What CO levels should I run on my 911 SC?
 

I was adjusting the mixture on my '79 SC and monitoring the CO levels. On the engine lid it says that the levels should be 2,5% +/- 0.5%. I am curious if the people here adjust their mixture to the 2.5% range that Porsche recommends or something else. I should point out that the car has been upgraded to 9.3:1 pistons from 8.5:1 and does not have a cat - it is otherwise stock.

Thank you

My 82 SC runs at about 3-3.5% CO, O2 sensor unplugged with a cat bypass, and passes emissions testing

Focus on the proper 13.5-14:1 air to fuel as your priority. Adjust before/after smog testing as needed.

How do you check the air to fuel ratio?

Why would you want your idle CO to be so rich?

No need to adjust. 13.5-14:1 translates to 1.5-2.5% CO. Close enough to stock.

I am running close to 2.0 in my 75S CIS, any richer that that and the engine starts to oscillate badly at idle. The upper rev-band may be somewhat "better" or "feel more powerful" with higher co, but the engine doesn't run good at lower revs at all. The CIS in my car really only runs clean at a very "narrow" adjustment range on the co.
My 2 cents...

dvkk, it does run a little rich which i think is because i runn with the O2 sensor unplugged. That said, I will be checking the AFR again.

Why not just plug the O2 sensor back in?

yes, why is your 02 sensor unplugged?

my wrench unplugged the O2 when he set the mixture. I am guessing to compensate for the cat by pass. He says it runs better a little dirty. I trust him, as he has been a p car mechanic for a long time. I do plug my O2 back in for emissions. Just seems to run better a little richer.

Ahhhh, I see. Your mechanic knows more than Porsche. :rolleyes:

Any CO you measure is incomplete combustion. It is incomplete because there's not enough free oxygen to form CO2. This is called rich.

The so-called perfect A/F ratio of 14.7:1 equates to 0.1% CO. So you see, Porsche already sets the mixture too rich at 2.5%. There's no need to go even richer. You're just wasting gas, and reducing the life of your CAT (if you had one). You're lucky to pass without a CAT. It is an instant visual fail if you don't have one.

I'm not sure by visual fail you specifically mean the ICBC system but I've heard of a number of local 911s with SSI's that pass Aircare. I aircared mine with cat bypass for the last couple years and both times the technicians just looked under the engine (surprise, the right end of the car!) with the mirror and either put "present" or "not applicable" for the "catalytic converter present?" check box. There was something in ICBC literature about vehicles produced earlier than 1990 not requiring a cat, but I don't know if that applies to a car that came with the "catalytic converter equipped" sticker.

EDIT Quote from ICBC:

i was playing around with the mixture and the LM1 a while back. i have a miss at about 4500 RPM. my mid RPM AFR was in the mid 15's, so i tried to set the mixture to bring the mid RPM down to 14.7. i got it around 15.1 or so but my idle mixture was too rich, so i reconnected my retard and the idle mixture leaned out about .5 points or more. so i was able to lower the mixture some more but never could get the mid RPM mixture below 14.9 -15.1. it mght touch 14.7 ever so often but not very consistenly.
does the mid RPM mixture really matter? should it be that lean?
CP's are dead on. WUR is only about 8 years old, just a babe

o2
 

cog, thats the beauty of an 02 car, to be able to richen the mix during open loop (past the 35 degree swith) and still run stoich at cruise and part throttle. any 02 car should have an adjustble wur, as the recomended warm control press. are 50-55 lbs, which is a little lean. (my car is set at 48 lbs warm pressure) the trick is to get you mix as rich as possible and still have your 02 be able to compensate at idle and part throttle. some guys dont understand that once you go past the 35 degree throttle swith, you are in open loop, and your 02 has no effect on mixture or perfomance at all, and when you disconect the 02 by pulling the relay, you are losing your only wot enrichment, the freuqency valve. although a crude system by modern standards, it works very well when all is sorted, and all being eqaul, you can make an 02 sc motor out perform any non 02 sc 3.0 car and still have excellent drivability.

This is a beautiful discussion and I am enjoying reading everyone's responses. My car does not have an O2 sensor, but I am still enlightened to see what people wrote about them.

My car is currently running around 3.5% CO - it runs well at all RPMs but it does occasionally stumble a little in the low RPMs. I will lean it out a bit to hit the 2.5% region and see if it makes any difference.

Many thanks.

Sorry to bring this up again, after reading about this, I checked my smog certificate and here is what I noticed:

CO(%) 15MPH MAX 1.41 AVE 0.20 MEAS 0.00
25MPH MAX 1.21 AVE 0.17 MEAS 0.00

Is this possible? It does "burp" a little below 2K RPM now that I think about it... What is the best way to adjust this?
Thx
SZ

There's no doubt Porsche knows a lot, but they were setting the car up to run the best it could 30 years ago with different fuel from today in order to pass emissions requirements.

It's hard for a gasolene engine to get to that elusive 14.7:1 because of the short duration of the burn, and w/o today's computerized sensors and controls, if things get just a little lean, then you're asking for trouble.

30 years on, w/ a different CR, different frictional losses, carbon buildups, and who knows what else, it's very possible for his engine to require something other than the stock setting to run right.

These engines seem to develop more power in the 3 to 3.5% range. On the street would it matter? Maybe not. And if your AFRs throughout the rev range are OK, then fine. On the track you may want that extra power, plus the assurance that you won't turn your engine into a hand grenade.

CJR-Driver,

You should have your AF ratio checked w/ a meter and your mixture set. If you don't have access to the equipment a shop can do this for you inexpensively. The mixture is set vi a a 3mm screw located in the mixture control unit, next to the fuel distributor.

Coglin,

What happened after you leaned the mixture?

Will do... Thanks Paul...

Hi, resuming this old thread cause I had my emissions checked this morning: 3,4% CO. Should I lean it out a bit? The test was done with O2 sensor plugged in and with frequency valve always on (my O2 relay is broken, so I temporarily jumpered the pins). My exhaust has supposedly been cut and emptied by previous owner, don't know if it matters...

Or maybe should I wait till I get a new relay and then do the check again (O2 sensor plugged or not)?

Thanks

i tend to go 3-3.5 with the O2 unplugged. if you get 3.4 with it plugged in, it would be richer than that when you unplug, which is how you're supposed to check it. as long as the freq valve is working, you're fine.