Quote:
Originally Posted by TJ's GMC
Turbobill. I fully agree with you on that Years ago I tested all 3 of these carbs on my 68 305E and the Holley 500 was still by far the top performer, it's all about the real world results  |
So if the stock Stromberg is in the 300 CFM range, math shows that it is too small. The normal old go to "(CID X RPM X VE)/3456 shows that we only need a 285 CFM carb at 3800 RPM with 85% VE. But, that is not the whole story. Do some looking around, and you will find that 4 barrel carb CFM ratings are at 1.5 inches of mercury while 2 barrel carb ratings are at 3 inches of mercury. At wide open throttle on an engine aimed at performance, we don't want 3 or even 1.5 inches of pressure drop. Eight to nine tenths of an inch is much better so we have to adjust our 285 CFM.
To do so, we need to divide the 285 CFM by the square root of the quotient of our desired vacuum (.8) over the rated vacuum (3). This shows us that we need a two barrel carburetor "rated" at
552 CFM.
Do the math with a 4 barrel carb and we would need one "rated" at 390 CFM - I think Holley even happens to make a 390 CFM 4 barrel...
So yes, you should see a large performance increase from your stock 300-ish CFM carb to a two barrel "rated" at 500 CFM.