Holley carb questions/discussion.

[Mark Bliss]

Moving this here because it is more general than FE specific...

General question:
In reviewing carb specifications while identifying some of the box of parts in my collections, I noticed that CFM ratings seem to be a little random.
For example there is of course a large group with the same sized throttle bores, and the same sized venturi, rated anywhere from 715, 730, 750, then another set of venturi rated 780, 800, etc. while the remaining variable- boosters, seem to appear identical.
Thoughts/experience?

Specific question:
The 3310-1 I have run on my "hot street/strip" 390 for years has "corrector tabs" on the primary boosters.
In reviewing my memory and vintage data sources, this was engineered to improve mixture distribution on a specific intake manifold- being in fact another brands factory dual plane which would have had an open plenum.
I am running it on a dual plane divided plenum (Vintage 427 Shelby "sidewinder")
Any technically minded folk here care to opine on the effect of this?

[Barry_R]

Most of the OEM carbs were rated on Holley's orifice box, and rounded to the nearest "5" cfm number.  Some of these were later converted to aftermarket carbs and the rat wings got tweaked as they changed boosters and such.  Somewhere here - buried in a box of old paperwork - I have a photocopy of many pages of the original hand written Holley part number assignment book.

Many (not quite all...) of the aftermarket carbs were rounded to the nearest "50" cfm number for marketing simplicity ie 700, 750, 800, 850 etc.

Later performance aftermarket offerings were rated by some arbitrary fantasy process as Holley reacted to the marketing pressures of the various carb modification places, and cannot be compared to older products other than by using Venturi bore, throttle plate, and booster data as you are doing.

[Mark Bliss]

Thanks for that Barry.
I kind of knew the aftermarket ratings were "nominal" but comparing specs and sifting through a box of parts got me curious.
The term "rat wings" is interesting. My old books called them "compensation tabs"

I am curious as to just how much effect they have and whether it is possibly detrimental to my use on a different manifold configuration.
In my research of old material (and attempt to refresh my old memory) I found notes that this specific model may have been originally used on GM 396/375's, 427/425's and possibly the '69 Z-28. If true it may have some value to a restorer and so I am thinking I don't want to modify it irreversibly. Instead, I intend to put together another from similar generic parts. I kind of want to play with an HP style with four corner idle for the tunability. You know, with all that spare time I have and all... (Sarcasm)

[Rory428]

The Holley 780 vacuum secondary carbs that were factory installed on the Chevy 396 375 HP engines, were 3310, with no suffix. The 3310-1, was the early "universal" 780, which used secondary metering blocks with replaceable jets, and down leg boosters. The later 310-2,3,4 etc (I think they are up to 3310-6 now), are 4160 models, with a secondary metering plate, with the sec. jets drilled into the plate, so no changeable, so you either need to change the metering plate, or drill the holes larger. These later carbs also used a straight leg booster design, which I suppose is a result of the -2 and later versions rated at 750, rather than 780 cfm.
Although the 428 CJ Holley 735 has the same sized venturis and throttle butterflies, they used primary boosters with "bell bottoms" on the bottom, which essentially makes the booster larger, which reduces the airflow.  By the way, I have a factory Holley carb from a 1970 429 Super Cobra Jet engine , which looks very similar to the 735 428 CJ carb, but does not have the bell bottom primary boosters, so is rated at 780 cfm.

[Mark Bliss]

Thanks Rory,
I admit that my sources and memory are scattered. It wouldn't surprise me to be off base here.
Heck, even the Holley spec data can be contradictory.
But the tabs in a generic carb app is even more odd to my mind, as it would certainly have varying effect depending on manifold specifics.

Anyway, onward with my experiments.