Carburetors with Annular Boosters

I currently have a new Holley street avenger 670 with Bob Oliver at Competition Carburetion in Sun Valley Nevada. He will be converting it to drop leg boosters and when finished it will flow approximately 730 and be blueprinted for the specifics of my 404 Cleveland engine.

I asked him about annular boosters and my takeaway was that he thought they had limited useful street applications. Due to their size they actually diminish the flow characteristics of a carburetor, and he says that needs to be taken into consideration when sizing a carburetor destined for conversion to annular boosters.

He didn’t say they were a bad thing in any way, but for my application he saw no distinct advantage.

My two cents.

Larry

A lot of hot rod types think of annular boosters as things that improve low rpm response for engines with big carbs that are operated on the street. And its true they do serve that purpose. And from that narrow perspective they wouldn't be of benefit in your application because the Avenger isn't "too big" to begin with. In fact, putting annular boosters in a 680 carb doesn't make sense from that perspective.

Autolite installed annular boosters in their 4100 series carburetors of the 1960s. Ford enthusiasts love those carburetors to this day, they performed well and were very reliable. They were not large carburetors, they didn't need annular boosters because of the carburetors' size. But Autolite engineers chose annular boosters for the 4100 series carburetors for other reasons (based upon good engineering) such as throttle response, fuel economy, low rpm and part throttle performance. My point is there are other distinct advantages for street engines (or any engine for that matter) to be equipped with annular boosters. And if I loose a little flow with a 680 carb, no big deal with an engine rev limited at 6200 rpm. If I want to rev the engine higher, I'll use the bigger 780 carb.

What I don't see is the advantage of making a 670 carb flow 730, especially when I could go out and buy 730 to begin with. That king has no clothes … not for me. I'd rather have a carb with a functional choke for my street engine. Mr. Oliver and I have different perspectives.

I think these particular QFT carbs with annular boosters will make good replacements for the discontinued Demon carbs, the best "street carbs" you can purchase. That's why I'm pleased with the reply I received.

QFT has "Super Street" vacuum secondary carbs with electric chokes in 4 sizes, 680, 735, 780, and 880 cfm.

Can you image a Blue Thunder manifold, exhaust heat blocked-off, with an 880 carb having annular boosters? A carb with the low rpm performance of a small carburetor, but the top end performance of the 880! With the annular boosters that carb would also be streetable on the Strip Dominator intake.

Carburetor boosters are perhaps the most important part of a carburetors performance, yet they are overlooked by most folks, and their functions are not understood. I realize now that I could provide more info about annular boosters, what they do, the various ways they can be employed. But not today, I'm not in the mood for composing something like that today.

Until then here's a link to some reading on the subject of Booster Venturis, an excerpt from David Vizard's book "How to Super Tune and Modify Holley Carburetors".

Very interesting your link and once again I regret not to read English more easily.

I found a supplier for a "Pantera" Blue Thunder intake manifold. he offers me to choose between two models, the "normal" and the "competition", the difference lies in the fact that the "competition" has a "Cut-down center divider for more high power RPM & no heat riser".

For a 450hp street engine with a 750 cfm Holley 9379 what do you recommend? I thought of something in between, without the heat riser but with a normal center divider?

René Hi
you are going to stroke it right?

Matthias

Are you building a competition engine, or a street engine?

You always want the full height plenum divider for a street engine, but especially for a Cleveland with 4V cylinder heads. You don't want to change the induction system in any way that would bump the power band to higher rpm.

If I were building a competition engine I wouldn't purchase a Blue Thunder with a cut-down plenum divider, I'd purchase a single plane manifold.

Blocking the exhaust is easy to do, you don't need to buy a manifold with that feature. They sell gaskets that do that! Blocking exhaust heat makes more power ... but makes having annular boosters in the carburetor more of a necessity to realize the full benefit and achieve the best possible drivability with a cold manifold. The second consideration here is which is more important to you, more power or driving in cold weather?

Food for thought, the manifold in the picture was installed for many years on my daily driver, a Mach 1 Mustang. Its designed for Holley Dominator carburetors. Big CFM. Holley Dominators have annular boosters. Drivability was good.

It also had a full height plenum divider.

So, as I thought full height plenum divider, but heat riser that I neutralize with the gaskets which will allow me to go back if the drivability is not sufficient in winter.

Thank you.