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 lower 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 means too big of a carb equals lower horse power/torque output, poor drivability, tip in stumble, poor low end torque 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, tyre size, trans type and converter 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 Calculator 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 companies is surrendering the optimum for low level mediocrity.

The art of fine tuning a carburetted engine is nearly lost, 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 carburettor around just to show our up and coming parts changers what they look like and as a comparison to modern fuel injection.....they keep them in a glass case like old bones.

If you’re not a carb tuner then you need to be sure and purchase your carburettor from someone who is, once you learn the basics and get it all dialled in keep a screw driver handy and the first person who attempts to "Tune" your carb....stab him!

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