Annular booster and jetting?

[TheUncle]

When you would switch from a downleg style booster to a annular booster,the signal would be much stronger.
So far so good,i can understand that,but why do you have to change the jetting so drastically?
Better signal,more fuel to suck in,in my small carburetor world.
Perhaps someone can explain it to me in easy words,cause my carb physics are not the best.
Might there be thumb rule to decrease jetting by x% of the current jetting?
Reading books might be easy,but the reality looks different.

[rgalajda]

The centre of an annular booster is typically much larger . This means much more air will travel through the booster instead of around it.
Wth more air travelling through the booster it creates more depression ( vacuum ) in the booster . This makes the booster draw ( suck ) more fuel. The greater the air flow the greater the amount of fuel drawn into the venturi.

It is the pressure drop ( suction ) caused by the air passing through the venturi that is used to draw fuel out of the fuel bowl
Mini annular boosters will not increase suction noticeably.
A high gain booster such as an annular booster can increase the main venturi signal by as much as 300 to 400 percent.

Below is a picture of a 3310-1 780 cfm Holley carb that I replaced the downleg boosters in the primaries with annular boosters. Doing this required reducing the main jets by 5 sizes when the air bleeds were untouched.

Annular boosters also atomize the fuel better because the fuel comes out numerous holes instead of one hole. Smaller droplets of fuel , easier to brake down and mix with the air. You can also atomize the fuel too much though, It is a balancing act.

[Right hand drive]

rgalajda has given a good explanation. I’ve found 850cfm Holley style carbs to also have a 4-5 jet size drop downleg to annular.

When you think of the main jet as a restriction rather than a flow device then it may be easier to understand how the stronger signal of an annular booster needs the amount of fuel from the main well restricted more than the less sensitive downleg or straight booster.

[rgalajda]

As an interesting note the Autolite 2100/4100 carbs in the 60’s used Annular discharge boosters ( mini annular ) . 1980’s OEM Holley carbs on Ford vehicles used a large annular. Holley did not invent Annular discharge boosters.
Quadrajet carbs used a triple booster design. Each of these were designed to give better fuel atomization/signal.
Better fuel atomization can also lead to better fuel distribution through out the cylinders.

When we use performance intakes without a heat crossover and /or air gap design , fuel atomization can be reduced. This is where a better booster design can help. Although a cooler intake charge can lead to better performance we may need to encourage more fuel atomization .
A stepped dogleg booster will help here because it adds a shearing effect .( better atomization ) . See picture.
Going to an annular booster will further increase atomization.
As a note hear ,if you use an annular booster with a heated intake this can cause too much fuel atomization , end result being a reduction in power.

Annular boosters are more often used on large venturi carbs to restore the lost signal ( suction ) of the large venturi.

[TheUncle]

Hello once again,i had to read it several times to understand your complete answers.
It took time to understand that.
Thank you and greets from Germany

[TheUncle]

Hello once again,during the season i made some test runs with jetting down 5-7 numbers down.
This doesn´t work really great,perhaps i made some mistakes.for the next season i will give the combination a new try.
On the photo you can sse the current set up jetted on weather conditions from 104-105. with .188 inlet and downleg booster.
Under the read line are the numbers of what i have on annular boosters and
different inlet sizes.
Perhaps HSAB are too big or small,with only changing jets down it doesn´t work.
Please take a look on the set up.All is running on ETS Z85 Perfo E85

[Right hand drive]

[GTO Geoff]

Looks like he might have mixed up the IAB & the HSAB.

[TheUncle]

Good evening !
I made a big mistake when writing it down,sorry for that.
So HSAB is .26 and ISAB is .62
Idle feed is .37.I have 2 carbs on a tunnel ram.
Sorry for my mistake!