Can you increase power and gas mileage by upgrading the stoc 2v carb on my 351m (1977) engine with a 4v carb and manifold setup? Is it really worth it?

 

It can increase milage as long as you keep your foot off the gas....which is pretty hard to do with with a 4 barrel. You'd probably want to add headers to see the full effect.

 

the theory, which is all i really know, is that you can gain performance and increase mileage...like jrj99ss said....keeping foot out of it.

In theory every engine/build will perform best with a specific cfm and mixture of air and fuel. A 2bbl carb will be lower cfm, maybe 450-500 or so but its all in the primary action of the carb.

A 4bbl carb will have a higher overall cfm rating...say 600 but will only give a portion of that...much less than the 2bbl...say half... in the primary and then, when requested, it will open the secondaries and dump in the other half. So if you do not request acton from the secondaries you will save fuel and if you do want more you have an additional 100cfm or so to add....which boosts performance...sometimes rather significantly.

From what i gather, many will over carb...ie a 400 would have to spin (rpm) above it's typical usefull range to use more than 600cfm...but some will use a 750 etc. Theres a a lot i don't get about how it all works as each individual build will respond in it's own way. For example, in my '72 351cj...max hp is at 4600rpm and thus, it should not benefit from a carb over 600cfm...maybe even 500 cfm. But ford put a 750 on it as it gave the engine the best numbers with the smog requirments etc at the time. I don't get it it but oh well...maybe someday.

hope this helps you understand the principle anyway.