dick, good points. a simple rule of thumb is 100 cfm of carb per liter of
displacement. this works across the entire carb/displacement range. ted
Sally or Dick Taylor wrote:
> E-mail message
>
> From: madmax_xx@hotmail.com (Shane Ingate) Date: Thu, Jan 9, 2003,
> Pack) Subject: Re: PI vs dual ZS vs triple ZS -- dumb questions
>
> Shane writes:
> A perennial sin (especially in the hot-rod world) is to over-carb an
> engine. The first major step to be made is to simply examine the air
> requirements of a motor to figure out how much carb one needs. How often
> do we see 750 cfm Holleys on a street-driven small block V8 when they
> really need only a 600 cfm Holley? One down-side of over-carbing is poor
> throttle response.
> I do not know what a ZS flows; I'm *guessing* it is around 150 cfm (Bob?
> Vance? Dick?). If my guess is right, then two of these will flow 300
> cfm. Three of them would be 450cfm, which is more than this motor needs
> except a worked motor at really high rpm. For comparison, 40mm DCOEs
> flow over 200 cfm, so triples would flow 600 cfm, which is WAAAAY more
> than this motor could ever Shane
>
> Dick, replying---I think Shane is right about some owners
> overcarburetting their engines. We know it looks cool to hang either
> larger or more carbs on an engine, as all but the most dedicated of us
> spend more time looking at such an engine and bench racing, than
> actually competing.
>
> Back to the "Cubic Feet per Minute" of air engines such as the TR Six
> require, I'll pass on the formula: ZZzzzz. Hang on, there will
> be something here for nearly everyone!
>
> Theoretical cfm = rpm x (cubic inches) displacement divided by 3456.
> (3456 is the constant for CF) So...
> 6,000 rpm X 152 / 3456 = 264 cfm.
>
> A good, stock engine operates at around 80% Volumetric Efficiency, so
> the "theoretical" 264 cfm is now down to requiring only 211 cfm.
>
> On a well prepared street engine, V.E. can go back up closer to 90%.
> (238 cfm)
>
> A race engine can actually exceed the 100% thru ram air phenomenia, so
> at say, 110% it can use 290 cfm.
>
> Forced induction, depending on boost pressure, can make this number jump
> up to 350 cfm, or more.
>
> IF the 1.75 carbs flow near 150 cfm, (I think it's higher) this would
> mean that two of them could easily take care of air flow.
>
> For those who race-shift closer to 4,800 rpm, which is a good point for
> the stock, or mildly modified engine, this gear change will bring you
> back close to the max. torque of 3,000. Going beyond 4,800 (in anything
> but top gear) will not only push the engine beyond where power starts to
> fall rapidly, but will then lose the place of max. torque when the shift
> finally occurs.
> Engines with cams that have their max. torque up higher, and can easily
> exceed 6,000 rpm can adjust accordingly.
>
> So, what would this mean if the TR engine had more air flow capacity
> than needed? Probably on either an SU or ZS setup with more than two
> carbs, it would suggest that the engine would not lift the air valves up
> far enough to take in extra air, nor expose the thinnest parts of the
> needles for max. fuel. These are "smart" carburetors. If the demand
> isn't there, it will not overproduce. This doesn't mean that one
> couldn't pour too much (or too little) fuel into the motor. The jet and
> needle size still has to be right for the application. I'd say those
> with Webers have their own sorting out to go through.
>
> Coffee time...
>
> Dick Taylor
--
Ted Schumacher
tedtsimx@q1.net
http://www.tsimportedautomotive.com
Pandora, Ohio, USA
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