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Effect of altitude on idle?

Started by raueda1, 04 August 2023, 08:50 PM

raueda1

I'm driving my car from Utah (4500 ft and much more) to sea level on the east coast.  I've noticed that there idle has been creeping up.  It was at 700 or so when I left and now 1100 or even 1200 rpm.  Would this be expected?  And does it mean rechecking CIS pressures, of idle mixture etc?  I haven't heard of this but what don't know would fill books.  Thanks and cheers,

Edit:  yes, altitude does effect idle.  But the other questions still apply.  Maybe recheck timing too on the ground that manifold vacuum might be effected by altitude?
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

Without delving into it too much, just wind the white  idle screw in until you get 750 rpm. When you return to your 4500 home, wind her out.
Frankly, i don't think the altitude difference is having any influence........I regularly take my 6.9 up into the hills which are about 4000 FEET and there's no perceptible difference.
I know there are altitude WUR's ...........

raueda1

Quote from: Randys01 on 06 August 2023, 03:10 AMWithout delving into it too much, just wind the white  idle screw in until you get 750 rpm. When you return to your 4500 home, wind her out.
Frankly, i don't think the altitude difference is having any influence........I regularly take my 6.9 up into the hills which are about 4000 FEET and there's no perceptible difference.
I know there are altitude WUR's ...........
Hmm, interesting, thanks, for the remarks.  I think it's a nice topic that doesn't get discussed much.  Anyway, yeah, I'll just go back and readjust it all. I moved, so the car won't be going back to altitude.  I'm also interested in whether the acceleration mixture is effected by altitude.  I'd think it would be, since the manifold vac is pulling against a higher pressure than up at high altitude.  In any case, my idle definitely did go up significantly for some reason.  After posting I googled (as always after posting, never before, duh ::) ) and learned that there IS an effect on idle in older engines without all kinds of electronic engine management, e.g. motorcycles, old carburetor cars etc.  So that's apparently a known thing.  As for the effect on K-jet, who knows?  And I actually do have a high altitude WUR out of a parts car that was bought in Salt Lake (4200 ft and lots of terrain much higher than that).  It seemed to be a dealer swap installation and so I also got original regular WUR.  I swapped it into my car a couple years ago and it didn't make any obvious differece.  But being at sea level sure does give the car a performance boost!  Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

I daresay the higher altitude is akin to a hot day at sea level. In both instances the air is to a degree rarefied.
Elevated temperature of the induction tract is epitomised with turbo charged vehicles....hence inter coolers...hence once step further...water cooled intercoolers.
In the elevated situation, to some degree, the air is of a lower temperature. So the loss of air pressure is some what offset by air of colder- therefore- denser composition.

K jet's behaviour in these circumstances is curious. Apart from the heat build up  as the air rushes thru  the inlet tract, the influence on the air metering baffle is probably academic.....and here's the difference with a carby.
The fuel metering is independent of the density of the air being drawn in. The same air is drawn in whether it be hot or cold...thick or thin....the throttle butterfly determines the vacuum applied to the metering plate. It is the position of the fuel piston in the Fuel Divider that determines the fuel volume.
With the injectors sitting in the mouth of the inlet valve and the fuel to the injectors being delivered by pipe external to the inlet tract- it makes for a pretty good situation.
So the issue is....to what degree does thin..or hot air... influence the fuel metering baffle.?
Because the engine can develop more vacuum than can be possibly satisfied, the fact it is pulling at any point in time air that might be thin or hot- requires consideration as to its influence on the metering baffle.
So we are talking about 1.5 psi at 15 de at 4000 feet.
Considering control press ranges from 50 to 58 psi I would conclude that "thin air" at these values has a negligible impact on performance.

HOWEVER I would argue that any perceived drop off  in performance is due more to the fact that thin air is less able to absorb the attendant heat in the inlet tract and so arrives at the cylinder a tad more rarified than cold or seal level air. So it runs a little lean..oh by an unmeasurable amount.

There comes a point where this phenomenon has to be dealt with . ie 9000 feet asl. Theoretically the engine would be running so rich it would be hopeless.
Thus the clever people at Bosch have dealt with this via the WUR.

Now I've not had the pleasure of dismantling a hi altitude variant but clearly it has to provide for a lower cold pressure range because the values are so low; and below the design parameters of a std WUR. For reasons outlined above, I dont see it necessary to fiddle with the hot control pressure characteristics..But?.
If some one likes to send me one, we will drill down  and find the difference!!