Quote:
Originally Posted by Tim Ayers
Makes sense and supports my thoughts on a stock Model A engine is under carbed.
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You are correct in that a Model-A engine is under carbureted
however many other carbureted engines are/were under-carbureted too, so that is not a big deal.
Colin's info above is kinda misleading IMHO though. To begin with, yes dyno testing has shown that at a steady 2000 rpm, the carburetor is drawing a little over 60 cfm, ...HOWEVER nothing in his comment mentions where the stock Zenith can flow more (-as in over 20 cfm more). The throttle blade that is partially closed is what is limiting the airflow and setting the engine's rpm level. If the engine needs more power, the driver presses on the pedal which opens the throttle blade and more air/fuel is added to each cylinder. It is worth noting that even though a stock Model-A engine makes 40 horsepower, it may only use 40%-50% of the available horsepower when driving 45 mph.
As mentioned above, the stock Zenith is capable of flowing in the low 80s cfm at higher rpms whereas the 32/26 Weber two-barrel carb and/or the Model-B Zenith are only capable in the higher 80s cfm at those same rpms.
There are two major limiting factors that enter in on this entire airflow scenario which IMO does not put the blame solely on the carburetor. First is the intake valve and bowl area are too small and they impede airflow. Second (-and equally important) is the exhaust manifold is too restrictive. Because of this, not all the spent gasses at higher speeds are evacuated from the cylinders. If we need to add a third reason, it would be the camshaft lift & duration are under spec-ed. Therefore just because we add a higher compression head does not mean our performance increase will be the same as someone elses. I can likely put a 5.5 CR Snyder head on an engine where I have enlarged the bowls, installed 1.750 intake valves, and an IB330 camshaft and outrun his present engine with a 7:1 Lion head added.