Greg, the recommended (by Ford, Holley and any other reputable carb maker) 4
barrel carb for the 351W is 600-650 CFM.
Sizing the carburetor is based on air flow in CFM at wide open throttle. It
is a simple calculation of engine size in cubic inches divided by 1728 and
multiplied by max RPM divided by 2. If you use max RPM at 6000 (for a mildly
tuned and balanced engine), for the 451W the number comes out to 783 CFM.
This is at 100% volumetric efficiency ( unachievable), an engine with a
sporty cam, headers, intake manifold and reasonable polishing should produce
about 80% efficiency which results in a CFM of 626, right in the range of
the Ford recommendation.
The stock '75 351W was probably set up very conservatively at 4000 RPM and
70% efficiency which requires 365 CFM. Note the horse power of the stock
'75 engine is around 150 while the stock '69 with a 4 barrel and higher
compression was 290 hp at 4800 RPM, (250 hp with the 2 barrel).
The smaller you make the carb for a given set up the higher the depression
in inches of water, in other words depression is a result of the carb
selected. The important criteria here is the need for depression at idle,
the bigger the carb lower the depression and poorer the idle, that's why a
really high performance engine idles so badly, one answer to this is the
variable 4 valve system used in modern cars.
The only basic parameters that improve engine performance are Volumetric
Efficiency and RPM.
Hope this helps.
Owen Evans
#1355
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