Don Terrill’s Speed-Talk

I had a post written shortly after the last one I posted and lost it because of the lock. Hopefully this can be kept on a more civil level. This is a continuation of the following subject.
viewtopic.php?f=1&t=37629

A couple people commented on the boosters, and that is my focus here. There are several types available, but we will focus on two.

First is the downleg used for 4150 carbs, and while you might think there are only a couple there is a selection to chose from. The standard downlegs from Holley BLP sells are sized starting at .140 in a couple sizes up to .178. Then is a Holley step with a .156 leg. Next is the custom BLP boosters, machined to tighter tolerances. The standard are either modified or legal for class restricted, and come with several leg sizes from .140 to .190, the sizes vary due to different carb sizes and fuel types. Next are the custom machined boosters with two larger diameters through the middle, one at .435 and the other at .450. They are also profiled on the OD to improve airflow. Last is a Braswell downleg, also a nice booster. The entrance and exit are cast shaped during manufacture and are also available in different passage sizes. Booster selection depends on the strength of the signal desired and the airflow needed. For a 4150 a downleg is my choice unless you are trying to put a larger carb on a smaller engine or in the case of a street or street strip that spends a lot of time at a lower RPM, annulars would be used and can improve economy and lower RPM performance.

Next is the annular, and I prefer over the earlier bar booster used in Dominators. Signal strength and airflow through the carb are improved, and droplet size can be controlled by the number and size of the holes. Boosters can be bought stock,, no holes, and I've seen up to 18 holes. With no holes the possibilities are up to you. Inserts also come in several ID sizes, I have seen .480, .580, and .600 as well as a couple custom machined from stock Holley inserts.

With the variety of boosters available, using a lot of bleed air to "emulsify" the fuel in my opinion is not needed except in maybe some extreme cases, as long as droplet size can be managed by the boosters, proper engine temps for the application, and the proper fuel is selected, using the bleed air to correct the fuel curve is all you should need. Using more may be used to tailor a limited RPM range, but does so at the expense some other part of the fuel curve. 1 to 2 bleeds will cover a larger majority of apps, 3 can be used if the emulsion bleed sizes are kept on the smaller side. Keeping some separation can help in an effort to prevent emulsion bubbles from coalescing, trying to keep the mix of fuel and air in the well equal. When bled air is excess, bubbles come together to form larger ones, and you get slugs of air and fuel early in the curve which makes delivery inconsistent.

You didn't mention the Braswell stepped-downleg booster which has an O.D. measuring .050" larger than the normal Holley downleg booster.
OR, the small diameter annular booster made from that Braswell booster.

I presume it is merely because you haven't any experience with those offerings so, I simply wished to mention that they DO exist.

I'm glad you did this JMark.
Love the good carb threads.

how do you choose a booster passage diameter?

Actually I did mention the Braswell downleg. The larger diameter may take a small amount of airflow away, but on my list is to see if a little work can improve signal and or flow. Have not seen the small annular version, I would have to test some.

rabbit wrote:how do you choose a booster passage diameter?

Fabulous question! How big do we want it, and what difference will it make? Any takers?

Any takers?

Where do you want it to go turbulent?

Hysteric

beside fuel type, its related to main well size and exit diameter for starters

jmarkaudio wrote:Actually I did mention the Braswell downleg. The larger diameter may take a small amount of airflow away, but on my list is to see if a little work can improve signal and or flow. Have not seen the small annular version, I would have to test some.

Mark, After a second reading I see you mentioned them, simply not their slightly larger outside diameter.
Those Braswell diameter, downleg "annular" boosters are made by "Stealth Racing Carburetors" in Cleveland, TN.
They have much higher booster signal gain at lower air flows but, at higher flows through the venturi it is only very slightly different.

Rabbit, The booster hole diameter should be large enough to NOT cause a restriction however, must be kept small enough to not dampen "start-up" or low venturi air flow signal.
Of course there are a few times where slowing "start-up" signal is preferable so, there is really no "cut & dried" method to what its size needs to be.
The criteria with different carburetors is always different.

For a 4150 a downleg is my choice unless you are trying to put a larger carb on a smaller engine or in the case of a street or street strip that spends a lot of time at a lower RPM, annulars would be used and can improve economy and lower RPM performance.

What about the BLP booster that I don't think you mentioned above? I can't find the part number but I was told it would outflow the annular booster with no signal loss and without the flow restriction of the annular.
Unless it is one of the BLP customs you mentioned. thought the ones I used were part numbered off the shelf but maybe not.

jmarkaudio wrote:

rabbit wrote:how do you choose a booster passage diameter?

Fabulous question! How big do we want it, and what difference will it make? Any takers?

Based on hp and engine size? Another question you relate droplet size and engine temp can you expound on this .

If we are talking about emulsion vs booster dont' you need to consider area of emulsion?

I always think of it like a MC card with a needle. The needle design (shape) is like the booster. The clip adjustments are like emulsion holes. If you lower the clip it is richer, sooner (depending on shape). If you raise the clip it will be leaner, longer. If you can't get part throttle cleaned up with clip changes and the air screw (air bleed) you need an different profile needle (booster). If it's an idle issue, you adjust the slide cut out (IAB) and retune the rest.

Am I close?

raceczar wrote:If we are talking about emulsion vs booster dont' you need to consider area of emulsion?

I always think of it like a MC carb with a needle. The needle design (shape) is like the booster. The clip adjustments are like emulsion holes. If you lower the clip it is richer, sooner (depending on shape). If you raise the clip it will be leaner, longer. If you can't get part throttle cleaned up with clip changes and the air screw (air bleed) you need an different profile needle (booster). If it's an idle issue, you adjust the slide cut out (IAB) and retune the rest.

Am I close?

A slide carburetor is a whole lot different than a butterfly carburetor.
With the slide you are always changing the area of the venturi throughout its travel.

What effect, if any, would/does booster ID sizing have on droplet size and distribution? Be it annular or downleg.

Walter R. Malik wrote:

raceczar wrote:If we are talking about emulsion vs booster dont' you need to consider area of emulsion?

I always think of it like a MC carb with a needle. The needle design (shape) is like the booster. The clip adjustments are like emulsion holes. If you lower the clip it is richer, sooner (depending on shape). If you raise the clip it will be leaner, longer. If you can't get part throttle cleaned up with clip changes and the air screw (air bleed) you need an different profile needle (booster). If it's an idle issue, you adjust the slide cut out (IAB) and retune the rest.

Am I close?

A slide carburetor is a whole lot different than a butterfly carburetor.
With the slide you are always changing the area of the venturi throughout its travel.

True. But does the analogy fit?