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TBI & high Altitude

N

ntsqd

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Is it likely or possible that GM used different maps or differently calibrated parts for TBI vehicles headed to high altitude states?

I recently drove my '91 SBC Sub up to 12,400 ft (hard to find roads that high in CA) and it ran about like it does at sea level. In the 6k-8k ft range it ran better than either higher or lower. I've suspected that something is a bit off with this truck, but last weekend has me wondering if the Sub's CO original point of sale might have something to do with it.
If so, I'm thinking that perhaps I ought to go looking for a "low altitude" ECM?
 
it's possible. but i took my 94 c1500 up there (crested butte, CO) several years ago with not problems. that's 12000+ feet, and i live at around 600. i felt some power loss, but nothing drastic. it seems the general just built a good setup.
 
There is no altitiude calibration in TBI.
TBI uses the 02 sensor to set and sense fuel to air mixture. No matter what the elevation is, the ECM trys to keep approx 14 to one fuel/air mixture or a BLM 128. The ECM has no idea what altitude is nor does it even care. No matter what the atmospheric pressure or the 02 content is. The ECM will try to maintain the proper AF ratio by adding or subtracting fuel to maintain a BLM of 128
 
My question stems from the knowledge that at least one other OE uses O2 sensors with different calibrations to 'tweak' the fuel map. I'm not ignorant of how EFI works in general, but am in terms of what GM may or may not have done. If GM sent Colo ECM's that were programed for, say, 12:1 A/F instead of 14.7:1 then the computer still wouldn't know or care what the altitude was - only that pre-programed target was now within it's narrow adjustment range.

Finally got thru to the local Chev parts guru. There are at least 4 different PROM's for my truck and one is labeled "Altitude Emissions" while another is a hodge-podge of "Fed", "CA", & "Altitude." So I guess I answered my own question.
 
how very interesting.... I know driving my '91 TBI 'burb from an altitude of 17 ft (texas gulf coast) up to crested butte, CO really made me want forced induction..... I didn't know power loss could be that bad before that trip
 
I was thinking they'd have to tune open loop differently, but it appears MAP handles the different altitudes even for that, since no reference to it.

Learn something new every day, MAP setups can actually re-calculate baro while the vehicle is being driven.

http://www.thirdgen.org/techboard/diy-prom/256564-baro-da-calculations-730-a.html

Edited for above post, GM has a few different O2 sensors, it's obvious they aren't all created the same. Some of that is probably application specific (physical shape, wiring configuration, etc) but IIRC, there are still different part numbers for sensors that are visually identical.

14.7:1 is best for emissions, that's it. Not best for power or anything else, seems to make sense there would need to be some modification of the code to reflect the fact that higher altitudes need slightly different parameters, I didn't think MAP was taken into account for open loop.
 
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Your experience was what I was expecting, and it really didn't happen. My sea level performance is also not spectacular. It runs better at a little elevation than it does at either extreme. Not by a lot, but enough to notice if you're tuned into how the truck runs.
 
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I highly doubt that GM made different cals for altitude based on point of sale. The MAP sensor should adjust for different barometric pressure. I belive that the MAP sensor checks baro as you turn the key from off to crank (during ECU on, but zero vacuum). If you are having issues try pulling over and shutting it down for a few minutes. When you re-fire, it should recheck baro and adjust accordingly. I know they calibrate the ECM at every altitube between Death Valley and Pike's Peak. They can't hit every elevation though and their interpolation may have better and worse points.
 
Then what would drive the existance of the "CA Emissions", Fed Emissions", "Altitude Emissions" and "Fed-Ca-Alt-(?) Emissions" PROMS in the Chevrolet parts system?

I need to open things up enough to find the various build codes. Then I can find out what I really have.
 
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vortec said:
it's possible. but i took my 94 c1500 up there (crested butte, CO) several years ago with not problems. that's 12000+ feet, and i live at around 600. i felt some power loss, but nothing drastic. it seems the general just built a good setup.
Click to expand...
I live 28 miles from CB, and it is NOT 12K elevation. More like 9K. I live next door to a Chevy dealer. If I see the shop boss, I will ask him about high altitude calibration, or lack of.
 
Hmm, never heard of "Altitude Emissions". I may be wrong on that point. Of course CA Emissions and Fed Emissions have nothing to do with altitude.

