Give that man the prize. I couldn't have explained it any better. But I might
disagree
with trying to set the redline on my brick at 6000 (I'm thinking 5200 would be
about
as wound out as I'd want to go. Also, with the comment someone made about
rather being overcarb'd versus under. I disagree. just a small bit undercarb'd
is
the way to go. It's better to take a small hit at redline than get the lag on
takeoff
(if you put a higher CFM carb than you need, you'll get that annoying lag when
you
stomp on it)
Seth
#1544
*********** REPLY SEPARATOR ***********
On 12/12/01, at 3:06 PM, Owen Evans wrote:
>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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