Adding a spacer under the carburetor effective increases the plenum area of the intake manifold.
Obviously, the amount of increase depends on the thickness of the spacer.
The effect CAN be that the engine thinks it has a marginally larger carburetor, with an increase of a few percent (maybe 2~3 percent) AT WIDE OPEN THROTTLE MAXIMUM RPM!
If the carburetor is TOO SMALL for the engine, then the spacer will help.
If the carburetor is not too small, and some means are incorporated to require a larger carburetor (better cylinder head, camshaft, etc.) then the spacer may help.
Remember that too large a carburetor will have lower venturi air velocity, which CAN negatively effect low RPM performance.
But 6 HP on a 235??? I'm from Missouri!
Also, while a spacer CAN help with a hot soak condition with modern fuel ON A VEE engine, the same spacer MAY contribute to ICING ISSUES in high humidity and temperatures lower than 40 degrees on an inline engine.
If you are referring to the 350 in your signature, you MIGHT get 6 HP at WOT MAXIMUM RPM, but if the carburetor is correct for the engine?????
With the 350, unless you are trying to go very fast around slow corners; a better bet for usable horsepower AND low speed driveability would be a spread-bore intake and spread-bore carb such as the Rochester Q-Jet or Carter TQ instead of a square-bore. If you are racing road courses with SCCA, then the square-bore is easier to make go very fast around these corners.
Jon.
