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High Altitude Carburation

13K views 51 replies 14 participants last post by  michaelfgoss 
#1 ·
I just moved to Colorado Springs, the locals here tell me that I may have to have my carb adjusted (as I had suspected) to be able to run at this altitude.

Is there anyone out there with a story of how they handled this?

Thanks,

Mike
 
#7 · (Edited)
Michael...listen to the locals. The fellas here from the coasts probably don't realize how high it is in COS.

South CO Front Range is an area that has an altitude that can vary from 5000 feet in Pueblo (nearest drag strip) to 7200 feet (Air Force Academy or Monument Hill just north of town on the drive to Denver). The city of CS can be as low as 6000 feet or as much as 7000 feet depending on where you are. Leaving a sea-level set of jets will cause it to run pig rich at that altitude. It'll certainly run, but it'll be rich.

Engines are (as a general rule) down on power 3% for every 1000 feet in altitude because there is less oxygen. You will notice this in your car if you came from sea level. Next time you land in COS, take a look at how long that runway is...it's not just because they land big planes on that Air Force base. It's because lift is lower because of the depleted atmospheric density at that altitude. It makes sense to put in smaller jets in your car. It won't make it slower...trust me, it's already slower!

Put an air fuel ratio gauge on it if you don't believe me...you gotta lean out a street motor at these altitudes.

Adjust the jets and throw in more timing (that lean mixture takes a little longer to burn). Generally, 1 jet size per 2000 feet so three sizes down primary and secondary is the rule. I run 64 primaries and and 70 secondaries in my 650 and it's just about right.

BTW...I live in Laramie, WY...altitude 7200 feet and grew up in Colorado Springs.
 
#12 ·
Pretty much spot on information!

Rick L lives in an area that is only 138 ft above sea level, not where it ranges from 5,000-7,500 ft. If he lived here he'd more than likely be re-jetting his carbs...or his car would run very rich and lack significant power...just saying:)
 
#8 ·
You don't need to lean out the main jets because a carburetor is self compensating. Atmospheric pressure on the vent tube is less at altitude - this means there is less pressure forcing fuel into the venturis through the main jets. The pressure drop across the venturi is created by airflow. That's a crude explanation of how that works.

On the other hand, the idle and transition circuits are unaffected by air pressure or flow, and they may need adjustment.

I know many won't agree with this, decide for yourself. If this was a real need, you would have seen car dealers and good automotive shops offering re-jetting service in high elevation locations for the last 50 years. You would also see guidelines for re-jetting for elevation in service manuals.
 
#11 ·
Depends on carb, Most have recommendations for metering rod size and jet size for over 5000' elevation. I have the info for Edelbrock Performer. Also have the complete kit of jets and metering rods. Live north of Denver, about an hour or so from you. As the others have said, less oxygen at this altitude. Easy to over fuel an engine. If you have to large a carb on a small engine, better off to go to a smaller carb.
EFI doesn't seem to have the issues as much as a regular carb.
 
#13 ·
"EFI doesn't seem to have the issues as much as a regular carb."

Warren, my other ride is a Corvette.
I'm near 5000 ft. here and love it when I can go up and down the mountains to sea level and not have to worry about it.
The computer does it all.:tu
 
#14 ·
"Rick L lives in an area that is only 138 ft above sea level, not where it ranges from 5,000-7,500 ft. If he lived here he'd more than likely be re-jetting his carbs...or his car would run very rich and lack significant power...just saying"

I don't think I'm quite at 138' but that's close. :)

What I do know is that when I was drag racing and travelling a lot, we'd occasionally race at Amarillo (about 4000') and at Denver (track elevation 5800').

Very rarely would we jet down the carb. In fact sometimes we'd jet up from the Houston/Dallas tracks we usually ran at because the air was drier and the termerature was cooler.

What we did do pretty aggressively for high elevation tuning was to increase the timing. Usually about 4-5º for Amarillo and 7º for Denver, relative to low elevation.

I learned most of my high elevation carb and ignition timing tuning from David Reher at Reher-Morrison Racing Engines, as I was a customer there when we first started travelling to places like that.

As for the EFI, yes EFI does need to be tuned to lean out at elevation. On an OEM vehicle, it's programmed into the computer from the factory. With aftermarket EFI, you have to get that from a supplied base map, tune it yourself, or use a wideband O2 sensor. EFI also advances the timing when the elevation goes up (MAP pressure goes down). It's the same principle as adding timing with a vacuum advance mechanism.
 
