Optimizing power production on high performance and racing engines is largely about air flow and cylinder filing - with regard to induction systems / carburetors. Optimized high performance and race engines will see not more than .5" lbs. of vacuum @ WOT. I've personally observed other engines which drop vacuum momentarily to zero (with a hand held, auto parts store vacuum gauge) at launch and perform exceptionally well. If the an engine is pulling more than 1.0" lbs. of vacuum @ launch there is power left on the table.
Not all carburetors are capable of metering / delivering fuel at these levels of vacuum. It does take an exceptional carburetor design and tune to take maximum advantage of an engine's pumping abilities. Enter, TMP Carbs...
Truth is, most carburetor size / application recommendations are sized based upon first; limitations of the carburetor's design and second; the extent or degree of tuning the carburetor will likely receive optimizing it to the engine / application to ensure customer satisfaction. The larger the carburetor, the more critical tune is which brings me to another part of the equation, baseline tune or 'out of the box' tune'.
I still hear people talking about how great "X" brand of carburetor worked right out of the box, that they didn't even need to turn a screw and it ran great. As a carburetor guy of 32 years, the only conclusion I can draw from those types of comments are that the person giving the apparent "unsolicited" testimonial hasn't got a clue about what 'running great' is. Certainly, there are those times when a 'bench calibrated' or carburetor which is built for a given application is on the money, but experience has taught me that by far it is certain there is more performance and throttle response left on the table.
I began my foray into building high performance / racing carburetors returning carburetors built by big name carburetor builders / modifiers and know that at best, all any of us can do is provide an educated guess.
Along those lines, I should point out with regard to high performance and racing engine's that the more "generic" a carburetor's design and tune or calibration is, the less performance that carburetor will optimize any given engine's performance. The more "aggressive" the carburetor's design and tune, be it pushing venturi size or in the fuel curve, the more critical and attention must be paid to tuning. If you are looking to make the most of an engine's performance or maximize consistency the results are are not only cost effective, but well worth the effort.
Back to carburetor sizing: if we stop and consider the "rated" CFM of any given carburetor divided by the engine's displacement you can get a more realistic idea of degree of "high performance" a carburetor offers a particular engine displacement.
For example, a stock early 1970's 350 SBC equipped with a four barrel Rochester Quadra-Jet rated at 710 CFM provides 2.03 CFM per cubic inch displacement. Granted, the Quadra-jet is a vacuum secondary carburetor, but let's also consider the specifications of a Camaro or Monte Carlo or whatever your favorite Chevy of the era is: weight in the 3500 +/- lbs. range; exhaust manifolds; stock non-performance mufflers and exhaust system; sleepy hydraulic tappet cam; cast iron intake manifold with heat cross-over; low flow air cleaner and filter, conservative rear gear ratio; and all designed for grandma to drive it cross-country; drive over the Rocky Mountains in the heat of summer with four or more people in the car; full tank of gas; luggage in the trunk and to do all this for 100,000 miles reliably.
A high performance / race car will most likely be lightened; have low gears; headers; high flow mufflers and exhaust system; aggressive solid flat tappet cam if not a hydraulic or solid roller using modern cam lobe technology (way beyond what existed in the 1970's let alone in an OEM applications); high flowing modern technology cylinder heads and intake manifold; modern racing ignition system; stall converter; etc.
Consider the following carb applications:
Carb Engine CFM /
CFM CID CID
710 350 2.03750 350 2.14
750 351 2.14
750 377 1.99
750 383 1.96
750 406 1.85
750 408 1.84
750 427 1.76
750 434 1.73
750 455 1.65
750 468 1.60
850 350 2.43
850 351 2.42
850 406 2.09
850 408 2.08
850 427 1.99
850 434 1.96
850 454 1.87
850 468 1.82
850 500 1.70
1000 350 2.86*
1000 351 2.85*
1000 406 2.461000 408 2.45
1000 427 2.34
1000 454 2.02
1000 455 2.02
1000 468 2.14
1000 500 2.00
1050 350 3.00*
1050 351 2.99*
1050 377 2.79*
1050 383 2.74*
1050 406 2.59*
1050 408 2.57*
1050 427 2.461050 454 2.31
1050 455 2.30
1050 468 2.24
1050 500 2.10
1050 540 1.94
Okay that's enough, you get the idea. Those CFM / CID numbers listed with the (*) Astrix will drop manifold vacuum @ WOT sufficiently to maximize the flow potential of a high performance / racing engine.
Here's another more generous calculators recommendation for the same 650 horsepower 406 SBC used in the previous calculator above. Interestingly, it seems a disingenuous as the web page this calculator is placed promotes two companies who DO NOT actually recommend the large sizes predicted by this calculator.
Unquestionably, racing engine's running in the NHRA's Super Stock / Stock Eliminator, F.A.S.T. or NMRA's Edelbrock Hot Street limiting carburetor venturi and butterfly diameter and the like are producing a significant amount of torque and horsepower. Nevertheless, similar racing engines with unlimited CFM and induction systems produce significantly more. Unless, you have the desire or need to limit carburetor CFM / venturi and butterfly diameters, more is better.
TMPCarbs.net carburetors provides the carburetor design to enable the successful use and maximization of torque and horsepower production over a very broad rpm range with carburetors providing well over 2.40 CFM per cubic inch displacement.
Weber Power Plates
