Altitude affect on Webers

[m5bb]

So if I were to drive my as yet un-built 3.5 engine from Atlanta to Colorado
and then drive in the Mountains, maybe up Pikes Peak 14000ft
with Weber 45 DCOE side drafts on a brand new motor with 10.5:1 pistons and a Paul Burke cam........... would I make it up the mountains without changing jets?

Whew, long sentence.

Gary

 

[Mike Goble]

David Reher of Reher Morrison Racing Engines has an excellent treatise on carburetors and altitude.

It is a misconception that you must lean out a carburetor at high altitude. The fact is that a properly tuned engine will use the same jets in Denver as it does at sea level....
In this age of digital everything, carburetors have an undeserved reputation as low-tech devices. In fact, a racing carburetor is a very ingenious system. A carburetor responds to differential pressure, and therefore it self-compensates for changes in barometric pressure. The gas in the float bowl is always subject to the prevailing atmospheric pressure; the jets deliver fuel in proportion to the differential between the pressure in the float bowl and the pressure in the induction system. So when the barometric pressure falls, as it does so dramatically in Denver, there is less pressure differential and therefore fuel flow is reduced accordingly.

http://rehermorrison.com/tech-talk-44-summertime-blues-how-to-live-with-bad-air/

 

[gwittman]

I don't agree with that on all carburetors. After racing almost 30 years I find to get optimum HP, many things including altitude effect the jetting needs for at least some carburetors. There may be some carburetors that are not affected by altitude but SU carburetors on my race car requires jet recalibration with change in altitude, temperature, air density, etc.

Fortunately, most of this tuning can be done with an adjustment of the jet needle on an SU without having to change jets or needles.

On a street engine, you can probably get by without having to change jetting for a drive up Pikes Peak. You may not be getting optimum gas mileage or power if is was tuned at sea level but I doubt you will have any problem with misfiring or not running. Just be careful when starting so as not to flood it.

If it is tuned for 14,000 ft and you take a trip at sea level, you could run the risk of running too lean. Then again, I don't know if Webers have self compensating features.

 

[Mike Goble]

I run my carbureted Bav pretty regularly over the Sierra to work in Nevada, and it acts just fine at 7000 ft. It's down on power, but it runs fine. On a hot day the DA on Donner Pass could be well over 10K ft.

 

[jmackro]

On a hot day the DA on Donner Pass could be well over 10K ft.

And "DA" means what?

District Attorney?
D-something Altitude?
Dumb A*s (who doesn't know what "DA" stands for)?

 

[Sven]

This is my understanding, which could well be tainted. Both carbs and EFI systems respond well to the pressure differentials surrounding the fueling. What they don't respond well to, is the effect the pressure differential has on the exhaust. At higher altitudes the volumetric efficiency of the engine is better because of the lower pressure that the exhaust has to "push" against (probably over simplified). In my case (with EFI) while tuning up in the Mammoth Lake's region 9,000 -10,000 ft, the engine was running very rich with AFR's dropping into the 10/11's. I had to set the barometric correction curve to take out 12-15% fuel.

So yes, you will likely run rich at high altitudes, which for a short hop over a pass or two is not a big deal. Obviously the power will be less in our naturally aspiration beasts, since at 10k you are only have ~75% of the air that you had a sea level.

 

[Mike Goble]

DA = density altitude

http://en.wikipedia.org/wiki/Density_altitude

http://www.dragtimes.com/da-density-altitude-calculator.php


One needs to remember that a fuel injected engine injects fuel into the engine, and if it injects too much it will be rich. How the system determines the fuel rate may be faulty. With a carburetor, it is the actual mass of the air itself that determines generally how much fuel goes into the engine, less mass flow through the venturi creates less vacuum, hence attracts less fuel from the bowls.
I don't think the exhaust enters much into the equation, you're burning less air, making less power, you have less compression, and your fixed size exhaust handles the flow better.

 

[m5bb]

Thanks so much guys, looks like I can do this.
Now I'm really excited. :lol:
Great info and explanation.

Gary

 

[DerSchwede]

One have to consider one more thing:

Even if the volume of air and fuel will adopt pretty good to the altered pressure at high altitudes, there will still be less oxygene in a given volume of air (at the higher altitude) and hence the engine will produce less power. You can gain a little of the lost power if rejetting but if it's necessary for a short mountain drive in a street application is questionable. The real problem if running rich over a longer period will be that the extra (unburnt) fuel will wash the oil off the cylinder walls and call for a new rebuild within a shorter time...

Cheers
A

 

[MMercury]

Pot stirring?

