Which carb ? Stock 383...

[Sly_drums]

Hello guys, I need a little help here.

The following combo is installed in my 71 automatic Challenger : '67 383 motor, stock HP manifolds and 2.5 inch dual exhaust. The intake manifold is an old Offenhauser Dual Port, which looks like an old 70s intake.
When I bought the car, a worn Edelbrock mechanical 750 cfm carb was mounted on the motor. The combo had great torque and was pushing good when you put the pedal on the floor.
The rear axle ratio is 4.29:1 which is WAY to short for a street application (bought the car like that). But now we are rebuilding the rear axle with a 3.23:1 ratio, much more streetable.
A few months ago I replaced the old Edelbrock carb with a rebuilt Holley Street Avenger 570 cfm (vacuum) that a friend sold to me, with the "quick change" that allows you to switch the vacuum springs.
The combo works good but I doesn't push as hard as before. I'd like to keep the motor quite stock for the moment (except intake/carb), but I'd like to go back to a 750 cfm mechanical secondaries carb. Questions are :
  - would you keep the Dual Port intake or change to an Edelbrock Performer ?
  - would you put a mechanical secondaries carb on this motor ? And which one ? (Holley dual pumper, 3310, Edelbrock...)
  - would you keep a vacuum secondaries carb and why ?

The car will cruise most of the time (no dragstrip) but I want to get the best performance possible out of it.

Thanks for helping 

[Chryco Psycho]

The best intake is the Holley Street Dominator but you would have to find one used as they are no longer made .
 The Dual port is actually not bad & has god response as you found out . The RPM would be better but may not be worth the cost to replace
 I have not seen any Edelbrock mechanical secondary carbs , it should have a linkage to open the secondaries but the air door will not open until there is enough flow to open it so in reality it operates as a vacuum secondary + to make things worse there generally is no spring to adjust the air door opening & they are set to heavy & open late . The only way to adjust this is to lighten the counterweight on the air door , I generally drill holes in the weight so it will open faster & if you need to add weight just thread the hole & insert a screw  or fill the hole with glue .
 The Holley is too small you need 750 CFM for a 383 .
 I prefer mechanical secondary carbs with dual accelerator pumps systems  , Not air door type as they are more responsive overall But you do have to learn to tune & drive them , most cannot be floored from idle without hesitation , but if you jump on it 1/2 way & then roll into it they will pull a lot harder or if you floor it at 2200 rpm or more they hit hard !
 Right now my Favorite carbs are the Proform mechanical , great performance at a great  price , [Mancini has some of the best pricing] Holeey Hp series are good , Also Quick Fuel Tech all are amazingly tunable .
 

[cudabob496]

see bottom of article

http://www.hotrod.com/how-to/engine/1306-vacuum-or-mechanical-secondary-carburetor/

[Racer57]

I have the same engine and exhaust you do, but with a 3:55 gear and Tremec 5speed. I'm using an Eldebrock Thunder AVS 650 carb and a Performer RPM intake. It runs great and I got 14.8mpg today with a lead foot.

[Sly_drums]

Thanks for replying

Most people say vacuum secondary is more efficient in a automatic car and get better gas mileage. I'm looking toward the Quick Fuel Slayer 750 cfm with vacuum secondaries, or the Holley Street Avenger 670 cfm, or the basic 3310 750 cfm.
As for the intake, I'm a bit skeptical about getting a new one, that Dual Port is working good but I read that there are better intakes even for stock motors out there, like the classic Eddy Performer :
http://www.edelbrock.com/automotive/mc/manifolds/chrysler/performer-bb.shtml
I'm a bit afraid of single plane intakes on a stock motor, and the lack of torque in the bottom 
What would you get considering my motor specs ? Would you keep the Offy ?
Thanks.

[Chryco Psycho]

Quick fuel or Proform  would be my choice , the Holley  HP series
 The Avenger series is ok but if you are spend for a new carb why settle for OK when there are far better out there ?
 Add the carb first & see if you are happy with the performance once it is dialed in . I would look for the Street Dominator if you are going to change the intake 

[dsburch]

For my 383, an 850 Quickfuel was on it when I got the motor.  850 was just too much.  I changed to a Holley 80508, without knowing the back barrels do not fully open from the factory due to the very stiff (black) vacuum spring.  Why Holley does that I have no idea.    With the Lunati 262/268, I'm pulling 17 inches vac, and running a 6.5 power valve.  I changed the rear bowl config to a jetted metering block, changed the spring to medium and jetted front 72 and rear 80 to set up like other factory HP configs.   This works great, pulls hard, and my best mileage leg got up to 15.9 mpg a couple years ago on a long trip and I think it will do better now with more tuning tweaks.  So - yeh, 750 HP, 770 Av, etc. 

[cudabob496]

you can change springs

[Bluemonster71RT]

My dad had a Holley 4150 on my 383 since as long as I can remember. It never ran right. Not that it ran bad but to me something didn't seem right about it. At the time my dad had it rebuilt and "mechanics" look at it. They drilled holes in in and what not. Anyway when I bout the car from my dad circa 2002 I said F'it and picked up a Holley Street Avenger 670. Best thing I ever did. I was a hole new car to me and my dad too. I went up 2 jet sizes for optimal tuning and it was great. That was on a stock engine less headers. So I don't know if the 750 was too much or what but it loved the 670. My new build is going to back to a 750 Edelbrock until I can afford a new Holley.