How to select the right carb boosters?

General engine tech -- Drag Racing to Circle Track

Moderator: Team

travis
Guru
Guru
Posts: 1611
Joined: Sun Mar 15, 2009 5:31 am
Location:

Re: How to select the right carb boosters?

Post by travis »

The reason I ask is because out of 3 different carbs I’ve tried on one of my trucks, the 650 annular is definitely weaker performance wise than a 670 straight leg or a 750 edelbrock. It’s not a small difference either. I’m wondering if maybe I should try a 750 annular.

A/F ratios are fine. This is on a 350-ish Hp 351w fwiw.
BradH
Guru
Guru
Posts: 1186
Joined: Wed Apr 03, 2013 11:34 am
Location:

Re: How to select the right carb boosters?

Post by BradH »

Can you borrow something like a 3310 Holley 750 vac secondary to try out? Depending upon the model and vintage, it would either have straight-leg or downleg boosters. It could give you a good feel for whether you need annulars for your combination using a carb of that size. On smaller venturi, an annular is definitely more likely to be a flow restriction.
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: How to select the right carb boosters?

Post by GARY C »

Considering the heat soak of a street engine would annular ever be the choice?
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
BLSTIC
Expert
Expert
Posts: 870
Joined: Wed Dec 05, 2018 7:14 pm
Location:

Re: How to select the right carb boosters?

Post by BLSTIC »

I would have thought that on a street engine you'd want all the vaporization and low speed manners you could get and damn the 15hp (or whatever) it costs.
travis
Guru
Guru
Posts: 1611
Joined: Sun Mar 15, 2009 5:31 am
Location:

Re: How to select the right carb boosters?

Post by travis »

BLSTIC wrote: Wed Jun 09, 2021 6:36 am I would have thought that on a street engine you'd want all the vaporization and low speed manners you could get and damn the 15hp (or whatever) it costs.
In this situation it’s more than a 15hp difference…it feels more like 50. Seriously.
ClassAct
Guru
Guru
Posts: 1024
Joined: Wed Dec 05, 2018 11:55 pm
Location:

Re: How to select the right carb boosters?

Post by ClassAct »

GARY C wrote: Wed Jun 09, 2021 1:30 am Considering the heat soak of a street engine would annular ever be the choice?
IMO...it depends. If the carb(s) are big enough, if the manifold is cold (no heat crossover) and if you can keep the engine temp down to 160-170 then I think yes an annular booster can work.

I’ve done it to one street driven tunnel ram engine that the owner keeps the engine temp at 160 and it was a big improvement.
Tuner
Guru
Guru
Posts: 3184
Joined: Sun Apr 01, 2007 5:26 am
Location:

Re: How to select the right carb boosters?

Post by Tuner »

travis wrote: Wed Jun 09, 2021 6:57 am
BLSTIC wrote: Wed Jun 09, 2021 6:36 am I would have thought that on a street engine you'd want all the vaporization and low speed manners you could get and damn the 15hp (or whatever) it costs.
In this situation it’s more than a 15hp difference…it feels more like 50. Seriously.
Were you using a WBO2 and tuning the carbs with annular boosters? They result in such a stronger metering signal it is necessary to use a jet size 10 or more sizes smaller than a down leg or straight leg in the same size main venturi would use to give the same A/F.

I have used #61 or #62 MJ for correct lean cruise A/F in a 750 (1-3/8" x 1-11/16") with annular booster.

A 650 with annular booster would require MJ in the low to mid 50s and a corresponding smaller PVCR, in comparison to the same 650 carb with straight leg boosters.

It is just nuts to use down leg boosters in the 600-650 and smaller carbs because the leg encroaches so much in the venturi minor diameter.
n2omike
Guru
Guru
Posts: 1067
Joined: Sat Sep 02, 2006 10:56 pm
Location: West Virginia

Re: How to select the right carb boosters?

Post by n2omike »

A had AED do a carb for my 363. It's a solid roller with a 255/263 0.700" lift, big ported intake, Trick Flow 225 High Ports, 11:1 pump gas.
Anyway, what they came up with, was an 850 (huge throttle bores) with annular boosters. I'm in the process of assembling it right now. We'll see how it works, as a dyno session is planned. If it was a 408-427 they probably would have used downleg boosters, but for the smaller engine, maybe the annular boosters will give it a better fuel curve and make it run more cleanly through the midrange. At least, that's what I'm hoping for. lol
BradH
Guru
Guru
Posts: 1186
Joined: Wed Apr 03, 2013 11:34 am
Location:

Re: How to select the right carb boosters?

Post by BradH »

I'd like to add some "visual context" to venturi size vs booster config.

1.375" v pri / 1.438" v sec w/ standard Holley straight-leg boosters
Holley 3310-4_2.jpeg
.

1.425" v w/ a "traditional" down-leg booster
BG Gold Claw v2_3.jpeg
.

1.50" v w/ a Braswell down-leg which has are larger OD than the "traditional"
QFT-Braswell 1.50 3.jpeg
.

1.59" v w/ a "traditional" down-leg booster side-by-side w/ 1.58" v w/ annular booster
BLP-1.59_HLY-1.58-AN.jpg
.

1.61" v w/ annular booster
Holley Ultra HP 1.61v AN_2.jpeg
.


My point is to (re)emphasize that just because you can stuff any style of booster in just about any size venturi doesn't mean you should. The small(er) venturi are absolutely going to see an airflow hit if you park an annular in them, just like the large(r) venturi are going to suffer from low booster signal if the booster selection doesn't take that into account.
You do not have the required permissions to view the files attached to this post.
plovett
Expert
Expert
Posts: 871
Joined: Sat Dec 30, 2006 3:49 pm
Location: Kansas City

Re: How to select the right carb boosters?

