4 Barrel Carb/ intake manifold?

I don't know how much of the airflow capacity is provided by the primaries of the 390 Holley, but less than 1/2m because the primaries were smaller than the secondaries.

I run a 600 vacuum secondary Holley on the six in my 77 Gremlin. I've done this before on a stock six coupled to a T-10 4 speed in a 72 Gremlin and it ran absolutely great. With the automatic there appears to be a long delay before the secondaries open. I do run dual headers and performance exhaust as I did in my previous 72 Gremlin.

With the stock cam, even with the headers helping out, there is a limit as to how much RPMs you can pull with power. Dragging it up to 5,500 RPMs doesn't seem to get a lot of production. I've seen slightly bored 258s (282) with forged pistons, performance cam, head work, headers, etc. crank out much higher RPMs using a 750 CFM carburetor. In fact, I've seen one 73 Gremlin turn in the 12's through the quarter mile using an automatic transmission set up something like this. In order to properly utilize a large carburetor you need to be sure to have the proper head work/valve work done and you'll need a cam upgrade for sure.

Good luck!

The 69 232 with a 390 Holley was my daily driver for 15 years, went thru 2 sets of headers. I got real good at patching headers.

The 600 cfm carb is a bit large, but if it's a vacuum secondary carb should work. The secondaries will only open as long as there's enough vacuum in the intake to pull them. If the carb opens to much vacuum drops so the secondaries start to close until vacuum comes back up. In other words, you were basically running a 300 cfm 2V carb, but the secondaries would partially open near WOT -- until vacuum started dropping. I bet those secondaries never opened more than half way, more likely more than 1/4.

The Holley 390 is a square bore, at least all I've seen are. They may have made a model specifically to replace a Rochester "spread bore" carb though. I've been looking for a 390 or 450 (vacuum secondary) 4V for my Jeep. Just now starting to get the 4.0L head done though! Finally plugged the water jacket holes for fitting to a 258 block.

farna wrote: The 600 cfm carb is a bit large, but if it's a vacuum secondary carb should work. The secondaries will only open as long as there's enough vacuum in the intake to pull them. If the carb opens to much vacuum drops so the secondaries start to close until vacuum comes back up. In other words, you were basically running a 300 cfm 2V carb, but the secondaries would partially open near WOT -- until vacuum started dropping. I bet those secondaries never opened more than half way, more likely more than 1/4. The Holley 390 is a square bore, at least all I've seen are. They may have made a model specifically to replace a Rochester "spread bore" carb though. I've been looking for a 390 or 450 (vacuum secondary) 4V for my Jeep. Just now starting to get the 4.0L head done though! Finally plugged the water jacket holes for fitting to a 258 block.

It is large for the stock heads and cam, but once the secondaries open they stay open even on the automatic.  We once locked up the secondaries to pop open full throttle mechanically and it didn't seem to run any different than when it was vacuum operated at high RPM. Just was a pig if you got in a hurry at low RPM so we returned it to vacuum. But once RPM is built this carb works great. If you floor it at low RPM it does take off on 2 barrels until it builds the proper vacuum.

With small displacement engines you can flow mega CFMs of air as long as you pull high RPMs. I learned that with Rotary engines. As long as you can keep enough fuel and air going in and getting it efficiently out, a 1200 CC Rotary engine will just keep slurping it up and revving, even way past 12,000 to 15,000 RPM. High RPM creates the need for more carburetion. For example, a tuner 4 cylinder at 8 grand can efficiently burn and process the same fuel as a small V8 at say 4 or 5 grand. It's all a matter of airflow dynamics and how durable the engine is constructed.

Myself, I've been educated by some of the Jeep technology and they actually have paved the way for the rest of us 6 guys to build engines that can rev high and flow efficiently though the intake, heads and exhaust. AMC sixes are great engines and hold up well. I'm surprised more people don't hot rod them aside from the Jeep guys. A good set of forged pistons and rods make these engines bullet proof.

However, if you leave the six internally stock and go conservatively on the carburetor using headers you will get great mileage. The performance cams designed to give high RPM performance will wreck fuel mileage.