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2bbl vs. 4bbl
https://www.slantsix.org/forum/viewtopic.php?t=11195		 Page 1 of 2
Author:  Guest [ Tue Dec 14, 2004 5:55 pm ]
Post subject:  2bbl vs. 4bbl
ive read many posts on this website and have found that many people run 4 bbl carbs. i cant find hardly any posts about running bigger 2 bbls like the holley 350 or 500's. is there a reason for this? does a 4 bbl offer that much more of a performance gain?
Author:  Ron Parker [ Tue Dec 14, 2004 6:54 pm ]
Post subject: 
If it is a daily driver it is not worth the change to a two barrel or four barrel. It is tricky to get a stock slant to run on a four barrel carb. If you have the four barrel intake and headers i would recomend the holly 390 cfm carb otherwise if it doint bog run with the one barrel carb and spend your money on some rear gears. Put in a 3.55 rear gear and you can see more preformace than a two barrel change. Thanks Ron Parker






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Author:  Guest [ Tue Dec 14, 2004 9:28 pm ]
Post subject: 
im not really asking for advice on my car. im just wondering why no one runs a bigger 2bbl. is there a problem with this? dusteridiot did it with what sounded like great results. a holley 350 flows considerbly more cfm that a bbd, right? is it a cool factor to run a 4 bbl?

how would a holley 350 or 500 cfm compare to a holley 390 4 bbl?
Author:  Dennis Weaver [ Tue Dec 14, 2004 9:35 pm ]
Post subject: 
One thing to consider is that, even though the overall carb may be smaller, bigger 2V carbs have larger venturis and therefore less velocity thru the carb at lower engine speeds than carburetors with smaller venturies, so a big two barrel may not have as good throttle response and off-idle characteristics as a small four barrel.
Quote:
how would a holley 350 or 500 cfm compare to a holley 390 4 bbl?
They are all three complete garbage in my most humble, non-threatening, non-biased opinion. :wink: :roll: :wink: :lol:

D/W
Author:  70valiant [ Wed Dec 15, 2004 4:01 am ]
Post subject: 
Big 2 barrels(from what I understand) are designed to be on big engines that can handle big shots of fuel coming from 2 large primaries and no secondaries. slants ares mall engines that can not take it untill rpms are up a bit so big 2 barrels cause a nasty bog. a small 4 barrel has 2 much smaller primaries and then 2 secondaries that open once the engine can handle the extra fuel. that equals much better driveability for the little slant.
Author:  Charrlie_S [ Wed Dec 15, 2004 6:22 am ]
Post subject: 
First, 2 bbl carbs and 4 bbl carbs are rated differently. A 500 cfm 2 bbl flows about the same as a 390 4 bbl. As mentioned a large 2 bbl will not have good velocity at low rpm, and poor driveability. A 390 4 bbl will only be using half the carb at low speeds, ie: 190 cfm. This give a stronger signal to the metering system, for better drivability. One of the main problems with using a 4 bble on a slant, is using too large a carb, with stock gears and converter.
I have a 500 holley 2 bbl on my 170, but I also have a 3,000 rpm converter and 3.91 gears. It would be a real pain in the butt, to drive with stock gearing. Can you say bog city. As it is I am using a lot of pump shot.
Author:  panic [ Wed Dec 15, 2004 7:34 am ]
Post subject: 
To compare 1 bbl. or 2 bbl. flow rating with 4 bbl., multiply by .707.
To convert 4 bbl. rating to 2 bbl., multiply by 1.414.
Author:  Charrlie_S [ Wed Dec 15, 2004 8:17 am ]
Post subject: 
Correct. I just didn't have the numbers handy. 500 2bbl compares to a 353 cfm 4 bbl.
Author:  Guest [ Wed Dec 15, 2004 8:25 am ]
Post subject: 
ive read up on every inline six website i can find, not just ones on slants. especially on the ford website, all they run is holley 350 and 500's. i know there are issues with the head/intake and such on a ford but i havent heard of them having problems with their cars and many of them are running 200 ci motors.

everyone has alot to say about this, but how many have actually ran a large 2 bbl and had problems? or is this just theory?

