the formula
CID x rpm/3456
works just fine.
Why it doesn't seem to work, is because most carbs have ADVERTISED flow-ratings that are grossly exaggerated, or are at non-standard pressure drops, or at pressure drops that are different between manufacturers.
Or worst of all, your street engine cannot produce the pressure drop in the first place. Or the engine under your hood cannot come remotely close to the required VE.
So you are forced into bolting something on that you guess might work, and until you figure out that the roar thru the air cleaner on the street, does not translate to performance across the board, you might extol the roar as power, when in fact it is just a roar.
I'll say it again
if you put your 225 on a dyno and measure the maximum air uptake at 235 cfm, then, you can install any larger than 235 rated carb on that engine that you might want to, no matter it's advertised rating; if the engine can only pull 235 cfm, anything bigger, will only pass 235 cfm; whether it have One barrel or six, 235 is all the engine will pull.
So now, your job is to wade thru the advertising hype, and find a carb that will flow the required cfm, at the measured pressure drop, and to heck with the advertised rating. And that carb still has to perform under all other rpms and load settings, to make it streetable.
Lemmee explain;
suppose you have a true and accurate 500 cfm carb rated at 500cfm @1.5 inches vacuum. But your slanty , with this carb installed, only pulls 3 inches. This 500 is now, a 354
But suppose you have a true 354cfm @3 inches rated carb, and your slanty pulls 1.5 inches thru it. This 354 is now a 500
The formula calls for 225 x say 4800/3450=313 cfm@100%VE. /
[email protected] VE
Now; a 250 2-bbl is so, only at it's rated pressure drop, usually 3 inches. So,assuming it was rated at 3, then at 1.5 is 354. Where it stalls I have no idea; but the dyno will show it.
Either carb will work at WOT, at the listed parameters. Just remember, the engine only pulls what it can pull, and as long as the carb can meet the demand, more is rarely a good thing.
The conversion factor I used is 1.414
for other pressure drops;
From Speed Talk;
