Question on Dual Carb setup

[Bigdreamsnobux]

Hi All,

I am still collecting parts for my satellite, and am planning on stroking my 400 to the magical 512 with edelbrock heads and a solid lifter. I bought a Offy dual carb low rise intake (including adapter plates) and really want to run a 4x2 setup. I don't have exact specs on the cam in my head to share (right now at work), but it will be healthy on torque backed by a 4 speed.

I am assuming that 2 500cfm Edelbrocks is the easiest/best way to go, but would dual 600's be too much? The Edelbrocks get pricey compared to some cheaper used Holley's, etc. I can find on Craigslist, etc.

What would you all suggest I run for carbs? Thanks in advance!

 

[69 Runner]

Not really apples and apples, but a Six Pack is 1355cfm.

Hemis used small carbs. But a Hemi woulda been stupid fast(er) if it had the Six Pack and intake available.

 

[Meep-Meep]

OK, a few things here. The Offy intake I believe is a divided plenum single plane just like the old 273 2 BBL and 4 BBl intakes (I have a 2x4 Offy for a 440 and I'm guessing it's the same as you have). Basically my Offy is a copy of the factory 343 HP 383 or the 413 setup. Both engines used inline AFB's on a low rise intake. From the factory these engines had very mild cams with I'm guessing a long LC angle for a broad power curve. I ran a factory 383 setup with original carbs on a 108 deg cammed engine and it was not very impressive. The Torker and 700 Holley DP would run circles around it. I also ran one of those Offy intakes on my Olds 455 jet boat. I had two 500 AFB's with progressive linkage, and again, I didn't notice anything earth shattering - but that's before I figured out my real problem was a cam/CR mismatch. I since changed the cam so will be up for a retest of the Offy setup soon. As a side note I picked up an old Edelbrock dual plane 2x4 for the Olds and hope to try that too. This might yield better results because of the divided plenum dual plane design similar to the six pack and Hemi.

The factory six pack setup is a bit deceiving. The intake is a divided plenum dual plane (with no notch) and that kind of fools the engine into allowing that pile of carbs to work as well as it does. If all those Holley's were on a single plane intake I think the result would be quite different. I noticed Edelbrock makes a dual plane 2x4 intake for the 440, which would mimic the 426 street Hemi setup. This one may be worth a try.

Vac secondary Holley's (assuming they fit inline) should be fine as these tend to be fairly forgiving, but you should vacuum reference them. I have a set of old 400 AFB's for the Olds and I'm curious if running 1:1 would help with fuel distribution. Looks like it might, especially on the Offy intake. Another thing you can do is get the mid 70's OEM Ford truck Holley out of the pick-n-pull. These are around 450 CFM and are vac secondary.

The 4x2 BBL setup would be cool, and if placed correctly, would give a fuel distribution advantage. Heck, for that matter go with an IR Weber setup!! If you want torque, make the runners long and use side drafts in a cross ram arrangement. Now that's good fuel distribution....but expensive.

 

[Bigdreamsnobux]

Wow, thanks for all the info. Being a fan of vintage race cars I would love a Weber setup like that, but as you mentioned the investment would be too much for me.

I will look into a set of Holley's like Meep mentioned, and attempt those. I just wasn't sure if performance would be as good with standard style Vac secondaries but the car won't be at the strip, just the street. I like the idea of running 1:1 as it would ensure even distribution throughout the rpm's and less headaches on tuning I would guess. Either way, fuel economy isn't the issue, as I wouldn't have any of this stuff if it was!!!

Thanks!!!!