It's not about HP, it's about induction velocity in the RPM range it runs in. Some heads and intakes are better than others, they maintain a relatively even velocity throughout the port. The 358 would work with a GOOD 1250, one with boosters that atomize the fuel well. Using fuel that vaporizes well will also help, like Q16 or C25. Being in Sweden I would lean toward a 1050, cooler temps there will make it harder to vaporize fuels.

The formula is just simple math. An engine burns fuel to make power and the fuel needs air to burn. It takes 1/2 lb of gas per hour to make 1 hp and it takes 12.5 lbs of air for each lb of gas. So with a little bit of math you can figure out how many cubic feet of air is required to produce one horsepower.

The size of the carb is a slightly different question than how much air the engine needs. How much air is a math question. The carb size depends on a bunch of other things such as throttle response and ability to vaporize the fuel.

The best motors I've built (perHP per C.I.)and dyno tested where a lot more effecient on the brake specific fuel consumption than the standard 440 motors, they where under .42 lbs per hour down to .35 lbs per hour

Yup .5 BSFC and I'd call it a pig with a capital P-I-G.

That is fine, you can adjust the formula for a lower BSFC if you want. Use HP x 1.2 = CFM for a more efficient engine. One reason the NASCAR guys get by on small carbs is because they have developed the engines to have very low BSFC. Burn less fuel and you don't need as much air.

Drag racers typically don't pay much attention to BSFC. You can go to the track and ask 10 bracket racers what their engine BSFC is and maybe only one will even know what you're talking about. So an estimate of 0.50 isn't far off for the average bracket race type engine.

As I said before those 1250 Holley's just have to much throttle blade for MOST ON HERE.

We run a splayed valve SB Chebbie with a 2.35 throttle blade. So engine size means nothing to be brutally honest.

That is true, engine size doesn't really matter when calculating how much air the engine needs.

However, the classic formula is Displacement x RPM x VE so people have that wired into their heads. Problem with this approach is that most guys don't know what their VE is.

The other way is to just cut to the chase and convert from HP to CFM. In this formula you need to have a rough idea of the BSFC as well as air density and fuel density. But it is easy to shorthand the formula if you know a little bit about typical operating conditions. When you use this formula then you really don't care about engine size or RPM. HP requires air and fuel regardless of engine size or engine speed.

Once you know how much air the engine is going to use then you can pick a carb. The carb is usually somewhat close to the max air used but not always. Street cars go smaller, drag race cars sometimes go bigger or even much bigger. There is a bit of art involved in the carb selection. The airflow requirement is basic science.

While that IS true in most cases.........for most basic calculations a .5 brake is still a decent number to use. It gets you in the ball park

Yeah I agree. I have a ton of dyno sheets that show typical hot rod or bracket type engines with BSFC right around 0.5. I'd say it is a very typical number for anything with a "normal" cylinder head.

Your right Monte...if I was calculating fuel system requirements I would use .5 BSFC just to do the quick math.

I was suggesting that on the dyno .5 BSFC would be junk and I still stand by that. I have had my stuff go under .42 most of the time and highly prepped stuff go under .40 and I am by no means cutting edge.

I'm trying to find a dyno closer to me, but I will dyno my junk here in the next month or so and I will post it up. If the BSFC is near .5 I will stop and fix it. I can't for the life of me figure .5 being the "rule" for even bracket crap after about 1995 or so (maybe earlier).