im curious because im at a point with my motor that i can go with a 4 bbl but manifold and carb pricing for 4 bbl's arent that enticing to me.

do you guys believe stock stall converter and 3.23 rear gearing would give me problems?
Author:  Dennis Weaver [ Wed Dec 15, 2004 9:37 am ]
Post subject: 
Quote:
ive read up on every inline six website i can find, not just ones on slants. especially on the ford website, all they run is holley 350 and 500's. i know there are issues with the head/intake and such on a ford but i havent heard of them having problems with their cars and many of them are running 200 ci motors.
I'm not surprised Phords and holleys would go hand in hand.
Quote:
everyone has alot to say about this, but how many have actually ran a large 2 bbl and had problems? or is this just theory?
See CharlieS' comments above. He da man, runs these things all day long, so he would know. I'm sure others here are running big 2V's, too, maybe some more of them should weigh in on this.

D/W
Author:  Charrlie_S [ Wed Dec 15, 2004 10:24 am ]
Post subject: 
If you wany to run a 2bbl on an almost stock slant, I would use the super six, or the 350 holley. Stay away from the 500 2bbl. Many ford v-8's used a 350 Holley or a similar motorcraft. It will take a fair amount of tuning.
Author:  FrankRaso [ Thu Dec 16, 2004 8:18 am ]
Post subject: 
To expand a bit on the previous replies, the main advantage that a 4bbl carb has over a 2bbl is the higher flow velocities through the venturis, at least with air-valve or vacuum secondaries carbs. The flow rating is simply the amount of air that produces 1.5" Hg or 3.0" Hg pressure drop across the carb. It does not mean that a 500 CFM carb will deliver 500 CFM to your engine.

When the throttle is cracked open from the closed position at idle, the carb moves from supplying the engine from the idle circuits to the main metering system at the venturis. Whenever the throttle is opened, however, air flow increases instaneously but there is a slight delay in fuel flow. The accelerator pump was designed to compensate for this delay.

The problem with using too large a 2bbl carb on a small engine is that the air flow through the carb is too low to have a smooth transition in fuel supply from idle to the flow at which venturi vacuum can start the fuel flowing through the main metering system. As Charrlie_S explains in his case, he has to dump a lot of fuel into the intake manifold when he starts.

With air-valve or vacuum secondary carbs, the air flow is initially through the primary venturis, which are usually smaller but never larger than the secondaries. The primary venturis are about the same size as the OEM 2bbl venturis so the drivability of 4bbl carbs should be as good as or better than that of the BBD 2bbl carb, when they are properly jetted. The advantage to the 4bbl is that the secondaries are available to supply more air when the engine requires it and not before.

I think the 'cool' factor is saying that you have a large CFM carb on your engine rather than having a 4bbl carb (although I'm one to talk with my quadrajet). The super six 2bbl upgrade is the simplest of the carb upgrades to make. Depending upon what you are planning to do with your car, the 4bbl may or may not be a performance gain over the BBD, especially with the amount of work and understanding to make it work properly.

Frank
Author:  Guest [ Thu Dec 16, 2004 8:47 am ]
Post subject: 
is a holley 350 that bad of a choice? most of the replies here are centered on the 500. i was never really interested in a 500, i guess i should have made that more clear.

by the time my engine is done (cam, headwork, etc.) im not gonna have an extra $300 to $500 to spend on an intake and carb.

so my question is, will the holley 350 be a good bridge for the gap between the bbd and a 4 bbl, till i save some more cash?
Author:  Charrlie_S [ Thu Dec 16, 2004 8:47 am ]
Post subject: 
Yeah, what Frank said.
Author:  FrankRaso [ Thu Dec 16, 2004 9:03 am ]
Post subject: 
Really, you can make any carb work with any engine. It's a question of how well they work overall. I have heard of people running Holley 350's on their engines as an upgrade over the BBD and it would probably work fine for you once you get it set up properly. I think everyone here was concerned that you might want to try the Holley 500 on the street. If you already have the 2bbl intake and the carb, you might as well give it a try. An easier upgrade would be the BBD's from 318s or 383s though.

Frank
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