeled.

There are three curves shown for SC engines, the first for 78, which I take to apply to ALL (US and RoW) 8.5 CR motors:
http://forums.pelicanparts.com/uploa...1302521859.jpg

The next one applies to US market 80-83, with Lambda and 9.0 or 9.3 CR (this one is confusingly labeled to suggest that it applies only to 80 SCs, for US and RoW):

http://forums.pelicanparts.com/uploa...1302521915.jpg

The final one applies to RoW 930/10 motors, with the 9.8 CR (this one is confusingly labeled to suggest that it applies to all 81 and later SC's):

http://forums.pelicanparts.com/uploa...1302521961.jpg

None of the SC distributors give 30 crankshaft degrees advance, and they all come on late. (Except for the 9.8 CR RoW motor, which probably has a conservative amount of total advance to go with the high compression ratio.)

From the advance curve for the 2.4 and 2.7 motors, I would suppose that it would be better to get a full 30 crankshaft degrees of advance, and I'm guessing that Porsche limited total advance to accomodate catalytic converters. Here is the curve for all 2.4 and 2.7 motors, except Carrera 911/53:

http://forums.pelicanparts.com/uploa...1302522301.jpg

Is there a recipe (swapping parts, say) that will get a 3.0 distributor to run the 2.4/2.7 advance curve? Is this a good idea, at least for a low CR, CIS motor? I am sure that I could get this done by Barry Hershon, but I'm a determined do-it-yourselfer, though I will have a local shop check the distributor on a machine, if I try any modifications. I have an early (78) distributor and a couple of later US (80-83) distributors at my disposal.

My motor is a stock late US market motor with early exhaust and 8.5 Mahles in it. (I got them free, how can you beat that?) My CIS system is an earlier, non Lambda system mounted on the later, small port intake.

Thank you for reading this lengthy post. I hope that I cleared up the diagrams in the shop manual for a few people.

Tom,

If you want to understand the factory's choice of US ignition advance curves, a little history helps a lot. Start with the fact that the 911 engine is a knock limited design with a wide VIA hemi head and domed pistons. The ideal advance for output is probably close to 40 BTDC, but it will be limited by the octane of the fuel. Porsche's initial response to the US regular fuel "mandate" was to lower compression and keep a near ideal advance. After the two oil price shocks in the 70's, the drop in efficiency was no longer acceptable and the factory chose to raise compression and detune the timing, so it could advertise increases in mpg. The later SC is clearly detuned with a 19-25 max advance spec. These engines can benefit from more timing, but the added heat of the cat and the vac retard, along with the long runner CIS manifold make detonation an issue with pump gas.