I'm carb shopping again, this time I'm looking at edelbrock's 4 barrels.

Here's the engine specs:

390 .040 over
9.0-9.5 compression
cam: Ford Hydraulic Flat Tappet Camshaft
intake: offenhauser 360

Considering this cam runs out of steam at about 4200, i probably don't need many cfms, I've been considering the 500CFM edels. Any input? I'll have this one dialed in by a professional shop (somebody with an AFR meter) I'm done with carb tuning.

Sam

 

Sam, Unless you are racing, 500 cfms should be enough.




John

 

The only thing I'd be concerned with is the ability to tune the secondary opening on that carb (never fooled with those, Holley's only for me) the 390 will suck the secondaries open really quick on a Holley 600, you'd need the heaviest sec spring on the 600 to keep em shut to get more mpg

 

I'm not dead set against a Holley, I was getting kinda mad at the one I had, but it turned out the be the intake causing the issue, not the carb. Would a 570 street avenger work ok on this motor?

I'd like to buy a carb from napa (got a 25% of cupon from them!) But it doesn't look like they sell any edelbrocks with tunable secondaries, so they're out.

Sam

 

A 670 would be a better match for a 390, or the list 80783 650. The 570 will work, but you will need the stiffest sec spring to keep the secs closed longer. I ran a 600 on a 390 with a purple sec spring and the secs would open the instant you nailed the pedal. I went back to the 3310 750, but this was in a 68 Merc Monterey that wasn't a dialy driver and I wasn't concerned with fuel economy.

 

I guess I'm not too worried about the secondaries, the motor pulls incredible vacuum, (about 20 in at idle) vacuum rarely gets below 10 in, even if I mash on the petal. I had to run a 10.5 power valve, anything less wouldn't open!

Sam

 

Are you sure the throttle plates are opening all the way?

 

I've got a factory replacement Holley 600 on my 360 with air cleaner, aluminum intake and headers. My father-in-law runs a 650 DP Holley on his 390 with rv cam, CJ heads, aluminum intake and headers. They both run great, ok his did, it hasn't ran in quite a few years.

 

600 cfm Holley should be just the ticket...I have run them on all my old 390's in my trucks..and the one I warmed over..I jumped up to a 750cfm holley...but I never built one for MPG purpose's...they all got around 10 to 12 mpg as I run a pretty tall gear on the rearend for pulling the race car at one time and the boat..I also run a tall tire for a 2wd..mine are 12.50 x16.5 on the 70 f250 with a 4:10 gear..and if you go Holley..(only carb) I'll run also..make sure to get the one with the auto kick down arm..well if you have the Automatic.. JMO

 

Since most 390's with a decently "streetable" cam (under 270 "advertised" duration) tend to quit making power/torque about 5000 RPM, going over 600-650 CFM carb is just not needed. On the street, most people seldom get over 4000 RPM, other than very short bursts. The minor drop in power up high, is more than made up by the improved throttle response of the smaller carb.
Drag racing is a different story. But for a street use truck, with a stock to moderately warmed up 390, a 600-650 CFM is typically the best overall fit. 750 would be the upper limit for a carb rating, but they work better for trucks with steeper gearing, like a F250/350 with 3.73-4.10's, compared to a F100/150 with 3.00-3.50 gearing.
It's all about the velocity of the air thru the primaries at cruising RPM. Matching the carb "size" to the application is the key to getting the best combo of power and MPG. Rear gear ratio dictates the cruise RPM, carb CFM rating dicatates the size of the venturi bores, and both of these have a direct impact on the velocity at cruise RPM. Get this combo right, and the engine performs well while yielding a bit better MPG.

It's like GM using the same basic q-jet for most any engine size. The small primaries always made for better velocity at cruising RPM, and the secondardies only opened as much as the engine could actually use.

 

As mentioned above, I adapted a 800 cfm qjet to my 390... the secondary's only open as much as the engine needs.. I used one from a '74 buick 455 and opened up the APT screw so I have full adjustment on the primary side as well... The carb works well for me...

 

Basic concept, cubes need fuel, no way around it. My stock 428 cj in a mustang gets 13 at best with a light foot on a 200 mile highway drive. If converted to FI you may get better but you have to weigh the costs to get there.

 

I got my intake and a carb, I found an old Offenhauser 360* at the junkyard and bought a brand new Holley 570 street avenger to top it off.

As far as mileage goes, I know it's going to be bad no matter what I do, so I'm not too concerned.

Sam

 

The Quadrajet is a spread-bore carb - the primaries are much smaller than the secondaries.

The problem most people have with a too-big carb is that they are using a square-bore carb. The primaries are the same size as the secondaries, and if you go to a way-too-big carb, the primaries are WAY too big for the low-RPM range.

 

Which is where a 3x2 shines. You get the benefits of a spread bore, but far better fuel distribution I used an 850 DP Holley on a mild 390 (was the only carb I had handy at the time, it did fine out on the "big road" but fell flat on it's face if you nailed the go pedal from a dead stop.