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Altitude and spark plug problems

2K views 10 replies 7 participants last post by  Rick_L 
#1 ·
I live at 8000 feet above sea level and have a stock '57 with a rebuilt 283 and 2gc carb.
I have major problems with it running rich with black smoke.

First problem found, was I givin and installed AC45 spark plugs instead of AC45S with the extended tip.

Then I was told to lower the float and advance the timing and it might help.

Next I was told to change to smaller jets and hotter plugs.

What do you guys think?
 
#2 ·
Advancing the timing is the best change you can make.

We used to drag race in Denver and Amarillo as well as at sea level. We would usually advance the timing 7-8º when we went to the "hill". Little or no change to the jetting. And we'd run more rear end gear.
 
#3 ·
Change the jetting for sure. Although Rick used timing advance in drag cars, where you pump all the fuel in you can handle for more power, a stock vehicle needs a proper air/fuel mixture to operate efficiently in everyday driving. Lowering the float should have no effect, unless it's so low as to starve the fuel flow though the jets. That would be a pretty fine line to tune to. Years ago I had a carburated Camaro that I drove to Golden, Co., for a few weeks. It didn't take but a couple of days to realize I had to rejet the Holley. I don't recall how much I dropped it, a couple of sizes at least, but it ran fine once I did. Back to the old jets once on the way back to the lowlands.
 
#4 ·
"a stock vehicle needs a proper air/fuel mixture to operate efficiently in everyday driving."

And a race engine doesn't need a proper air/fuel mixture to run fast? Sure it does.

The reason the carburetor doesn't need any changes is that it's self compensating. Less air means less fuel, no changes needed. Altitude is the same thing as using less throttle. When you open the throttle more, you still have the right a/f ratio - and that's what altitude requires, more throttle.
 
#5 · (Edited)
Well, I don't want to start a ******' contest here, but, I have to disagree with you, Rick. When racing, pedal to the metal, you pump all the fuel you can through that engine, open the exhaust, and go. Everyday driving constitutes something much different. Carburetors are not self-compensating for air fuel mixture, the mixture remains fairly consistent throughout the throttle range, jets detrmining the the fuel portion, throttle position determining the air. Open the throttle, you get more air. More air pulls fuel through the venturi at a higher rate, but the ratio remains the same. This is why cars in the carburetor day were sold with high altitude packages. Today's fuel injected, computer controller cars adjust the fuel delivery rate based on oxygen content available in the environment. The throttle continues to let in the same volume of air for a given position. Mention of such here:

http://web.archive.org/web/20051214181804/http://www.saabclub.com/242/altitude.htm#physics

http://racingarticles.com/article_racing-33.html

Mike
 
#9 ·
Rick, you're not seeing the whole picture, the volume of air flowing through the carb might be the same but the air at higher altitudes is less dense so you have a lower amount of oxygen at high altitude making a richer fuel mixture...


Info from Mortec:
JET CHANGES AND ALTITUDE AND TEMPERATURE
Holley carbs are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 foot increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carb's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather variables, temperature, barometric pressure, dew point and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in" (if they are bracket drag racers).
 
#11 · (Edited)
Chevynut, you are correct on the mass air flow. The air density difference at altitude affects mass air flow.

And you're right, the entire difference is not made up, though most of it is.

When we were racing, one reason we didn't have to change much if any was that typically
we'd be set up for a lot of humidity when running at sea level (or at least less than 1000' elevation) at tracks in Texas and Louisiana in the spring and summer. This also requires less jet size than dry cool conditions at those elevations. Since the dew point and humidity is typically much lower at higher elevations, they tend to cancel out.

My opinion, the one jet size per 2000' recommendation is too much. My recollection is that we'd run the same or one jet size smaller at Amarillo (4000' elevation) and 1-2 jet sizes smaller at Denver (5800'). Part of the jetting decision was also based on temperature and the actual barometer. Keep in mind, we sometimes changed the jet size the same amount in a given day at sea level based on weather changes.

Timing is where you can get some performance back. The octane requirement is much lower at higher elevations, so you don't get into detonation issues.
 
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