[Tom Vaught]

Going to talk a little bit about "Uniform Distribution".

"Uniform Distribution" means "Equal" Fuel and Air Distribution to the various cylinders.
"Non Uniform Distribution" means "Unequal" Fuel and Air Distribution to the various cylinders. It causes a loss of power and efficiency in the engine.

"Uniform Distribution" assumes a number of things:
a) Good atomization of the fuel from the carburetor to the runners and cylinders.
b) Use of proper fuel of the correct volatility for the "seasons" the engine and fuel system will be exposed to.
c) Using proper methods of engineering/ techniques to assure vaporization of the fuel.
and finally
d) Using a proper intake manifold design.

One thing that a lot of people miss is that Idle Fuel is typically 10 to 20% richer than "Ideal" to offset exhaust gas dilution.
If the carb were to be required to pass emissions requirements that would demand that ALL of the cylinders were receiving basically the same fuel/air ratio.
A Intake Manifold with poor distribution would basically cause the engine to be excessively rich on some cylinders to be able to run and that would cause the engine to fail the emission standard. ALL of the "Pieces of the Puzzle" are important.

If you were after maximum fuel economy then the fuel/air ratio would be about .06 and all cylinders receiving the same fuel/air.
If one cylinder was lean then the rest of the cylinders would have to have a richer fuel/air ratio for the lean cylinder to have the proper fuel/air mixture to fire properly.
So the lean cylinder (poor distribution) has messed up the operation of the other cylinders. This is where EFI with an injector for each port and proper fuel/air distribution is actually better vs a typical carb and normal intake manifold.

A fuel/air distribution of .075 is very close for the maximum power target.
You have the same issues at max power as at idle with the one lean cylinder except that now that lean cylinder need to be made richer to avoid detonation.

Of course again the other cylinders will now be over-rich as far as making best power from the engine.

One final comment tonight, Unequal distribution not only affects idle and power but also has an impact on timing used in firing the spark to the cylinders.
If you have unequal fuel/air distribution then the ignition timing curve for the engine can only be a compromise at best. The amount of Timing Advance can only be the amount the leanest cylinder can support without producing knock. Again a EFI and Modern Individual Ignition System for each cylinder can bring back a lot of the power lost in the above discussion.

Have a good night. I was an Engine Systems Calibrator for several years in my previous job.

Tom V.

ps People always tell you that Flow Benches don't really tell you anything except for a FLOW number to brag about.
That is true if you use the thing for bragging rights in intake and head FLOW discussions.
If you use it as a tool to optimize the fuel/air distribution in the engine then you are using the tool properly.