Steve and others,
I am looking for some advice on how to obtain optimal performance from a 1980 911SC. This car is NOT part of the bi-annual Ca smog program because it is in a rural county. First some background:
Car has now has approx 100 miles since I rebuilt it. What started as a two broken head studs ended up being a complete (everything !) top to bottom rebuilt. Partly due to my curiosity. Current spec are:
964 Cam profile put onto sc cams
SSI heat exchangers
recurved dist by Jerry woods.
MSD 6200 ignition with MSD coil
I have added an innovate wide band AFR meter which has a narrow band output hook to the CIS analog computer. Additionally I am using the innovate data logger to record freq valve duty cycle and RPM. All vacuum lines, hoses, boots etc on the CIS are new. I have been experimenting with control pressure and primary pressure and I have stacks of interesting data logs that I can make available to any interested parties.
First question- Idle richment adjustment. O2 sensor unplugged on a 1980 puts the duty cycle at 85%. If I set the idle richness with it unplugged the car leans considerably when the O2 is reconnected because it defaults to a 65% duty cycle. See attached jpeg for an example. If the 15C switch is open this car starts at with 85 percent duty cycle but not for long. Once it goes to the 65% setting it is pretty lean. FYI the primary pressure for this graph was 5.4 bar and the cold/warn control pressure were 1.6 and 3.5 bar
What is the best way to set this? O2 connected or disconnected? What CO or AFR?
I have since modified the warm up regulator to make it adjustable. Based on a WOT test and the duty cycle of the car at constant rpm under light load I concluded the car wants more fuel. See attached jpeg " 5.4 primary and 3.5 control". In this log at time=17.0 to 17:10 I am cruising at light throttle at 3000 RPM (black line). The AFR is is between 14 and 15 (appox) (magenta line) and the duty cycle is between 65 and 75 % which I think is high. Just after 17:15 I accelerate at WOT for a few seconds and you see the duty cycle go to 65% because the 35 degree (throttle plate) switch has put the system in open loop. The AFR during this period is lean as shown. 5 or so seconds later i do the same test up to about 4400 rpm with the same result.
Question for Steve- I have read that during break in I should stay under 5000 rpm. Is this test safe or should I wait until I have more miles on the car to be doing this experiment?
Based on these logs I have dropped my control pressure to 2.9 bar and things have improved. I also have experimented with bumping the primary pressure a little. It is currently at 5.8. Interestingly the AFR variation (high frequency changes) at idle and during operation is increased with higher control pressure. My first logs were with primary pressure at 4.6 Bar. I upped the primary because it was quick and easy to do. This was before the WUR was modified to be adjustable. I plan to repeat the tests with lower primary and control of 3.0 if it is safe to continue this type of testing.
The last log for now is for a primary pressure of 5.8 bar and control at 3.0. In this the WOT response is much better all the way to 4500 RPM. Duty cycle at cruising constant RPM has dropped to 55 to 65%.
My concern is that although this might be acceptable what will happen when I operate at WOT to red line? I suspect the 65 % duty cycle will not be able to provide enough fuel. Comments? I could lower the control pressure to compensate but I what point will that cause other problems. I also had an idea that i could add a limit swith disconnect the o2 sensor signal thinking that would trick the CIS controller to going into 85 percent duty cycle for extra enrichment. I tested this theory with a toggle switch on that signal and discovered that once the 35 deg throttle switch puts the unit into open loop mode (65% duty cycle) it doesn't see the O2 switch being disconnected. So I say what about using the 15deg c switch? Will that for 85 percent duty cycle? Wiring would be simple. Add a switch and use a relay to switch the 15C circuit. The last option is to disconnect the 35 deg throttle switch and use the O2 or 15 C switch circuit to get 85 percent duty cycle at WOT. Has anyone ever tried or done this?
Thanks for you help and wisdom.
Matt