Upgrading my 340

273 said
2nd since the smaller the carb gets we give up hp over the 1050. So different size carb will make the engines air flow needs vary slightly but for this discussion let's say it don't ando for our imaginary engine say it needs 550 cfms of actual air flow not carb size. And the formula cid x rpm ÷ 3456 does estimate the engines actually consumption pretty well. Now no matter what carb you run the engine will pull 550 cfm through it. So theoretically if you put a 550 cfm carb on it. It should pull a vacuum of 1.5" which is a fine choice if you don't mind leaving power on the table.

But most will think that this is the correct choice cause our engine needs 550 cfms and the carb is 550 cfm a no brainer. But what we don't realize that a 750 cfm carb at around .09" of vacuum is also a 550 cfm carb. And a 850 cfm carb at even less vacuum is a 550 cfm carb. No matter what carb you put on this engine the vacuum should change to make them all flow 550 cfms.

So really the right carb for the job should be the biggest carb that full fills all the engine requirements.

The underlined; YES! But only top end power. Or via the formula, a whole lot from mid way to the top.

The bold type; Not exactly. The carb is still the carb at it's rating. The engine is pulling or consuming that 550cfm. So the carb is not smaller, just not used to it's fullest. Down grading the carb cfm size (850 to 550) is IMO a wrong way to state it and go about writing about. This wold just confuse people. It is not accurate.

The italic; YES! Because the engine is only consuming that much. But where it happens should change, slightly. Nothing to argue over. The bigger the throttle bores the less velocity.

One last thing, a carb doesn't need vacuum. The air rushing through the venturi(s) is ether done by vacuum OR the atomshpereic pressure. OR turbo/supercharger. Fuel is drawn out from the carb from the air rushing past the fuel openings in the carb.