#17 ·
Rick's usually not wrong so I respect his opinion.

Rick...do you think it's different for street applications vs. race applications?

Not going to smaller jets at higher altitudes seems totally counter-intutive. Asking an honest question...not trying to start a ******* match.
 
#18 ·
I understand your concern.

There is no real difference between a race carb and a street carb in the context of this discussion, except for one thing.

Our race carbs (Holleys) had the power valves removed and plugged and were jetted up the usual 6-10 jet sizes that you do when you plug them. For a street carb with power valves, you'd want to select a power valve that wouldn't open at normal cruise. That would probably require a lower number power valve than the carb would come with, or need at sea level. An Edelbrock or AFB carb would want the same. That way you wouldn't go richer too soon.

We ran engines with single 750 Holleys double pumpers, single 1050 Dominators, and 2 Dominators. They all responded the same.

I was hesitant to name drop. But David Reher does know what he's doing, they won multiple season championships back in the day in NHRA and IHRA Pro Stock. And they won multiple races in Denver and Amarillo. And he's not the only one, every competitive racer I knew that raced up there knew what to do. It took me a long time to understand what to do with weather and elevation tuning on carbs with a degree of finesse. As I said, with a carb you jet up and down with temperature and humidity, you increase and decrease timing with elevation/barometer and humidity. Notice that humidity is the one in both columns.
 
#19 ·
I agree, Rick does know what he's talking about, but why do the manufactures tell you to change the set up for altitude? Maybe they just want to sell more parts.
I did buy my 57 from down south, low altitude. It had a holley 750 on it. When it came off the car hauler here in Denver, it hardly ran. I down sized the carb as recommended, re-jetted as recommended, and changed timing and all was good.
 
#20 ·
"why do the manufactures tell you to change the set up for altitude?"

Please post or link specific written guidelines from Holley or Edelbrock. I'd like to see them.

I'm not posting what I post to BS you guys. I am trying to keep you from doing something unnecessary that will reduce performance rather than help it.
 
#22 · (Edited)
"why do the manufactures tell you to change the set up for altitude?"

Please post or link specific written guidelines from Holley or Edelbrock. I'd like to see them.

I'm not posting what I post to BS you guys. I am trying to keep you from doing something unnecessary that will reduce performance rather than help it.
Regarding Holley, Altitude above 4000' - Power Valve www.holley.com/data/Products/Technical/199R7903-2.pdf

Normal tunning - www.holley.com/data/Products/Technical/R7849-2.pdf
 
#21 ·
You need to re-jet leaner for high altitude, that is a fact. The problem is most don't know what their specific air/fuel ratio was to begin with. If you were tuned spot on for sea level you will need to jet down as the carb manufacturer's suggest. Most everyone runs "out of the box" and has no idea where their carb is really at.
 
#24 ·
Rick, your own words seem to tell the story about elevation, doesn't it?

"As for the EFI, yes EFI does need to be tuned to lean out at elevation. On an OEM vehicle, it's programmed into the computer from the factory. With aftermarket EFI, you have to get that from a supplied base map, tune it yourself, or use a wideband O2 sensor. EFI also advances the timing when the elevation goes up (MAP pressure goes down). It's the same principle as adding timing with a vacuum advance mechanism."

As to carbs, the difference is with street rides, you have to worry about idle settings and cruise jetting. Where at racing it isn't needed.
 
#25 ·
I respect Rick L's knowledge and he is usually right on in his assessments.
Yet there is a big difference in tuning for daily driving vs. a quick run or two down the quarter mile at altitude. What one can achieve tuning a carb for the track for a short period of time can not in any way be compared to what will work on a street driven driver.:)
I'd rather take the carb manufacturers recommendations for tuning for a street driven car any day...seems they rather know what and doesn't work with their carbs, for performance, fuel economy, etc..
 
#26 ·
If you guys had read all my posts, I did bring up power valves and idle settings.

And there is no question that the fuel requirements change with elevation.

Think of the carburetor as an analog computer if you know what that is. It's programmed with orifices and springs. It does a pretty good job of adapting to various conditions. On the other hand with EFI you have to tell the computer what to do in every situatuion or condition by programming it.
 
