What's the difference between the annular booster and a 3 circuit annular booster carburetor?

Annular 2 vs 3 circuit refers to the addition of an intermediate circuit. It is a discharge tube placed about half way between the venturi, minor diameter of the throat, and the throttle shaft. It adds the most fuel at part throttle, but still contributes at WOT. It was designed to be used for an individual runner application, when pulses from the runner are so high they disrupt part throttle booster operation. 4 and 6 cylinders can benefit even with a plenum as pulses are strong. I have found only rare occasions that need a third circuit.

Here is a picture of annular boosters. The fuel comes out of the tiny holes inside the booster.

Andy, do you have a pic of that adapter off the manifold?

Here is a close up. It is fairly simple open to open with just a slight taper outward.

I just tested the 850 on my car last week and holy chit it was way responsive but WAY too much emulsion and WAY lean down low so after tweaking it, it rocks but it just had the stock down leg boosters........

Thumperdart and all,

When would you recommend an Annular Booster over the smaller down-leg booster?

Also, I run my engines in a Strip/Street configuration, so would two smaller CFM Annular booster carbs in a dual quad configuration be correct? and would a low vacuum signal be another reason for Annular boosters?

Thanks!

I havn`t changed em out but Mark W. is the guy to ask so hopefully he`ll chime in. I know that the annulars give a cleaner perimeter fuel spray instead of wicking like a downleg does and apparently automized better.......

Mark changed my 950 over to annular boosters because I was running a low RVP fuel in cold weather. The change to annular boosters picked up a lot of torque down low (20 to 30 ft-lbs) but the carb didn't make anymore power up top.

I'll rerun the tests this summer to see if the torque advantage stays there or if it is a cold weather thing only.

Thanks for the info. I do run an annular booster now, needed to know if the 3 circuit was worth getting. Never ran one before.

If you're asking about a Dominator then I'd recommend sticking with a 2 circuit carb. The third circuit can be handy for specialized applications but for the average guy it just dumps extra fuel into the intake and causes problems. I think Holley should stop selling the three circuit carbs and offer them only as a special option but I'm guessing they don't worry too much about my opinion!

i agree! if ya do have a 3 circuit thats giving you fits try running carb without int. air bleeds or i have found a 90-100 air bleed helps.

Most racers and builders prefer the 3 circuit and we sell a LOT of them, especially for STOP racing. We offer both and with different options. It's up to the consumer to either KNOW what he needs, or call the tech line and get some help. But why would we stop making something just because many don't understand them or what they need

We have had this debate several times. A few builders like Mark, prefer 2 circuits and that's fine. But more yet prefer 3 or more circuits. Does it take more understanding to make it work at part throttle?.......yes, without a doubt, but if you know what you are doing, they work just fine. NOT suggesting that Mark or anyone else doesn't understand..........they just prefer the 2 circuit. I am sure he can make a 3 work as well as anyone else can.

I just watched Mike Laws personally tune a 3 circuit on a 170+ mph Super gas combo at Orlando. Watching the car and looking at data, he made the curve silky smooth and a lot of that was done on the 3rd circuit

I'm sure tthis will sound like a broken record, as I've both stated and asked, if the metering blocks still have the idle fuel supply in the main well. THIS, is the design flaw to beware of. Anecdotal. 🙈

It`s been well documented that even if you plug the int. circuit on the 3-circuit blocks, the main well can`t flow enuff fuel due to the emulsion tubes as they take up needed space/volume and why guys go w/the BLP widebody blocks............

I understand this, and went 2 circuit because of this problem. I want to understand why Holley made them that way to begin with. Every 3 circuit success story, seems to inclube BL metering blocks. Only my experience, of course😉

Change to a 2-circuit block and problem solved........

Hi 68dodge,

What size carb are you running or planning to run? And is this a strip car or Street/Strip?

I am also interested in your questions, as I was looking at the Quick Fuel 750 SS Carbs w/annular boosters for a dual quad in a progressive linkage.

Since I am running on the street some, I thought of running the above carbs instead of a QF Dominator 1400 CFM...same cfm, but would be more streetable??? maybe???

Mark

Annular boosters provide superior atomization of the fuel, this leads to more equal distribution. For a 4150 as long as the carb is sized to offset the airflow difference then peak power will not be hurt. There are also options available like the custom annulars in Andy's 950 on page one, BLP also has a custom mini annular that installs in a modified Braswell down leg booster that works well. Also with the annular it allows a larger carb than normal to be used, this can improve power across the board.

As far as the intermediate circuit it is rare that you need it when the carb is tuned correctly. There are cases with a throttle stop where it can be useful, when an under carb stop with 1 to 1 linkage is used the blades may be closed too far to get a consistent signal to the boosters to meter well. And most of the time with the 1 to 1 linkage it's on a larger engine that has to close the blade a lot to get the stop RPM down where they want. With a progressive linkage under carb stop or inline linkage stop it isn't an issue, and the intermediate can actually hurt. I do have several guys running 2 circuit carbs with 1 to 1 stops with no issues, so it's just dependent on the combination. For anything bracket raced or run on the street there is no reason for it at all. My feeling is the more fuel you run thru the booster the better atomized the fuel is, the better distribution is, the more fuel gets vaporized early lowering induction temps... more overall power.

I have some 2 vs 3 testing scheduled for later this year. My 1250 has been modified by BLP with the upper fuel jet modification and extra metering blocks so I can switch back and forth between 2 circuits and 3 circuits.

BLP provided me with multiple setup sheets so I can run the carb either way. I just pull the jet area out of the intermediate circuit and route it thru the main circuit by using a larger main jet and larger upper fuel jet. So then we can see the power difference between dumping the fuel from the intermediate nozzle or having it flow thru the boosters.