I am currently working on a 396 BB chevy (more like 402.23something).  It has closed chamber heads off a 1969 chevelle, slightly agressive cam, and some raised pistons, am guessing around an 11.0 to 1 compression ratio.  I am currently running an Edelbrock 750 carb, which is driving me NUTZ!!!!  it is running rich and has been a complete pain to adjust.  A friend is wanting to sell be a modified holly 850 double pumper, forget the company, but they do alot of work for the World of Outlaws.  The Chevelle that the engine is in has the M20 tranny, (4spd manual) and is used for fun, on the street and on the track.  I am worried that the 850 might be a little 2 big, but I also heard if tuned properly, it will run just as good, if not alot better than what I got now.  SHould I stick with my edelbrock or get the 850 holly?  And if this is not an ez question to answer, how do I find out how big a carb I need for my application?

almost every engine we dyno test ..we find a way to hook
a vacuum port or fitting to either the carb , carb spacer,
somewhere on the intake manifold....we do this to check vacuum ...the dyno will record vac readings every 100 RPMs
and do it to .000 decimal places if you like in inches HG

with vac readings you can see REVERSION spikes ,
determine the real CFM of the carb ..on a live full-throttle run with fuel coming out of the venturis

back in early years..Holley and others offered only 1 or 2 barrell carbs ..and flow benches back then could probably only pull 3 or 4 inches H2O out of those 1 or 2 barrell carbs ...so when technology progressed far enough to make 4 barrell carbs ..the larger CFM carbs on the same flowbenches could only get around 1.5 inches Hg ..so Holley used 1.5 to rate 4 barrells and 3.0 for 1 and 2 barrell carbs.

but now in modern times with a bunch of dynos all over the country .....just about every engine you find makes peak HP
at or below 1.0 to 1.1 inches HG ....once you climb above (Butterfly Valves)
1.3 inches Hg...the carb becomes like a "RESTRICTOR-PLATE"
to the engine.

if i were Holley ..i would rate all my carbs at 1.0 inch of Hg ....i know the customers would complain because for example  an 800 cfm carb at 1.5 would now be 653.2 cfm at 1.0 inch hg. ...racers would frown on this ..but it would be closer to real live engine Peak-HP conditions !!!

if you dyno an engine with an 800 CFM carb on gasoline
and then modify that same carb for methanol or purchase another 800 cfm methanol carb to test...on the flowbench both carbs might flow exactly 800 CFM ..but on the dyno with gasoline coming out of the venturis  -VS- methanol coming out of the venturis..the methanol carb will
actually flow LESS by 10 to 15 percent measured VE
..so you would see only 700 to 720 actual dyno CFM with methanol.

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