That formula works best when you determine what application you're going to use the vehicle for. In a drag vehicle, the RPMs at WOT should be used. In a street vehicle, normal RPM range should be used. Also with that formula, you normally round up to the next largest off the shelf size carb.
Hell, many nascars have 750 horsepower and spin between 8,000 and 9,000 RPM for 500 laps and they run a 390cfm carb.
But like I said, I don't know everything and most of what I know is from reading about how/why carbs work the way they do. You guys have a lot more experience with this than I do and if the practical application works better than the theoretical, awesome. Honestly, I'm not trying to be an a$$ or disrespectful to anyone here. Not trying to troll or jump threads, just trying to learn like everyone else. I do appreciate the lessons and it even helps me learn more. Thanks.
Might want to check some of your specs quoted there...please view this webpage.
http://bleacherreport.com/articles/...76-the-new-nascar-engines-a-quick-look/page/9
Nascar engines also use 12:1 compression (increases combustion efficiency) outrageous camshafts, and produce far less torque than horsepower-that's what they're built for.
Carbs-again-are only a fuel/air mixing device at the mercy of outside barometric conditions, the parts installed in them, the engine below them, and the lead content of the foot actuating them. They operate on the Bernoulli principle where increased velocity of a gas in a tube produces a low pressure area also known as a venturi-this low pressure area is where carb makers put the boosters (and can be optimized by placing the boosters further in or out of the throat of the carb). The boosters are fed through passages and fuel is regulated by jet sizes which means if you're drawing in too much fuel-it isn't the carb's fault-it's the tuning intrinsic to the carb.
While I would agree that running a 900cfm carb on a 360ish cubic inch small block isn't the greatest idea for street use, it obviously works in the OP's experience. And I would agree that for any app, the carb should be matched to the purpose.
When you speak of bigger TBs on fuel injected engines, you're comparing mandarin and naval oranges-while the principle of allowing air into the engine through a port (throttle body or carb throttle plate) is the same, the execution of fuel management is different. Most modern EFI systems use one of two forms of fuel management: speed density, wherein the manifold absolute pressure (engine load) determines fuel demand, or mass airflow where the air drawn into the engine is measured as it flows over a heated platinum wire. Either way, if you put on a bigger throttle body, that in and of itself doesn't make the engine consume more fuel-it ALLOWS for greater air intake at a given throttle position. HOWEVER, if the engine will only draw in 548 cfm at max load, with proper tuning, it will consume no more fuel with one tb compared to another as long as they are both large enough to flow at least that amount of air. If you have one that only is capable of supporting 400cfm, and another, 750cfm-that's another story altogether.
The previous combination that resided between the fenderwells of my Mustang was a Vortech supercharged 302 with aftermarket cam, heads, etc...the 70mm TB on it was large enough to flow well over 700cfm, and it still made 22mpg on the highway with 3.73 gears on a 26.5" tall tire and pushed over 400rwhp. But, before I played with the fuel tables, the best I could get was closer to 16-because it was running too rich at a given load level. Long before I had that combo, I ran a 600 Edelbrock. I could change rods and jets (and I did on many occasions to find best power) and within a tank of gas I could tell the difference it had just by those small adjustments-and I never even changed the carb. There was a short time I ran a 750DP...but, I didn't have the right size jets, so it ran pig rich. Drop a few jet sizes primary and secondary, and voila...mileage back to where it was with a 600...as long as I wasn't being too jolly with the go pedal.
So, go ahead and tell me again about how the size of the carb [or throttle body in EFI's case] will automatically make you consume more fuel. This discussion is ages old. Combination of the rest of the vehicle powered by the engine notwithstanding, the only things that will make an engine consume more fuel is altered components, higher loads, increased throttle transients, higher rpm operation, and proper control of fuel management, i.e., tuning throughout the rpm band and load conditions.
Will a bigger carb allow for more fuel to be consumed *if* the previous one was not enough to feed the engine? Yes. Will an 850 cfm carb make an engine that only needs 600cfm consume more fuel than that same engine with only a 750cfm carb? Only if the tuning isn't properly done, and the operator has too much of a lead foot...otherwise, this discussion is purely academic...all that being said, I have to run out to the PO to send off some parts
