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Re: Is this a good deal?



>I don't get this argument.  If your engine takes 750cfm to run at WOT/5000
>rpm, and 500cfm at
>WOT/3500 rpm, how can a 750cfm TB be better than a 900cfm at anything but
>WOT/5000?  Anything
>under 5000 rpm and you are over-sized with a 750cfm unit, right?  Is a
>900cfm unit at WOT/3500
>worse than a 750cfm unit at WOT/3500?  Or WOT/2000?
>
>Squash
>
This is the mistake of looking at this stuff based on only one point on a
curve, as is done with the cfm ratings on carbs or TB's! For most things
four barrel, the cfm is tested at 1.5" of HG pressure drop. Your best
performance will happen at a lower pressure drop. Best to figure about 1.5
RATED cfm per PEAK horsepower that you are aiming at for a hipo engine.
Otherwise, it will be somewhat restricted at the top end.

Having an oversized TB SHOULDN'T make any difference to an engine, other
than making the throttle feel pretty "sudden", UNLESS the injectors do not
atomize the fuel very well. With poor atomization, then the higher air
velocity will help atomize the fuel, and make the engine run enough better
to compensate for most (or maybe all) of the lost airflow.

There will ALWAYS be some pressure drop across a carb or a TB, no matter
HOW big it is. To say otherwise indicates a pretty deep misunderstanding of
the principals of fluid flow. Yes, there are sometimes standing wave
phenomena present which create fuel "standoff" from a carby, but these are
much better described as being anomalies resulting from a two phase flow
than as being the result of a "positive" pressure in the intake tract!

Greg



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