Posted by Ken (32.102.153.29) on April 10, 2001 at 22:48:13:
In Reply to: Re: carb/intake combo posted by Fastman on April 10, 2001 at 15:44:57:
Without regard to the make of carb, the following!
The ability of an engine to injest air is set by its cu/in size. Then subtract all the losses that affect air flow into the engine.May they be intake valve size, runner size/shape/routing,carb cross section area and design of venturie,air cleaner and anything else that offers restriction and you end up with about 70% filling ability.
Now then the total air intake will always be less than 100% except special cases of very long cam timing with special headers that extract exhust thereby allowing the cylinder to fill to a positive pressure then trapped by the valves closing.
Of course the air intake is directly related to RPM with the limit being reached when the restriction start adding up to create this limit.
The 302 at 6000 rpm will injest only about 550 cfm so carb sizing in based on this fact. There in some benefit to increasing carb airflow rating at wide open throttle in engines that have been cammed to run in the upper rpm band as long as the heads will flow the extra air (case for aftermarket heads).
As for absolute carb size on the engine,it has little to do with size and more to do with jetting as far as richness. These engines come with 2 barrel from the factory and did not run rich if there was no jet change. The small carb only limits the air flow into the engine therefore the power that could be produced.
Its called...volumetric efficiency.. If one increases airflow by a larger carb cfm rating then power goes up to the limit of the engines ability to take in air and is limited by valve size etc as mentioned previous.
When changing camshaft to longer timing specs, the ability to injest air in increased at the higher rpm and allows an increase cylinder filling again up to the limit of any restrictions be they carb,head or other.
Of course there is more to this subject if one get into fine points involving valve seat angles and other elements. For street engines concerned with fuel milage, the carb needs to be sized for high air flow at cruising speed for optumin fuel atomizing and burn and will be greatly influenced by main jet size.
At wide open throttle, more air at a air-fuel ratio of close to 12.5/1 in needed for max power. This richer fuel ratio is the result of the combination of the main jet size plus the power valve in the case of the Holly carb. Every engine parts combo will be somewhat different in reqirements for max power.
So carb size depends on what you want the engine to do and there will always be a need to compromise at least on a street car. Some other thing that will have an effect on all this for fuel mileage is gear ratio,torque converter (auto trans),weight,and of course how heavy the right foot gets and how often.
Said enough to cause sleep to set in for now.
Ken