[Biscuit6447]

Lets say you have have a built 327 sbc 11.1 cr, 292/300 adv. dur mech. cam, 2.02/1.60 valves, and a single plane intake.

If you need XXX cfm at sea level, then what CFM you need at 5000 ft??

Is there just a universal conversion table, or does it depend on other factors?


On the topic of air flow... At what CFM will a K&N 14x3 air cleaner element start to restrict flow??? at what point will a x-stream top lid element be beneficial???


Thanks, Josh

 

[BugRod]

I am not aware of any cfm change for an engine at altitude, I do know that you have to lean down your mixture as you climb.

 

[curtis73]

Agreed. The engine ingests air based on a pressure differential; less pressure in the cylinder means the atmosphere pushes air in through the carb. There is no change in CFM requirement, just a change in how many molecules of air are present per cubic foot of air.

Reducing CFM rating of the carb won't move the same amount of air as sea level simply because you go up in altitude, CFM is CFM anywhere you go. Most carbs do a pretty good job of keeping accurate metering; since there is less mass of air flowing past the venturi, there is less fuel being sucked in and its self-regulating to a point. I find that at more extreme elevations (like above 8000) you start getting rich enough that its wise to jet down... but that depends on the carb.