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There is a great deal of confusion when people think about airflow in a engine. Carburetors are rated in CFM and engines require a certain amount of CFM for a given RPM. But what does this really mean? How much air is really being used by an engine? The confusion is probably due to a misunderstanding of the relationship between volume flow and mass flow. Let me show you what I mean with some examples using my favorite engine, the slant six. Let's start off by comparing an engine under no load and one with a
load. When the transmission is in neutral, the only load on the engine is
the internal friction of the engine and accessories attached to it.
Intuitively, revving the engine with no load should require much less airflow
than if the car is accelerating on the road. Now let's compare two identical 225 slant six engines with the only difference being that one is equipped with a 2bbl carburetor and the other has a 4bbl. Let's assume that the 2bbl carb is rated at 325 CFM to produce a 3.0" Hg pressure difference at 5000 RPM while the 4bbl carb is also rated at 325 CFM to produce a 1.5" pressure difference at 5000 RPM. At 5000 RPM, assuming 100% volumetric efficiency, our engine will require 325 CFM. Like our example before, the density of air at 3.0" Hg & 70°F is 0.0674 lb/ft³ while at 1.5" Hg & 70°F, the density is 0.0712 lb/ft³. The mass flows of air that the engines are using are: Mass flow with 2bbl carb is 325 ft³/min x 0.0674 lb/ft³ is 21.94 lb/min. With all things being equal, the 5.6% more air being ingested by the 4bbl-equipped engine should result in a similar gain in power. As you reduce the manifold vacuum (related to carburetor restriction) at wide open throttle, you increase the mass of air getting into the engine, thereby increasing its power. With a more restrictive carburetor (like the factory 1bbls), the engine produces less power because the pressure (and density) of the air finally reaching the cylinders is lower. |
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