I came accross this the other day in my old TRW Hi-Perf book.
I thought you guys would find it interesting.

 

That guide suggests the 327/365 motor is slightly under carbed in stock config. Holley 2818 is a 585CFM I believe.
Brian

 

I think one of the most common misconceptions is bigger is better where carbs are concerned. I ran a 600 CFM on a 402 BB, and everyone laughed because they thought it justified a 900 CFM "double pumper". I pick the smallest carb that I can get away with. Seems odd, but throttle response will be better as well as "tune-ability".

The other tuning myth is listening for the secondaries to "Open" by the "Bog" sound. The "BOG" sound is the secondaries coming in to soon. A well tuned carb should have no bog, and all engine scream.

Mark

 

Your splitting hairs! This is a general guideline - 325CID/6500 revs calls for 611 CFM.

GM was right on the money for a good high performance street engine.

Duke

 

Darn!
So you think I should pull the 1050cfm Dominator off the small block to go quicker than 10 seconds...?

 

Its too bad GM wasn't aware of this chart and what they were supposed to do. It would be sweet to have 680+ cubic inches under a tri-power!

 

What is the combined cfm of the 56-61 2x4 carbs? I am thinking that it is around 550 cfm (total of the two), which is not too far off of the chart (albeit a little higher). Plus, I'm not really sure how this formula applies when the front carb is only opening up with heavy throttle application.

Out of curiosity, what is the total cfm of the tri-power set-up?

I always thought that 650 cfm was about as large as you needed to go for almost any small block, and that 750 or 850 was sufficient for most big-blocks (not counting blown/supercharged engines).

 

1282 CFM - 350 center, 466 outboard [ea]

 

Context, guys, context?

Two-barrel carbs are rated at 3" Hg. depression, not 1.5" like four-barrels, so to convert two-barrel flow to "four barrel equivalent" you have to divide by the square root of two. Flow responds to the square root of depression so if you double depression, flow increases by only 1.414. Airflow numbers are meaningless unless you know the depression at which it is measured. I can make an 800 CFM Holley flow 1600, but the depression - or pressure loss across the carb. - will be 6" Hg., which is restrictive.

Run the numbers and the '67-'69 3x2 setup is equivalent to a 907 CFM four-barrel. Given all the design compromises of a multi-carb manifold, I'd much prefer the '66 L-72 setup with a single 800 CFM four-barrel.

The WCFBs flowed about 400 CFM at 1.5" Hg. depression so the 2x4 setup on 283s was about 800 CFM total at 1.5" but the engines actual demand was only about 360 CFM for the 245 HP version and 426 for the 270. (I'm assuming 80 percent VE at the respective redlines of 5500 and 6500.) And you wonder why these systems are so finicky - talk about overcarbureted - holly molly, but they sure look cool!

The multicarb setups of the fifties pretty much gave way to the single four barrel in the sixties with a few exeptions, most of which IMO were marketing gimmicks rather than actual performance enhancements.

The actual air flow through an engine is purely a function of its efficiency as an air pump. The carburetor merely represents a slight restriction at the inlet, and the air flow ratings are a general way to determine the relative restriction. On a good high performance street engine you want a carb that will provide the engine's air flow demand at not more than about 1.5" Hg. pressure loss. A higher flowing carb.may not provide a good metering signal for good throttle response, idle and off idle driveability and reasonable fuel economy. Any less will be restrictive.

For a racing engine the carb should be sized to provide peak air flow demand at no more than 1" Hg. depression since you're not concerned with idle and low speed driveability.

At the other end of the spectrum are racing engines that are intentionally restricted to limit horsepower. NASCAR superspeedway engine restrictor plates result in manifold vacuum of 7-8" at 7000 revs at which point they make peak power an are completely "out of breathe".

Duke