Timimg for twin plugs, advise needed

[1SIK930]

My 87 930 has twin plug heads. When I bought the car, the timing was set at around 15 deg. BTDC. I changed it, to pass smog in CA to 26 deg. BTDC. It ran better, so I left it. I recently checked the timing, and found out I was only getting spark from half the plug. I figured out that one of my CDI boxes was bad, so I replaced it. The engine response is night and day. I reset the timing to 17 deg. BTDC. I talked to somebody (very knowledgable about Porsche turbo charging) who said it doesn't matter if its twin plug or single plug, it should be set at 25 or 26 deg., and then I heard other knowledgable people say you should set it at 21 deg. then from another I heard 16 deg. I'm so confused, any advise?

[Flieger]

Run less advance with twin plug. The mixture burns quicker because you start it burning at two places instead of one. This is the reason you get more power, because you can run less advance and so have less gas pressure pushing down on the piston while it is coming up while still getting full pressure after the piston starts moving down.

[A930Rocket]

^^^ +1

[voitureltd]

What distributer or management you running? I have two different twin plugs built way back when. Both have the 12 lead distributers from 934-935 era and they are both set @ euro spec 29 before TDC @ 4000 RPM. They all have enrichment dvices from that time and I use 108 leaded octane gas if on track touring.. Run great for street and light track use. Really high HP turbo Porsche motors were built by the factory even late 70s but it took awhile for this
engineering to trickle down to be affordable.

[1SIK930]

It's a daily driver, that gets 91 octane fuel. How much less advanced should I run?

[jwasbury]

Its not clear from your post whether you have any advance/retard mechanism or is your timing static across all rpms?

My twin plug ignition is powered by an Electromotive XDI2, with a laptop programable ignition table (RPM vs MAP).

Idle timing is set at 14 BTDC

During light load operation (MAP < 100kpa), advance steps up progressively from idle to 28 BTDC (full advance) by 3000 rpm.

The timing begins to retard above 100kpa, and decreases progressively to about 15 BTDC at full boost (approx 180kpa)

HTH

[voitureltd]

To be safe I would use the factory spec or just a little less if using 91 octane ( if using factory distributer, ) as USA Porsche spec 26BTDC is based on 95 ron euro gas. I do not have that value comparison between the two fuel ratings, however some others on this forum may be able to enlighten on this equation.

[Flieger]

You are aiming to get the maximum cylinder pressure at a certain few degrees after top dead center. As the revs rise, the flame still burns at the same rate so you have to start it earlier. The advance curve should be the same, but shifted down.

Less advance is less negative work, so that is good. If you run too little advance, however, the mixture will pre-ignite if the octane is too low. If you have high octane, you can set the advance to as little as it takes for the max pressure to be where you want it. If you have street fuel, you have to advance it so that it stops pre-igniting, but not too much more.

You have to decide on the safety factor for pre-ignition.

[1SIK930]

It has a distributor that has been set up the work 12 plugs. It has vacuum advance and boost retard. And I set the timing by removing and plugging the vac and boost lines then revving the engine to 4000 rpm, then setting the timing