#30 ·
A two stage power valve was used in some Holley carbs for RVs. Since these are heavy vehicles that tend to cruise at fairly low vacuum levels (and tend to lug), they have a "medium" enrichment that will let this type of vehicle richen up and make a bit more power (and maybe hold some vacuum) on a slight grade. Then if the manifold vacuum drops even lower, the usual WOT power enrichment kicks in. Keep in mind that this is more about richening the fuel mixture from cruise. I suppose that it might be of advantage on a car at very high elevation. You need a carb with a metering block that accepts a 2 stage power valve. Or swap in the metering block and 2 stage power valve to a carb without that.

To me this just demonstrates that the basic jetting doens't need to change. Maybe others don't think that way.
 
#31 ·
To me this just demonstrates that the basic jetting doens't need to change. Maybe others don't think that way.
Spot on as usual Rick, hence the reasoning I used for posting that little tidbit.

Rick, you learned from one of the best out there. My tutor was Franz at Chaparral Race Cars, Midland Texas along with Gerald Davis at Race Engine Service in Midland, TX. What you say is gospel, some just don't believe.
 
#32 · (Edited)
Rick, here is some stuff you asked be posted out of the Edelbrock owners manual pertaining to changing rods and jets for altitude.
As with you, we aren't making this stuff up. Holley has similar info. What you stated makes perfect sense, but the various manufacturers are also saying re-jet for altitude...

From page 9:

High Altitude
Altitude has a direct effect on the operation of most carburetors. As the altitude increases, the air becomes less dense so a carburetor, originally calibrated at low altitude, delivers too
much fuel and the engine runs richer. If the preceding tuning procedure is performed on an Edelbrock Performer Series carburetor, a proper high altitude calibration will result. If the vehicle
was calibrated at lower altitude, however, and is to be driven at high altitude temporarily, it is not necessary to repeat the complete calibration procedure. Instead, use the rule of: “2%
leaner per 1500 feet” and the Calibration Reference Chart for your model carburetor. For example, with a #1405 at baseline calibration (location #1 on the chart) and intended operation
at 6000 ft. altitude, you would want 6000 divided by 1500 x 2% = 8% leaner calibration. That would be location #24 on the chart which would require only a rod and jet change.

Model #1407 & #1410
ROD/JET REFERENCE CHART
REF # MAIN JET METERING ROD CHANGE FROM BASE
1....................1433 (.113")............................1453 (.071" x .047") ..............................none - stock calibration
2....................1433 (.113")............................1450 (.070" x .042") ..............................Rod
3....................1433 (.113")............................1449 (.070" x .037") ..............................Rod
4 ..................1434 (.116")............................1454 (.073" x .037") ..............................Rod & Jet
5....................1431 (.107")............................1443 (.063" x .047") ..............................Rod & Jet
6....................1432 (.110")............................1445 (.065" x .047") ..............................Rod & Jet
7....................1433 (.113")............................1452 (.070" x .052") ..............................Rod
8....................1432 (.110")............................1441 (.062" x .052") ..............................Rod & Jet
9....................1433 (.113")............................1447 (.068" x .047") ..............................Rod
10....................1434 (.116")............................1456 (.073" x .047") ..............................Rod & Jet
11....................1432 (.110")............................1443 (.063" x .047") ..............................Rod & Jet
12....................1433 (.113")............................1445 (.065" x .047") ..............................Rod
13....................1434 (.116")............................1453 (.071" x .047") ..............................Jet
14....................1434 (.116")............................1449 (.070" x .037") ..............................Rod & Jet
15....................1433 (.113")............................1443 (.063" x .047") ..............................Rod
16....................1433 (.113")............................1442 (.063" x .037") ..............................Rod
17....................1434 (.116")............................1444 (.065" x .037") ..............................Rod & Jet
18....................1432 (.110")............................1452 (.070" x .052") ..............................Rod & Jet
19....................1433 (.113")............................1457 (.073" x .052") ..............................Rod
20....................1433 (.113")............................1456 (.073" x .047") ..............................Rod
21....................1433 (.113")............................1454 (.073" x .037") ..............................Rod
22....................1431 (.107")............................1445 (.065" x .047") ..............................Rod & Jet
23....................1431 (.107")............................1444 (.065" x .037") ..............................Rod & Jet
24....................1432 (.110")............................1449 (.070" x .037") ..............................Rod & Jet
25....................1431 (.107")............................1447 (.068" x .047") ..............................Rod & Jet
26....................1432 (.110")............................1456 (.073" x .047") ..............................Rod & Jet
27....................1432 (.110")............................1454 (.073" x .037") ..............................Rod & Jet
 
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