Upgrading my 340

rumblefish360 said
I'm having problems wih the page loading. It keeps resetting. But I think I read enough to understand what you mean. I think this boils down to just how much the engine is eating rather than carb size. I do not see a carb as a limitation as you putt unless it is pulling to much vacuum under load. (W.O.T.) IIRC, (LOL there....) a bigger carb is needed if it is pulling .7 or .5 on the vacuum gauge at WOT.

Considering carbs are rated at a vacuum draw of a set amount, how often does the carb pull that much vacuum? It varies as you stated and I agree. But it is the overall size of the carb that has to be choosen for the intended purpose at hand. To this end, not to add to the mix, but, this is the reason I like certain carbs for certain jobs. In example, the TQ or AVS for a dual purpose machine. Holley's for a race effort. The Holley has many different cfm sizes available to suite the need. The TQ is super flexible and the AVS only has a few sizes. So a short coming is hard to avoid sometimes, cfm wise. Theres worse things ..... than a carb to small.

I think the easiest way too explain how I see it is.

1st is we know carbs need some vacuum level to operate but for the sake of this discussion lets say they don't for a second. So then we would want zero restriction from our carb which would be zero vacuum at WOT any bigger wouldn't offer anything more since the carb has no restriction in this case.

So for this case say a 1050 cfm carb offer zero restriction and has zero vacuum at WOT on our imaginary engine.

Since we do need some vacuum to operate were gonna need to run a smaller carb. And as we go smaller and smaller the carb cost us hp over the 1050 because or restriction.
So only want to go small enough that the carb can operate. Since a daily driver, street strip and race car have different requirements the size will be different for each. But from what I understand the sweet spot for a hot street or street strip engine is a vacuum level of .08" to 1". In this case probably a 750-850 carb.

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.