high to low altitude

[cwhubb]

I moved from 6300ft elevation to about 770 ft, I read a few sites that said if a carb was running great at one elevation that it should also run great at any elevation high or low and keep the jetting you have because the venturie signal ( ?) carb doesnt recognize altitude ...this is counter intuitive to me, if its recommended that you decrease jetting 2 numbers for every 2000ft increase wouldn't you increase jetting size for every 2000ft you decrease in elevation? I'm running a holley 950 (830 actual cfm) carb and it ran excellent at 6300ft

 

[Ron Ward]

You are correct. Increase the jet size for the lower altitude. You'll also want to pull about 6º timing out of the ignition lead.

 

[cwhubb]

thanks, the timing tips appreciated, I forgot all about that, I hope it runs as strong down here in the humidity as it did in new mexico this 112 degree heat index adjusted temperature is new to me

 

[Roughbird72]

You are correct. Increase the jet size for the lower altitude. You'll also want to pull about 6º timing out of the ignition lead.

Why would you pull 6° of timing out if the air/fuel mixture is corrected?

 

[mopar 3 B]

Also you lowered the boiling point of fuel, water and every liquid in the car.

 

[cwhubb]

@roughbird, I think the timing thing is octane related, at altitude you can run lower octane gas

 

[Ron Ward]

Why would you pull 6° of timing out if the air/fuel mixture is corrected?

At altitude, the cylinder pressure drops. Quite simply, there is just less air in the cylinder, and thus, you need a bit more ignition lead to compensate.

 

[lewtot184]

Also you lowered the boiling point of fuel, water and every liquid in the car.

actually increased the boiling/vapor point due to a higher atmospheric pressure.

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i don't look at jet number sizes anymore. i figure the square inches of area that a given size has and then figure the percentage of change i need and then find a jet orifice size to match up. i know holley claims flow numbers on their jets and maybe the old ones were made good enough to use flow but i don't think the new stuff is made that good. i'd increase jet area by 1% for every 1000ft elevation change as a starting point. timing may need to be reduced due to the increase in cylinder pressure from increased air density.

 

[cwhubb]

I always wondered why you could use less octane, less cylinder pressure...makes sense now. dave at hughes once told me I should be running around 13 cr at 6300ft, I thought he was nuts, guess not.

 

[69a100]

thanks, the timing tips appreciated, I forgot all about that, I hope it runs as strong down here in the humidity as it did in new mexico this 112 degree heat index adjusted temperature is new to me

So where you located now?

 

[cwhubb]

I was running 78's front and back in the carb I pulled some plugs and the timing mark on the strap was perfect, the thread base ring showed a slightly rich condition but only by a hair, 6300ft explains that. porcelain looked good. I kept the 78's in front and put 79's in the back, it should run good. havent messed with the timing yet, should I back it down one degree at a time or set it back 5* at a time?

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I'm in ne oklahoma, right on the border of mo, and ark. grove/jay area. really humid and average mid 90's temps with out the heat index, with the index it sometimes hits 102 and higher, 112 was the highest so far.