I am not sure I subscribe to the theory that one jet fitting will work at all altitudes. Factory jet settings were always a compromise for best all around performance, fuel economy, and in later years, emissions. So were most “tune” settings. I can think of plenty of cars I have driven or owned that benefited from some minor carburetor and ignition changes - at near sea level altitudes. By analogy, most piston-powered and carburettor-equipped aircraft also enabled the pilot to alter mixture settings - generally - to lean the mixture out at altitude. As an over-generalization, the more power under the hood, the more easy it is to overlook slightly degraded performance at sea level or altitude. Altitude may make the degraded performance more pronounced to the more sensitive operator (unless the engine is verging on a mixture that is too lean at sea level :wink No one would care to hill climb with a misfiring engine but the loss of a cylinder in a Model-T would likely be far more apparent than the loss of a cylinder in a lightened E3.

Having infrequently crossed the Continental Divide in a variety of vehicles, I recall on most inclines wishing for more power and traction. I cannot determine whether this had more to do with the incline or the altitude, but I do know most of my rides tuned at or near sea level “seemed” to perform better there than up in the Collegiate Range. In retrospect, the power loss comparisons were probably exaggerated even when driving a so-called "performance” vehicle, since the cars were probably too laden with gear and passengers to appreciate any meaningful power differential at higher altitudes. Temperature (Brrrrrrr) and prevailing conditions probably had a major effect too.

Science.

Weber-related sites seem to offer the same rule-of-thumb advice concerning high altitude jetting. You can determine for yourselves if it makes time-tested barometric sense. It might be interesting to hear from a few who live in the higher climbs and what changes, if any, they ordinarily make to migrating city cars.:wink:

From: http://classicinlines.com/WeberHighAlt.asp (A restriction in text length requires two separate posts.)

Stelvio

 

[MMercury]

Continued.

From: http://classicinlines.com/WeberHighAlt.asp (A restriction in text length requires two separate posts.)

Weber - High Altitude Tuning

​

While there is no exact formula for tuning or re-jetting a Weber carburetor for high altitude operation (4000 feet or more), the following steps should help to maximize the available power. Keep in mind that there is nothing you can do to stop some power loss at high altitude, as this is going to happen, due to the lack of oxygen at high altitude. Simply put, there is less air to mix with the fuel. However, jetting the carb properly to get the correct fuel mixture is the key in maximizing performance. This applies to both single and multiple, side-draft and downdraft carbs.


Factory Calibrations and Set-Up Before you go tearing into your carb, the very first thing you need to do is make a record of the factory calibrations or set-up. In other words, what parts were in the carb when you pulled it out of the box. Make certain you designate the differences, if any, between the primary and secondary circuits. Never rely on a list you found somewhere on a website or in a book, that says what your carb might or should have came with. Instead, remove the top of the carb and actually remove the jets, emulsion tubes, air correctors, check the sizes, then WRITE THEM DOWN. Don't use a small piece of scrap paper that could get tossed out in the trash, write them down inside the front cover of your Ford or Weber service manual, or anywhere you'll have a permanent record that won't get lost. This way you'll always have the original factory settings if you ever need to refer or go back to them.

Idle Jets: You will normally need to drop 1 jet size for every 2000 feet, once you go above 3000 feet. For example, if you used a jet size of 50 at sea level, you would install a jet size of 40 at 4000 feet, or 35 at 6000 feet. Of course, this is assuming the engine ran properly at sea level.

Main Jets: You will normally need to drop 1 jet size for every 1000 feet, once you go above 3000 feet. For example, if you used a jet size of 160 at sea level, you would install a jet size of 140 at 4000 feet, 135 at 5000 feet, or 130 at 6000 feet. Again, this is assuming the engine ran properly at sea level.

Air Jets: You will normally need to increase 2 jet sizes for every 1000 feet, once you go above 3000 feet. For example, if you used a jet size of 140 at sea level, you would install a jet size of 220 at 4000 feet, 250 at 5000 feet, or 270 at 6000 feet. Once again this is assuming the engine ran properly at sea level.

Fl[o]at Levels and Fuel Pressure: No changes for High Altitude operation.​

There is no shortcut or magic formulas for high altitude operation, you'll just have to work through it. The guidelines above are just starting points, or a rule of thumb. In the end, you'll probably need to invest in a range of jets, and tune the engine accordingly.

 

[Stevehose]

I am not convinced the idle jet needs to be smaller, this will restrict the volume of fuel getting to the engine and therefore choking it, if it needs to be leaner then the air bleeds need to be larger for a given idle jet size.