More Power from CIS

Hladun · 03-17-2002, 10:41 AM

I have a exhaust gas analyzer and in looking at the manual I notice that maximum engine power is obtained at a rich A/F ratio of about 12.5 to 1, CO level of 5 to 6%. I have a 78SC with emissions optimization by removal (no air pump or cat). Has anyone run a CIS system at 5 to 6% CO for more power? If so, any results to report? I suspect you could also advance the ignition timing with a richer mixture. What you give up is fuel economy, but what the hell....it's a Porsche.

stlrj · 03-17-2002, 01:08 PM

Sounds like a good idea in theory, but be prepared to see your wrench sooner that you would expect when your valve guides wear out prematurely due to higher than normal cylinder head temperatures that will result from rich of peak mixture settings.

Seems to be a lot of data from general aviation that supports that fact. You may not find that information in automative circles since most car engines are water cooled, so most of your answers will come from the air (General Aviation) where these issues are of critical importance.

Joe Garcia

Hladun · 03-17-2002, 01:39 PM

Joe, you're right of course. But won't higher cylinder pressures to produce more torque, by definition mean higher temperatures. That's the definition of power. Isn't that what the guys with the carbs, trick cams, and exhausts are trying to achieve. I'm sure Webers would run rich.

sammyg2 · 03-17-2002, 02:07 PM

The air metering plate is installed in an air horn that is not linear in design. It is actually a three step (approximately) progressive design, where it adds fuel at an increasing rate just off idle, then slows down in the mid range, then again becomes more aggressive near full flow. I didn't explain that very well, but that is how it works. By making the idle richer, it will make the mixture richer all the way through the range, but it is not linear. In order to calculate the actual mixture at full throttle/full flow, you will need a dyno. The system is already designed to go rich at full throttle. The bosch designers had to meet emissions specs at idle and cruise, but not at full throttle. They set it up for full horsepower at full throttle, and making it richer will probably not help very much. You may get a little better throttle response, but i wouldn't expect much more.
As far as combustion temperatures go, the highest temperature should be reached around 15 to one. lean makes more heat than rich at the same throttle setting/load.
Running at around 12.5 to one at full throttle would reduce the combustion temperatures as well as the exhaust gas temperatures. The extra horsepower comes from the extra fuel, not higher temperatures.
If the valve guides wear more than normal, it would be due to the extra fuel washing the oil off of the valve stems. This does happen under extremely rich conditions, but is more prevalent at rich idle, not full throttle.