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Carb Sizing
Considering that an engine is only an air pump...air in and air
out, with every cycle of the engine it inhales a measured amount of air
based on the cubic inches of displacement and exhales exactly the same
amount. The variable is the amount of fuel you add to the air and
what the combustion chamber does to it.
When you go WOT your opening is a big hole to the intake port,
the bigger the hole the lower the velocity or air speed....too big a
hole and you lose velocity this velocity and because the valve open time
is so short it can't fill to it's max potential or sweep the spent exhaust
gases out of the combustion chamber...too big of a carb equals lower horse
power/tork output, poor drivability, tip in stumble, poor low end tork and
for the Drag Racer a time slip that shows a mediocre 60' time.
If the carb is too small it creates a restriction or vacuum condition
in the intake and again the intake valve can't deliver a full charge.
These variances in the vacuum signals send a message to the carb
and dictate how much fuel it's going to add to the air rushing through
it.
This balance is critical to achieve optimum performance.
This is not a mystical theory but a calculated scientific formula
based on:
Load: Weight, gearing, tire size, trans type and convertor
stall.
Demand: Cam lift, duration, manifold type, intake runner size,
valve sizes, header efficiency, basically the ability of the engine
to breath.
Combustion Chamber efficiency: Size and shape of the chamber, compression
ratio, swept volume and combustion chamber quality.
Application: Drag Racing, Road Racing, Off Road, Street, Circle Track, Trailer
Queen, Grandma's 67 Polara Station Wagon Grocery Getter.
Here's a quick Calculkator designed for Demon Carb's:
CFM Calculator
When all these things are considered the correct size and series
of carb can be chosen and optimum efficiency will be obtained.
To think that you can carb the worlds performance market with 3 carbs
like some company's is surrendering the optimum for low level mediocrity.
The art of fine tuning a carbureted engine is nearly lolst, there's only
a few of old timers left that can listen and feel the performance of an engine
and determine which screw needs to be turned, which way and how far. Modern
automotive tech schools keep a carburetor around just to show our up and
coming parts changers what they look like and as a comparisom to modern fuel
injection.....they keep them in a glass case like old bones.
If your not a carb tuner then you need to be sure and purchase your carburetor
from someone who is, once you learn the basics and get it all dialed in keep
a screw driver handy and the first person who attempts to "Tune" your
carb....stab him!
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