recordmanv

I am adjusting my mixture in my 83 SC with CIS injection. I am using a CO machine, I did a before and after test, leaned out 1 click, and it equaled a drop of about 0.5 % CO.
My question to the group is what does one click on a K injection mixture equal or suppose to equal? I know there are a lot of variables and external factors and of course the accuracy of the test equipment itself, but surely one click = +/-approximately %CO?

Thanks in advance.

Houpty GT

My '81 doesn't click.

What CO% tester are you using? Some measure in 0.5% increments and this would be to inaccurate to use on the 1980 and later cars. The earlier cars should be alright since they run a higher %.

I am looking into buying an Innovate LM-2 fuel ratio data logger and then just convert those air-fuel ration numbers to adjust for CO%.

recordmanv

I am using a Gunson. It will measure down to 0.1 %, but has a accuracy of +/-0.5%. My reading was steady and I knew the AF ratio going in from dyno runs was about 13.5 over the rpm range. I wanted to click it one notch toward leaner to get closer to the magic 14.7. I measured somewhere around 2 to 2.5 % before I leaned out. I ended up around 1.1% CO on the final. But I don't trust my first readings because it was my first time using the device and I don't think I let it calibrate long enough in air in the beginning.
Anyway, I trust my final reading because I let the thing calibrate for 20 minutes in air before the test, the car was fully warm ( almost too hot actually) and the result numbers were not jumping around like my first test.
I don't use a O2 sensor or a cat, I have racing headers, so I stuck the sniffer up the muffler (2 in 1 out) . From what I could tell from Wayne's book, an '83 should be between 0.4 and 0.8% CO right? FWIW, I am a big bore 3.2 with CIS. Should I be richer? Car runs fantastic at old setting, haven't driven the car 'hard' with the leaner setting. Last thing I want is detonation, that's why I turned the CIS mixture maybe 1/8 of a turn ( smallest amount I could CCW), sure felt like a stepped attenuator, that's my '1 click of a turn' reference. Anybody have a comment? Please? Thank You!

Peter Zimmermann

0.6% +/- 0.2% is the spec for Cat + O2 sensor. Without those, and modified exhaust & displacement, I would put the mixture back to where it was (approx 2.0%). Your car will not pass smog anyway, and, as such, is technically illegal to operate in CA, so... Taking a reading from the tailpipe is OK.

No "clicks" for the mixture screw; the resistance you feel is because the screw uses a thread locker to resist moving.

recordmanv

Pete, I respect your opinion and expertise on the subject. Please forgive my ignorance, and maybe I don't understand it right, but I thought the spec for a stock 83 911sc was to measure BEFORE the cat and have the O2 sensor disconnected? I know my setup is different, with exhaust, 98mm PC & 7:31 R&P, but since the injection is the same ( CIS), wouldn't the CO% setting be the same as a stock 911 SC 3.0?
Why do you recommend 2% then, especially since my AFR was around 13.5 before I leaned to 1.1%? Aren't I too rich at 2%? I am trying to squeeze every last bit of power out the motor for tracking, what's the best setting? Is a 14.7 AFR ideal for 911? Or is the 911 special and like richer settings?
Update: I drove the car today with the leaner CO and it seems much smoother and quicker on the low end and less "peaky" in the upper RPMS ( post 4k), but pulls very smooth and strong. Torque monster. Very hard to quantify, but I think it's faster.
Thanks again, any insight is greatly appreciated! Sorry to beat this to death, I just want to understand it correctly and I want my car to have the proper CO mixture. Cheers. Kent.

Houpty GT

15% rich is going to be max power and 5% lean is max economy.
14.7 AFR is to maximize the emissions equipment.

recordmanv

Now i am really confused. I thought 14.7 AFR was the best ratio for complete gasoline combustion, not emission. 15% seems out of this world too high, but clearly I don't know what I'm talking about. Houpty GT, don't take this personally, but I am going to wait for some more feedback before I change a thing, but what do you have your 81 SC CO set at? Thanks.

Peter Zimmermann

The following excerpt is from some work that I'm presently involved with, and it should give you some insight regarding this issue.

...that left the 911SC (1980) to share Porsche dealership showrooms with the 928 and 924. Once again, emission control requirements take center stage, not only did they send the mighty Turbo packing, a new development for the 911 SC is probably the highlight for 1980. It’s amazing how quickly emission control devices moved from an almost afterthought status to an engineering challenge that played a part in almost every engineering decision. Porsche, often at the forefront of automobile development, installed a three-way catalytic converter fitted with an oxygen sensor on all ’80 SCs. Chemistry was never one of my strong points, but, basically, a three-way catalyst has three jobs:
1. Carbon monoxide (CO) is considered a toxic, non-greenhouse gas. The 3-way cat will cause oxidation of that gas to the less harmful greenhouse gas, carbon dioxide.
2. Hydrocarbons (HC) are unburned, carcinogenic compounds made of carbon and hydrogen. The 3-way cat will cause oxidation of those compounds to carbon dioxide and water.
3. Nitrogen oxides (NOx) are a combination of nitrogen dioxide and nitric oxide, both are poisonous gases. The 3-way cat will cause a reduction of those gases to nitrogen and oxygen.

The three-way catalytic converter replaced the already in-use two-way cat, which would be ineffective in reducing NOx emissions to meet regulatory changes for the 1981 model year. Enter the stoichiometric point, the theoretical point at which combustion is complete. In other words, all the carbon (C) is burned to become CO2, all of the hydrogen (H) is burned to become H2O, etc. The 3-way cat works best when it receives exhaust from an engine that is running slightly lean, above the stoichiometric (ideal) point, which is between 14.8 and 14.9 parts air to 1 part fuel. When more than the required amount of oxygen exists the engine is running “lean,” which favors the above two oxidizing reactions. When excessive fuel is present the engine is running “rich,” at which point the cat favors the reduction of NOx.

Enter the oxygen sensor. Originally called Lambdasonde by its developer, Bosch, it is a sensor probe that installs into the exhaust just before the catalytic converter. The oxygen sensor measures the remaining oxygen content in the exhaust, and then sends corresponding signals to an electronic control unit (ECU). The ECU is then able to make fine adjustments to fuel delivery, and keep the fuel injection system as near as possible to the ideal stoichiometric point. - Peter Zimmermann, Copyright.

Now, the car in my avatar was built for PCA Club Racing, and was fitted with a stock 3.0 CIS motor with SSIs, and a two in, one out muffler. I ran that car at 2% CO in order to protect the engine following removal of the O2 sensor. I felt that 0.6 - 0.8% was flirting too close to the edge without having Lambda management available. Your car might "feel" quicker/faster at 1.0 - 1.2%, and it actually might be, but I felt that the potential for a lean condition to occur from that point, along with detonation and extreme cylinder head temps, just wasn't worth the risk.

whalebird

This is a good thread. I feel that an engine in its finest tune and well maintained is a reliable and efficient engine. Thanks Pete for the info.

recordmanv

Many thanks Pete! I get it now. I had a great conversation with my mechanic and he suggested 2.5 as a starting point, maybe 3.0 or 2.0, depending on how the car feels to me on the track. Again, thank you for the response, I learned a lot this week on AFR and CO!