Ideal Dual-Carb FE setup for a street 390/FE?

[Cobra-Radjah]

Duane,

There is a way to calculate the flow demand of an engine, based on a nr. of engine specifics, such as bore, stroke, compression ratio, con-rod length, flownumbers of the intake manifold and the cyl heads, cam advance and duration (at 0.05") and the desired RPM. If you post these properties, I'll be happy ro run them through my calculator.

Off hand, I reckon that even 2 - 450 CFM carbs ma be a little big for a 390 street cruiser. Holley usually recommends to stay a tad on the small side for better respons and accelleration in the low and mid range, sacrificing a little at the top. However, properly set up, dual 450’s should work just fine.

[Carnut427]

If the 450s are vacuum secondary carbs, the secondaries will only open as much as the engine can use. Also, with dual quads, no matter what size carbs you have on, you'll get a similar rush as with a single double pumper when the primaries on the second carb open.
Food for thought,

Dan

[edwhite7]

If you feel the secondaries open then the carbs are most likely not set up correctly. Feeling the surge as the secondaries open usually means that the engine is not making enough power prior to their opening. If you plan to use the Snellings and Hellings air cleaners you may want to mill off the choke air horns so that the carb can breath. I have two 390's with mechanical secondaries and I like them a lot better than the 600 vacuums that I used to have. If you ask me on a cold morning I will tell you their perfect but on many hot summer days I get a flat spot at about 1500 RPM as I get on the gas. It is a work in progress.

[Carnut427]

Originally Posted by edwhite7

If you feel the secondaries open then the carbs are most likely not set up correctly. Feeling the surge as the secondaries open usually means that the engine is not making enough power prior to their opening.

Not sure about this...I've had single Holley 4bbls that you could feel a surge when the secondaries opened, and the car ran excellent. The dual 600s on my 496, however, seem to open without the surge...probably because the car is surging from the time I stomp on it. Seriously, I really don't know if it's different with dual quads or not, as everything is happening so fast with this motor...I'm a work in progress in getting used to it.

Dan