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RClewett RClewett is offline
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Join Date: Apr 2010
Location: California
Posts: 15
911st,

The CIS system does/did a great job for what it was designed to do. It met emission standards and had reasonable performance. The CIS system does have its limits. The biggest problem is that after 20+ years mechanical parts wear out, and good 930 fuel distributors are hard to find. Good modified fuel distributors are even harder to find. With modified fuel distributors, low speed mixture control can be difficult to keep from fowling plugs. Warm up regulators are also a contributor to this problem. When you add the performance minded individual into the equation, the mixtures are adjusted richer than originally designed; and with no way to adjust the fuel range of fuel delivery, problems can arise.

For the rich idle condition at low speed, it's not so much how long is the spark or how many sparks you have. But do you have enough heat in the chamber to burn the fuel? When the mixture is too rich, there's not enough heat to burn the carbon off the spark plugs, and they foul. To maintain enough heat in the chamber, the mixture shouldn't be richer than about 13:1 AFR at idle, hot.

In my opinion, the fix for CIS is to go EFI. I admit, I'm spoiled. It brings the engine up to modern day ignition and fuel control technology. It is very easy to make changes and provides the means to use a wide variety of performance components. With EFI, much of the CIS and turbo lag can easily be tuned out, making the engine much crisper. By adding some slightly more aggressive cams, a 930 will wake up and start making some real power. That's why I'm spoiled with EFI.

Twin plugging might help the off idle response some. But the real fix is to have the correct mixture and timing.

With timing, the goal is to have peak cylinder pressure at 14 degrees ATDC. Earlier than 14 degrees develops excessive heat and beats up the top end of the engine. Later than 14 degrees leaves power on the table. The dyno is the best way to find the sweet spot. I typically recommend for a single plug 930, 20 degrees at .8 boost. On the dyno, I've been everywhere from 18-23 degrees, so I pick 20 as a happy medium and starting point.
The timing at .8 and 1 bar are about the same, maybe 1 degree less at 1 bar.

Twin plug is usually about 2 degrees less than a single plug timing. This number also depends on RPM. With EFI I'll add a couple of degrees @3000 and back timing down a couple degrees by 7000. There is certainly a power difference between single plug and dual plug engines.

I hope this helps,

Richard
Old 04-28-2010, 12:53 AM
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