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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Was reading about CIS and ignition on another thread.

CIS seems to have some challenges with efficient air fuel mixing. If there is not a good mix it is possible that the molecules or mix at the spark plug may be rich or lean and more difficult to ignite.

It makes some sense as we know, CIS is delivering fuel 100% of the time to the injector and it collects at the intake valve in wait. I am guessing that a CIS injector also dose not atomize the fuel as well as an EFI injector.

Additionally, there is a significant amount of intake volume between the metering assembly and the throttle plate. This makes for a lag in fuel delivery and a lean spot with acceleration. Many other CIS WUR's lower control pressure with loss of intake vacuum upon acceleration to help add fuel. The 3.0 Turbos also had this. 3.3 CIS turbo's only add fuel with significant boost.

Add to that low compression and even lower effective compression with an SC or C2 cam, and getting ignition 100% of the time with CIS is a known challenge.

I am accepting that Porsche went with such a small intake ports in an effort to mitigate a CIS ignition issue. Increasing volocity of the air entering the cylinder would help promote better mixing.

As such, if we balance the intake port size by making it larger, we need to be aware of this.

Concentrating on good AFR's pre-boost or improving ignition with MSD or Twin Plugs could be part of this.

Last edited by 911st; 02-04-2010 at 01:49 PM..
Old 02-04-2010, 01:44 PM
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