Carb choice for high altitude?

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MopaR&D

MopaR&D

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I currently have a 625 cfm Carter AFB on top of my built 360 with Magnum heads and am considering getting a Holley double-pumper to use when I race the car. I have read people have had good luck with 750 cfm carbs on 360s BUT I also read that engines don't need as much carb airflow at higher altitudes; I am at ~6000 ft. above sea level so would a 650 cfm be a better choice? Here's my setup I intend to have once I start racing my car at the strip:

Engine: 360 LA block bored .060", factory iron (untouched) Magnum heads, KB flat-tops for 10.5:1 compression, Voodoo 256/262 dur. cam (hope to upgrade to Howards 275*/.470" cam later on), Magnum Air-Gap intake, and Hedman shorty headers.

Driveline: 904 trans with factory higher-stall converter (2200 RPM I believe) and shift kit, maybe upgraded internals if I blow up my current one :glasses7:... also 8 3/4" rear with sure-grip and 3.91 gears, I may give slicks a try depending on how well my street Firestone Firehawk Indy 500's hook.

This is in a '70 Duster btw
 
I probably shouldn't comment here, since I lived my whole life close to sea level...:D

But on the surface, it looks to me like the bigger carb would be a advantage to getting more air in at altitude? It's really the fuel ratio that needs to be tweaked at that level.

I'm sure someone will let me know if my theory is correct or not...
 
See I would agree with you, mostly based off the fact that carbs are rated at the volume of air that can flow through, not the mass or density which is what changes at higher elevations. However I have also read that because engines pull much less vacuum at higher altitudes a smaller carb is better to maintain the vacuum signal at the boosters. I still don't know though it makes sense either way LOL... I want to just go with the 750 but I don't want it to give me a nightmare trying to tune it to run smooth and responsive esp. off the line.
 
dont change the carb, just tune it, pull jet out to lean it, open air bleeds for proper transfer AFR, Readjust idle mix after bleeds.
 
Being a pilot A/F mixture is critical to maintain proper performance of a naturally aspirated engine. Mopekid is right about vacuum at higher altitude. Later larger carb won't give you more HP at higher altitude due to air density. Fuel injection is the way go go if you can afford it for operating at higher altitude. Lean out your AFB to achieve optimum H/p and go for it. Or you can always slap a turbo on it to increase the amount of air you can force into the cylinders as well as fuel.
 
Keep in mind you loose about 1" hg per 1000 ft in altitude.
So the cam you have at sea level has already lost 6 in/hg at your altitude.
 
Late model TQ with the Alt. adjuster on the front.

PS, your tires won't hook.
 
the DP will out run a non adjustable air door AFB.....
 
Street

Demon

...

If you go to a traditional Holley, check out a 670 Street Avenger.
 
I already know about the changes in a/f going to higher altitude, i'm planning on getting a wideband o2 sensor before i start tuning. My current carb runs great it's set up more for economy though which i want to keep; i just wanna get a holley dp and set it up for racing then i can swap the carbs back and forth depending on what i'm doing. My question really just had to do with the airflow capacity and whether that changes with altitude...
 
This is my take on it.

more HP more CFM right!

Any given eng will have less hp at altitude(N/A)

aka smaller carb wins.
And I live at 6700 foot.
 
So it looks like Cudafever and i were kinda on the right track. Did some googling and found a tech article written by a guy who races and restores classic mustangs in colorado... he concluded that because of the lower airflow requirements of an engine at high altitude a smaller carb must be used to maintain a good vacuum signal. According to the article for an engine operating at 5000 ft. above sea level the required cfm airflow of the carb is multiplied by 0.912, meaning my 360 requires not even 600 cfm to make power up to 6000 rpm...

Interesting stuff, looks like a 650DP will be plenty for me
 
MOPEkidD-3 said:
So it looks like Cudafever and i were kinda on the right track. Did some googling and found a tech article written by a guy who races and restores classic mustangs in colorado... he concluded that because of the lower airflow requirements of an engine at high altitude a smaller carb must be used to maintain a good vacuum signal. According to the article for an engine operating at 5000 ft. above sea level the required cfm airflow of the carb is multiplied by 0.912, meaning my 360 requires not even 600 cfm to make power up to 6000 rpm...

Interesting stuff, looks like a 650DP will be plenty for me
Click to expand...

When I lived in Colorado (racing at Bandimere), I used a Holley 650dp on my mild 360 (eddy heads, 509 Mopar cam). I did a Proform main body swap on the carburetor to make it a 750dp and the car picked up 3 tenths and 3 MPH right off the bat. It picked up a little more after tuning.

Your article sounds logical from a drivability stand point, but the drag strip results speak for themselves. I would go with a Holley 750dp carb with the HP style main body (no choke horn) for max power potential.....IMHO.
 
You have the right idea.keep the aft for street .get a 4779 for racing....
 
With the converter you have, I would consider something more along the lines of a 750 vacuum secondary. The DP may be a bit of a handful to tune with a stock-ish converter...if you're planning on also using it on the street.
 
flyfish said:
When I lived in Colorado (racing at Bandimere), I used a Holley 650dp on my mild 360 (eddy heads, 509 Mopar cam). I did aProform main body swap on the carburetor to make it a 750dp and the car picked up 3 tenths and 3 MPH right off the bat. It picked up a little more after tuning.

Your article sounds logical from a drivability stand point, but the drag strip results speak for themselves. I would go with a Holley 750dp carb with the HP style main body (no choke horn) for max power potential.....IMHO.
Click to expand...

I think i will take the same route you did (start 650 and upgrade to 750 later), my engine currently is not built up as much as yours was it has untouched magnum heads and a small voodoo cam.
 
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