AFR change going to higher altitude

[quattrorunner]

Has anyone else seen this?
I drove from vegas 2100 feet, to Salt lake city 5000ft with some driving between of about 6000 ft. I noticed the afr changed lower the higher I went. This was kind of a shake down road trip so I adjust AFR on the road at stops just to do it. I wanted safety and good milage. At cruise I want to see high 14's to low 15's and got to low 16's after I started dropping back down to lower elavations.
Car ran great but runs better with richer mixes like 14's in cruise.
I ended up with 21mpg going up and 22.5 mpg coimg back to vegas. Great trip, did it at night so I didn't worry about heat.
I'd like to just set the mixture at a nice range like low 14's in vegas and let it be. Ideas?

[quattrorunner]

No comments?

[mark houghton]

No big surprise, since CIS doesn't have an active automatic adjustment to compensate for barometric pressure. Ya go higher, ya get less air, ya get a richer mix.

With that said, perhaps the WUR is able to compensate somewhat for altitude and pressure changes. Plenty of others here that have a boat load more in-depth knowledge on the subject matter.

If you plan to modulate between altitude extremes, then set your mixture in the middle of the two extremes. Probably not an issue to be really concerned about, though. Just consider the Mother Country and some of the elevations that Porsche's were known to run in.

[copbait73]

I'm driving my '73cis turbo a lot this summer and find more AFR change due to the fuel I buy.

The CIS system has the air metering plate to weigh the air so within it's design limitations it does adjust for altitude variation.

[RarlyL8]

Thinner air affects the AFRatio. If you regularly travel varying altitudes you might consider swapping your WUR for a -160 or -165 from a 965. They automatically adjust fuel pressures for altitude.

[mark houghton]

Originally Posted by copbait73

I'm driving my '73cis turbo a lot this summer and find more AFR change due to the fuel I buy.

The CIS system has the air metering plate to weigh the air so within it's design limitations it does adjust for altitude variation.

Less dense air at altitude would have less mass, which in and of itself would result in a richer mixture whether or not it impacts the movement of the metering plate. But theoretically it could also result in the metering arm deflecting less with less mass/force against it, all other things considered equal (such as air velocity) so I suppose the two combined would passively counteract each other (though probably not in a linear fashion) and keep mixtures somewhat stable. For active control, Brian's suggestion of a 965 WUR would be the ticket.

Sounds like a study in Physics (it's been a zillion years since my last class on that subject and I never was any good at it)!

[zaidigonzo]

Interesting.. So a 965 wur adjusts itself? You can just replace it on a 930 motor and no problems? So a Leask 965 WUR would then be ideal right??

[zaidigonzo]

Anyone?
Im asking cause Im moving to kingman az from florida
Elevation 3300 feet. I have a leask wur on the 930 with great afrs but i have the 964 cabrio with 930turbo motor and stock wur. I wanted to do the same but if sourcing a 965t wur is better for driving up and down elevations then Id rather do that..
Im not yet on the efi slope but that may change if the cost of 965 wur plus sending to leask etc costs near an efi conversion then maybe...

[quattrorunner]

Originally Posted by zaidigonzo

Interesting.. So a 965 wur adjusts itself? You can just replace it on a 930 motor and no problems? So a Leask 965 WUR would then be ideal right??

Based on my experience this weekend, I wouldn't worry about it. Just find a nice happy setting and all will be fine. even when my AFR seemed lean, it ran low 12's-nmid 11's on full boost which is safe. Only difference when it was lean was more popping decelerating and maybe not quite as smooth? Not bad at all.
If you need to make the adjustments, it's super easy.

[zaidigonzo]

Cool thanks!

[quattrorunner]

Yeah, i'd be more concerned with your A/C

[RarlyL8]

If you already have an adjustable WUR you can tweak it when you get to the mountains. If you plan to drive up and down elevations often and you want to maintain your perfect fuel tune then the 965 WUR can do that. Cost to make adjustable is the same as the 930.

[zaidigonzo]

How much does a 965 wur run?

[zaidigonzo]

never mind. once the 965 wur is made adjustable it looses its auto compensation so its in turn a 930adjustable wur . so ill stick with that
thanks!

[RarlyL8]

Adjustability only changes the set points, not the general operation of the WUR.

[zaidigonzo]

Brian
U sell 965 adj wurs?

[RarlyL8]

Yes but I prefer to do it on an exchange basis to keep cores on hand. Find a -160 or -165 core and we can set it up for you. This really is only useful if the car frequently travels varying elevations. If not your adjustable 930 WUR will be just fine when tweaked for the new altitude. My old car, the red rocket, is now in CO so it has the same challenge. I put a newly rebuilt adjustable 930 WUR on the engine for the new owner so he could compensate for the change when he got there.