Annular vs downleg boosters

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maple ridge puller
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Annular vs downleg boosters

Post by maple ridge puller » Mon May 16, 2011 12:56 pm

What is the difference between the annular boosters and the downleg boosters? Thanks in advance

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Re: Annular vs downleg boosters

Post by Troy Patterson » Mon May 16, 2011 1:13 pm

maple ridge puller wrote:What is the difference between the annular boosters and the downleg boosters? Thanks in advance
Annular boosters discharge the main circuit air / fuel emulsion from typically eight (8) or more discharge holes in the booster whereas, a down leg booster discharges the air / fuel emulsion from one.

Annular boosters are greater in area and mass and therefore reduce the total of CFM. How much depends on the specific annular booster.

Annular boosters are more sensitive to the metering signal allowing a better tune and with a cold / cooler intake track will make more horsepower.

Annular boosters typically deliver smaller fuel droplet size which provides a number of benefits. An engine with a hot intake may make less horsepower when using annular boosters, but may still make more torque across the curve.

I'm still waking up, but that's part of it.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

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Re: Annular vs downleg boosters

Post by T-flow » Mon May 16, 2011 8:36 pm

Check it :D
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I have had great luck with Annular style 4150's, thats all I run.


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Re: Annular vs downleg boosters

Post by 6sally6 » Mon May 16, 2011 11:43 pm

Sooo which kind of boosters do Edelbrock carbs have?!
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Re: Annular vs downleg boosters

Post by lightning-jr » Tue May 17, 2011 1:48 am

6sally6 wrote:Sooo which kind of boosters do Edelbrock carbs have?!
6sally6
I assume you speak of an AFB type "Edelbrock", they have standard boosters, neither down-leg nor the very special annular.

Down-leg boosters are like standard boosters, they are just lowered further into the venturi to enhance their performance.

Most everyone who has the chance to work with annular boosters loves them, I do.
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Re: Annular vs downleg boosters

Post by DaveMcLain » Tue May 17, 2011 11:11 am

Annular boosters in a 4150 can be a mixed blessing. In some cases they work well but in others it's problematic. I ran into one a few years ago with a 750 annular Holley. It was on a Chevy with a reasonably good cylinder head and intake manifold. For whatever reason that carburetor would NOT work well on that engine. I could use a down leg carburetor and it was fine OR change to a different intake and it was good with the annular Holley. Also, running the Holley annular on a different engine produced good results, weird but true.

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Re: Annular vs downleg boosters

Post by Troy Patterson » Tue May 17, 2011 2:11 pm

DaveMcLain wrote:Annular boosters in a 4150 can be a mixed blessing. In some cases they work well but in others it's problematic. I ran into one a few years ago with a 750 annular Holley. It was on a Chevy with a reasonably good cylinder head and intake manifold. For whatever reason that carburetor would NOT work well on that engine. I could use a down leg carburetor and it was fine OR change to a different intake and it was good with the annular Holley. Also, running the Holley annular on a different engine produced good results, weird but true.
It was just the way the annular carb was set up.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

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Re: Annular vs downleg boosters

Post by jmarkaudio » Tue May 17, 2011 2:48 pm

Troy is right, you have to calibrate the bleeds and emulsion different given the same size carb when comparing a downleg versus an annular booster.
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Re: Annular vs downleg boosters

Post by DaveMcLain » Tue May 17, 2011 5:15 pm

I know of a couple of Nascar truck teams that experimented with using annulars in 4150 carburetors and their experiences were the same. On some engine combinations they were dynamite yet on other ones they were awful when compared to the same basic carburetor with a down leg booster which seems weird. I called my carburetor guy when I ran across this problem engine and after I told him the symptoms but before I told him what carburetor we were using he told me, 750 annular.....

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Re: Annular vs downleg boosters

Post by treyrags » Tue May 17, 2011 5:27 pm

Makes you wonder if it has to do with the fact that the annular booster in a 750 is a larger percentage of the venturi cross section (restriction/turbulance) than larger carbs using annulars?
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Re: Annular vs downleg boosters

Post by T-flow » Tue May 17, 2011 6:04 pm

Most that I have or had tried are, 1 3/4" butterfly or bigger with 1000cfm body or larger.
"Yes" I think it has alot to do with the set up and the venturi size.


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Re: Annular vs downleg boosters

Post by Troy Patterson » Tue May 17, 2011 6:28 pm

treyrags wrote:Makes you wonder if it has to do with the fact that the annular booster in a 750 is a larger percentage of the venturi cross section (restriction/turbulance) than larger carbs using annulars?
The annular booster used in the 750 isn't real sensitive honestly. It come back to carb design / set up / fuel curve / calibration. I personally never look at a carburetor as a whole, for me it's a combination of items / parts / settings and so forth and each needs to be correct, but I am a carb guy not the end user so my perspective is a little different.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

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Re: Annular vs downleg boosters

Post by DaveMcLain » Wed May 18, 2011 11:08 am

"Most that I have or had tried are, 1 3/4" butterfly or bigger with 1000cfm body or larger.
"Yes" I think it has alot to do with the set up and the venturi size."

I'm sure that's true and on the larger 4150 body carburetors with a larger than 1.375 venturi I too have had good results from an annular booster. Again though, the 750 annular in my example ran perfectly fine on some engines but very poorly on one particular combination for some reason it was very weird how it could be either a hit or a big miss.

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Re: Annular vs downleg boosters

Post by JDR Performance » Wed May 18, 2011 6:25 pm

One type of booster does not fit all.

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Re: Annular vs downleg boosters

Post by stilhead » Wed May 18, 2011 8:27 pm

What would be considered a "cold intake tract' versus a "hot intake tract" in relation to booster type used?

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