Technical Do any carburetors compensate for altitude changes - sea level to Pikes Peak?

Wondering if maybe carbs with metering rods will lean out at high altitudes? Have had experience with a sea level jetted carb barely being able to power a sbc at 7000 feet elevation.

 

No

 

computerized ones with injectors.

 

I have to go along with that no. Remembering my car stumbling a bit going over the pass on US 50 in Colorado in 1969 and being told by the gal in the cafe at the top to step outside the door and look at the sign above it when I said something about it. At over 10000 ft you couldn't expect one to run perfect.

 

The 80s Quadrajets do..

 

Airplanes had to have it. But for cars Holley had a miser or a power carb with a solenoid to control the needle in the early '80's but they went away quickly. Miser could lean and the other enrich from a standard setting. Early computer carbs used this.

 

The emission carbs of the 80’s would require a lot of work, but probably doable although outside the scope of the HAMB. If you’ve been there before and had to make carburetor changes you should have a handle on setting up a spare carb and taking it with you if it’s in the budget.

 

"Constant vacuum carbs" such as SU's (british) and, I've forgotten the name, but squarish and once made in Napa are self compensating for altitude.

 

Predator carb

 

Airplane carbs that bring 2 different grades of Avgas do...

 

Why worry about it?
In 2013 I drive my rod from northern Illinois to Vancouver to L.A. and back home through Denver. Had an AFB style carb on the 350 at the time.
Going out we crossed the mountains on I90 in Montana. Coming home we crossed on I70 in Colorado. Not the most extreme altitudes, but well above the 700' we tune for at home.
The only adjustment was to take a dime and increase the idle speed slightly.
No problems.

 

I'd agree if you're just scootin' over the hill it isn't a huge deal If you're going to spend some time at those elevations, or if you live in places like Leadville, then it becomes important. Note that the reverse is not the case at all - engines tuned specifically for high altitude will run lean at low altitude and will likely burn a valve or two.

It isn't just jetting to think about, the power circuit is maybe even more important. I remember as a little kid flatlander cars and station wagons lined up near the summit of Pike's Peak in a traffic jam and watching a thin but steady stream of pure gasoline dripping out of the tailpipe of the cars in front of us. Power valve or carb power circuit opening points are based on engine manifold vacuum, for example at high altitude a Holley power valve suitable for Iowa will never close under any operating conditions up in the High Country.

 

One jet size per 1000' elevation or 20 degrees Fahrenheit. Brian Wilson in two lane blacktop must have had OCD.

 

Common carbureted piston engines used in a high percentage of light aircraft do not automatically compensate for altitude. But they are equipped with a mixture control operated by the pilot to lean or enrich as circumstances require, which is not always altitude related.

Even the (mechanical) fuel injected engines typically have manual mixture control for the same purpose. Same for turbocharged engines.

Older piston engine commercial and military engines, I believe, often had some altitude compensating carbs, but they also commonly had Flight Engineers who handled the engine functions in most phases of flight.

Many aviation people think that it is long overdue that certification standards be amended to include EFI and electronic ignition systems. The Experimental category can legally use those systems.

Ray

 

I lived in Flagstaff AZ for a while and ran my 57 Chevy LT-1 with Edelbrock C3B and a Holly DPer. I think 720 cfm it ran fine up in Flag and down in Phoenix or Sedona and even drove up to the ski area and down to the Colorado river out on the Havasupi reservation. I never raced it or raved to 7 grand but normal stuff and going through. the gears it ran fine.

Pat

 

Not automotive but the Schiebler on my Indian has a mixture control knob and I can reach down and adjust it richer it leaner a click or 2 when riding..

 

I believe a Ford variable Venturi would as it is dependent on vacuum as to how the venturis work. I wouldn’t run one though. They were a miserable POS to get right.


Sent from my iPhone using H.A.M.B.

 

Predators ?

 

Blow through turbochargers with a carb inside a pressure box
The air/fuel ratio will remain constant, but the boost pressure will drop off at altitude.

 

2150 Motorcraft 2 bbl had a high altitude compensation valve on some models.

 

NOPE -

If you are racing, you will be practicing at different altitudes. You need an 02 sensor for proper information on the setup.
An average (bottom to the top)...of fuel metering is the best that you can do with a carburetor.

Been there, done that, back in the old "dirt" surface days..!

Mike

Note - Actually they do, only a VERY small amount. Not nearly enough to make much difference when going to 14,000+ ft..!

 

This is what I was going to say. I have had a couple on some FSJ's.

 

I agree! Except it was Dennis Wilson, who is no longer with us. Brian is though, kind of.

 

No.

If one studies the Carter (Carter introduced metering rod technology in 1929) documentation, one will find that Carter released standard rods, rich rods, and lean rods. Typically:

Sea level - 4000 feet - standard rod
4001 - 6000 feet - 1 size lean rod
6001 - 7000 feet - 2 sizes lean

While a very few 3 size lean rods were offered, generally the 2 size lean rod was suggested above 7000 feet. The rich rods were typically offered on export carburetors to countries with poorer fuel than the USA (how times have changed! )

Holley, Rochester, and Stromberg used fixed jets (at least in H.A.M.B.-friendly carburetors). Typically, a 0.002 inch diameter change per 5000 feet. Thus:

Sea level - 5000 feet - standard jet
5001 - 10000 feet - 0.002 smaller diameter jet.
10001 - up - 0.004 smaller jet.

We have custom-made and sold lots of lean metering rods for folks that live in altitude areas.

Jon.

 

I'm sitting here at my kitchen table having my coffee and looking at Pikes Peak, it looks like about 14,115 feet high from here and I can tell you any carburetor that runs good at sea level would have avery hard time up there

 

Brian had his own "altitude" issues at one time...

 

If one does one's homework ahead of time, AND uses Carter AFB carburetors; one can carry the lean rods in the glove box, and change them as needed.

It takes longer to open the hood, and remove the air cleaner than to change the rods. While I do not recommend this, to prove a point, I changed a set while the engine was running. Very easy calibration change.

Jon

 

Dennis...

 

Great responses! Going to go for the cross country trip, Mich to Cal, 47 Fleetline 2 dr sedan, 350 sbc, Edelbrock carb. Got spoiled the last few trips with computer controlled rigs.