Proform carb
 

What more power out of my 670 cfm street avenger holley carbed 345 HP 302 crate. My speed shop said I'm under carbed, he is recommending a 850 cfm Proform carb...it seems like it my be too big? Any carb experts in the club?

way too big. you need a 650 double pumper. the avenger is not a performance carb

Agreed. The 850 Proform is basically a clone of the Holley 850 HP 4brl with a few billet pieces thrown in for tuning latitude and visual appeal. Either way, you'd have a venturi diameter (1.562”) to throttle bore (1.750”) diameter ratio that is just plain awful for a street performance 302 cu in engine operating at anything under about 5500 rpm.

Assuming that a functioning choke mechanism isn’t a necessity for you (you were considering a Proform, which doesn’t have choke provisions), the Holley 650 HP double pumper would be a much better choice. The smaller venturi diameter (1.250”) will have the carburetor’s main circuits (which are far more efficient that the idle and transition circuits) active much earlier in the RPM range, giving this carb a huge advantage over the 850 in throttle response. And don’t be fooled by the fact that the 650 HP is officially rated at less airflow capacity than your 670 Avenger…the radiused shape of the 650 HP venturi entries is much more efficient than the “funnel” entry shape used in the 670 Avenger; you’ll end up making more power and torque throughout the entire RPM range.

It might help you to know that several of the oval-track sanctioning bodies across the country that now require the use of that same engine in their “spec engine” classes also require the use of the Holley 650 HP carb, having found it to be the best overall performance match for your engine’s breathing requirements. If you do decide to go with the 650 HP, PM me and I’ll give you some additional info regarding some fairly simple calibration changes to this carb that will optimize it for this particular engine.

Note: I wouldn’t advise considering the Holley “old” non HP style #4777 650cfm double pumper carb. Geometrically, it’s identical to the 670 Avenger you have now (identical venturi size/shape, throttle bore diameter, and booster design) and while it might provide some general improvement in overall performance, it certainly wouldn’t provide the best increase for the $$$ spent.

Stan Justus
Stallion Racing Components

To figure out the correct cab size - take the cubin inches x the RPM of your max. HP and divide that by 3456.
Example - 302 x 6500 = 1963000 divide by 3456 = 568 CFM.

As a fellow Ohioan, I hate to think there is a speed shop that is this incompetent. Whoever told you this hasn't a clue. Never go back there. At 8000 RPM and 600 hp, you still would only need about 700 CFM.

Putting a bigger carburetor on an engine rarely makes more HP, as most engines are already over carbed. The carburetor doesn't force more fuel/air into the engine. The engine has to pull more air through the carburetor. Until the carburetor becomes the bottleneck, putting a larger sized carburetor on will do nothing for power, other than kill low rpm drive-ability. You're not even pulling 600 CFM into your engine (likely 500 CFM range).