Special altitude calibrations don't make any sense to me. They can't possibly assume a truck sold in Kansas for example won't find its way to the top of a mountain someday. I live in Kentucky and my fleet of crappy cars include three we bought in California, one we bought in Michigan, one we bought in Tennessee (first sold in Georgia), and one we bought in Kentucky (from a guy who moved it from Florida). We didn't buy any of them new - probably not even the second or third owner of most of them. Who knows what altitudes they have been at since they first came off the assembly line.

Altitude compensation should just be another table in the calibration. I don't know why GM would want to create, validate, emissions certify, and service any more calibrations than they had to - and all the calibrations would require altitude compensation. I would have only expected CA and Fed calibrations.

Maybe somebody here can set me straight.
 
"Altitude Emissions" were the words of the parts guy when he looked up what the PROM options were.

I agree, as an OE you wouldn't want to have any more permutations than absolutely necessary, if only from a cost standpoint, let alone possible drivability problems due to one calibration making it's way to another area.
 
The problem more than likely was capability of the ECM/hardware at the time if there are different calibrations.

There used to be a website where you could research many of the PROM's GM used, but it doesn't appear to exist anymore. :(
 
You're thinking a limitation in range of adjustment forced different calibrations?

That is what I would expect to drive such a thing. No doubt a cost/benefit said that multiple configs cost less than an ECM with enough adjustment ability or the needed AI to compensate.
 
I'd *imagine* it has to do with base stuff, like open loop, where the O2 sensor isn't a factor.

Since you can disconnect the MAP sensor and still have it run, to have that same functionality at say 10000ft and sea level (or whatever is "typical lowland" elevation for the rest of the country) would require different calibrations, since the MAP is the only way the ECM "knows" where it is eleveation-wise.

I doubt it was cost-savings, GM probably just hedged their bets and decided to hope that most vehicles didn't experience problems when they WERE taken into higher elevations. That would make for some bad PR if there was a real performance loss based on the engine management at altitude, not the normal loss of performance due to altitude. My understanding is that OBD2 requires MAF *and* MAP, which would eliminate the need to "hope" that one doesn't fail and altitude can't be compensated for.
 
<~~ still trying to figure out where in CA does a road go over 9K?

I know my NA 6.2 is a dog (more of a dog than usual) when I go up to lassen peak. I need turbo!!!
 
Mikey von said:
<~~ still trying to figure out where in CA does a road go over 9K?

I know my NA 6.2 is a dog (more of a dog than usual) when I go up to lassen peak. I need turbo!!!
Click to expand...
Bristlecone Pine Forest up 168 from Big Pine on Hwy 395. If the gates are open you can drive to Barcroft Research Station at 12,500', otherwise you top out at about 11,700'
 
my friend's '99 grand prix gtp had more issues in colorado than my truck ever did. you'd think having the supercharger would help a lot, but he snapped the belt and it actually ran better after that. very odd.

next time you're in crested butte, you should stay at the old town inn (no it's not actually old). and my endorsemtent is in no way tied to the fact that my aunt and uncle own the place :wink1:.

http://www.oldtowninn.net/
 
Mikey von said:
<~~ still trying to figure out where in CA does a road go over 9K?

I know my NA 6.2 is a dog (more of a dog than usual) when I go up to lassen peak. I need turbo!!!
Click to expand...

Cal State route 108 over Sonora Pass is well over 9000 feet.
State route 120 over Tioga Pass is 9945 ft. It is the highest pass in Cal with a highway over it. Both of them are pretty good pulls.

PS Guess i should have thought about things a little more when I stated "the ECM does not care what altitude you are at" . I was wrong. Map measures Baro pressure when key is turned on at startup. Altho it does not measure altitude per say. It does know the atmospheric pressure and how dense the air is. It will make changes in timing, fuel curve, and IAC settings according to the baro readings.


But I still dont think there any special calibrations for altitude. The ECM can make any necessary changes and learn to run at any altitude as long as it is within its program.
 
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my friend's '99 grand prix gtp had more issues in colorado than my truck ever did. you'd think having the supercharger would help a lot, but he snapped the belt and it actually ran better after that. very odd.
Click to expand...

Very odd indeed. There was definitely something wrong with your friends car if that were true (no matter what altitude).

Regarding the truck and altitude, you may want to send an email to the tbichips.com guy. He knows that stuff inside out.
 
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