Altitude Question

[jaxxon99]

I live at 7000 feet altitude and my new to me 1981 SC comes from sea level. Should I have the air/fuel mixture adjusted for its new home? What about timing or anything else? It's running a bit rougher than I was expecting and wondering if what sort of altitude adjustments I should make.

[Winter]

I am dealing with a new-to-me 76 911 with CIS. Car was set up (well) for Missouri, but started having idle issues upon arrival in Boulder, Colorado (5430').

You can read about my issues here - CIS expert in the house? - and I saw your post on my way to edit my thread and add some links to CIS and altitude at the end.

It is my understanding that early CIS systems (through '79) do not have an O2 sensor and thus must be adjusted for large changes in altitude. Your '81 SC should have an O2 sensor and thus be able to manage altitude better than the early systems. You can learn more here: https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/31-CIS_Fuel_Injection/31-CIS_Fuel_Injection.htm

But . . . I am not an expert (just trying to learn as much as I can about CIS, which still is a bit of a mystery to me.

It is my understanding that CIS can run - and sometimes "run well" - even when there are issues with it and it's not been set up optimally.

So . . . You first need to have your CIS totally dialed in. No vacuum or air leaks, proper fuel pressure, etc., etc. Then the system should work as designed and your car should be able to "adapt" to your altitude and your Sunday drives if they take you over high mountain passes, etc. If the car was totally dialed in at sea level, I don't see why it couldn't adapt for the change in altitude to it's new home. Since you say it's running rough, I'd imagine that the CIS experts here will advise you to check for air leaks and do all the other CIS trouble shooting first.

Good luck and enjoy your car!

[jaxxon99]

Interesting, I did find in the service records a note that my car does not have an O2 sensor, wonder if it was removed at some point or if 81 also did not come with one? Car seems stock in all other aspects.

[jaxxon99]

@Winter thanks for the links, I'm unable to search in the forum for some reason (just signed up), otherwise would have gone there first!

[targa72e]

You should have the CIS checked at its new altitude. Remember set to "specs" is all about emissions. You can do little tweeks to the CIS to have the mixture dial into "runs and drives" best. In general you can run a little more timing at altitude because your effective compression rate is lower (around 20% in Boulder).

John

[Schulisco]

If its a model year 1981 US car with the 930/16 engine, then it has a catalyst with lambda control. In this case, you can set back and relax if the CIS is tight and all components work properly. The lambda control notifies if the oxygen is too less or the mixture too rich it will reduce the fuel being injected. On non-lambda CIS versions prior to MY 1980 they don't have any lambda control at all. What then happens you can read on in the neighbour thread here:
https://forums.pelicanparts.com/porsche-911-technical-forum/1168980-cis-expert-house.html

But also a lambda controlled 1980-83 SC may have some mixture issues when riding in higher altitudes than before where it was adjusted. Will say that if the mixture is a bit more rich than necessary (to sort out some vacuum leaks e.g.) then also may run into trouble.

Further reading on the 930/16:
https://nineelevenheaven.wordpress.com/english/
https://cis911primer.com/home.html

If the car is new to you and you're experienced on vintage cars like these I recommend to check over time all the things like mixture on a certified gas tester and ignition timing which are the most important parameter. The rest comes over time.

Have fun with the new toy! I also own a 1981 US SC ;-)

Thomas

[Dronen719]

Thanks for sharing this, I'm also having same issues and don't know what to do before.

[Schulisco]

Just a background information.
The Carreras got an altitude sensor which is basically a simple switch to differ between altitudes lower than ~1000m / ~3250ft and above. On higher altitudes than 1000m / 3250ft the injection pulse width gets decreased by 6%. I don't expect that the pulse width corresponds to the mixture, but -5% mixture seems a good approach to me too.

https://www.swchips.com/fuelsys.html

Thomas

[Tommy Z]

When I moved from sea level to 5300 feet, my 79’ SC was idling poorly and slugging a little. A quick adjustment to the idle and Air Fuel mixture made everything better. It even runs equally as well up at 10,000 feet now.