Join Date: Jan 2005
Location: Wake Forest, NC
^Dollars to doughnuts says that in real life, a larger carb will out-perform that 456cfm one your calculations came up with.
Yes, the 456cfm will be more responsive or "snappy" off idle, but shortly after that, the larger carb will be spanking it like a frat pledge.
I agree about using a spread bore carb, such as the Holley spread bore, or a Quadrajet. Best of both worlds there, you get the responsiveness of the smaller venturi at part throttle, and the power of the larger carb when you get into it.
But overall, most anyone will be more pleased with the larger (600cfm in this case) carb.
Vacuum secondaries I also agree with....you can even put a stronger spring in the diaphragm to prevent it opening until whatever RPM you deem it necessary....again, gets you best of both worlds.
Basically, if you have a square bore (e.g., a standard Holley) that's 600cfm, you'd be running a 300cfm 2bbl until your secondaries open....that gets you that responsiveness everyone seeks, plus the power over 3000rpm that most folks also want.
Again, this setup will outperform that smaller carb pretty much every time, if both are set up the best they can be.
I should say that I'm speaking from the viewpoint of a street-driven vehicle...not a primarily off-road Jeep. If all you want to do is rock crawl, then by all means get the small carb, because you'll never see the other side of 3000.
But I'm thinking this guy wants a Jeep that will run really well on the street, and be good off-road....and in that case, the 600 Holley is his best bet outside of a well-tuned Quadrajet.
I'm also assuming the setup he wants to run....304, with mild cam and aftermarket intake/headers. A 500cfm carb will choke that engine. The same engine with the 600 will run off and leave the smaller-carbed one.
And yeah, it's correct that the majority of driving is idle-3500, but that's not what a guy builds a V-8 with a cam/intake/headers for. If one were to run a dyno test on his engine with a 450-ish cfm carb, you'd see it falling on its face over 3000rpm. Definitely after 3500. But the 600cfm carb with vacuum secondaries would probably only be behind the smaller carb under 2000-2500, and after that it'd be killing it.