Post by plovett »

BOOT wrote: Mon Jun 07, 2021 2:41 am
jmarkaudio wrote: Sun Jun 06, 2021 3:43 pm I use annular for most new builds, but also size the carb accordingly. Small carbs and engines with a lot of heat may not need an annular.
Another excellent point, I set my cars up to run cool and that's why I decided to get my first mainbody converted to annular. Still like many things(ie spacers) sometimes you just gotta try each and see what works for your app.
What is the connection between heat and annular boosters?

Thanks,

paul
PRH
Guru
Guru
Posts: 1501
Joined: Fri Sep 08, 2017 4:16 pm
Location: S. Burlington, Vt.

Re: How to select the right carb boosters?

Post by PRH »

travis wrote: Wed Jun 09, 2021 6:57 am
BLSTIC wrote: Wed Jun 09, 2021 6:36 am I would have thought that on a street engine you'd want all the vaporization and low speed manners you could get and damn the 15hp (or whatever) it costs.
In this situation it’s more than a 15hp difference…it feels more like 50. Seriously.

In my mind, for there to be anywhere near that kind of power difference, the carb that’s making that much less power just has problems.

On a typical 350-ish cube V8 making roughly 1hp/ci(this is going to be a pretty mild build), there wouldn’t be 50hp difference going from a decent 750 to a Holley 4412(500 2bbl....... which is dimensionally the size of 1/2 a Holley 4779).

In the last 30 years I have dyno tested piles of various kinds of carbs that just worked like crap.
Somewhat handy with a die grinder.
User avatar
BOOT
Guru
Guru
Posts: 2895
Joined: Fri Apr 09, 2010 6:23 pm
Location: Michigan

Re: How to select the right carb boosters?

Post by BOOT »

plovett wrote: Wed Jun 09, 2021 4:28 pm What is the connection between heat and annular boosters?
From my understanding this is how it works

Heat aids atomization but hurts performance, less heat(cooler engine) increases the need for other means to aid atomization. This is why injectors are great at cold start-up and driving w/o warming up first, also why OEM's used the exhaust heat to warm the intake faster for carbs. Heat does causes expansion, cool more dense air and atomized fuel make more power. On a hotter running engine dumping(not really) fuel cools the intake so it doesn't heat up n expand the air(NA it's harder to increase air than fuel) and that causes the fuel to atomize, if the fuel is already atomized then it doesn't cool nearly as well decreasing the density of air.

To be clear a carb does not dump fuel. Opening the throttle allows atmospheric pressure to drain into the engine and that draws fuel from the bowls.
Channel About My diy Projects & Reviews https://www.youtube.com/c/BOOTdiy

I know as much as I can learn and try to keep an open mind to anything!

If I didn't overthink stuff I wouldn't be on speedtalk!
travis
Guru
Guru
Posts: 1611
Joined: Sun Mar 15, 2009 5:31 am
Location:

Re: How to select the right carb boosters?

Post by travis »

PRH wrote: Wed Jun 09, 2021 5:45 pm

In my mind, for there to be anywhere near that kind of power difference, the carb that’s making that much less power just has problems.

On a typical 350-ish cube V8 making roughly 1hp/ci(this is going to be a pretty mild build), there wouldn’t be 50hp difference going from a decent 750 to a Holley 4412(500 2bbl....... which is dimensionally the size of 1/2 a Holley 4779).

In the last 30 years I have dyno tested piles of various kinds of carbs that just worked like crap.
I don’t think there is anything wrong with it…I just think maybe it is incorrect for the application. I’ve tried this exact carb (edelbrock 650 AVS2) on several milder engines and it runs great. But…I never compared it to anything else on those builds either.

Tuning for all the carbs tried was done with a WBO2. Also, this engine has no provisions for exhaust crossover and temps run in the 165-175 range.

This engine is a 351w built copying a 425hp 350 sbc. It feels like a 350hp engine with the 650 annular on it, and MUCH stronger with any other carb I’ve tried
PRH
Guru
Guru
Posts: 1501
Joined: Fri Sep 08, 2017 4:16 pm
Location: S. Burlington, Vt.

Re: How to select the right carb boosters?

Post by PRH »

365” SBM, 9.7cr, .500” lift solid cam, dual plane manifold with no spacer, 1-3/4” headers.
Pump gas with a Holley 4779 on this pull.
Solidly over 400hp....... under 600cfm air consumption.

A Ede 750 made within a couple of hp.

The only way a carb with a flow capacity that is actually only 100cfm lower making a bunch less power on a build like this is........ if it doesn’t work well....... it’s not the “size”.

I can’t tell you how many times I’ve tested carbs where the fuel flow and A/F ratio numbers looked good....... yet they didn’t make good power.
8C906A07-5FF9-4273-9A2A-B09E113E4C44.jpeg

On a 450+/- hp 383 Mopar I tested 13 carburetors back to back.
A bit of a mix of sizes and styles. The smallest being the factory Carter AVS which flows 585cfm, up to a ported 850 Holley with annular boosters flowing 905cfm.
That’s a 320cfm spread......... about 23 hp difference across the board(443hp to 466hp).

On a FAST(factory appearing stock tire) legal 511 BBM, an Ede AVS 800 was about 30hp better than a correct Carter AVS for a 69 383(585cfm).

That motor used about 720cfm@6500 rpm.
You do not have the required permissions to view the files attached to this post.
Somewhat handy with a die grinder.
travis
Guru
Guru
Posts: 1611
Joined: Sun Mar 15, 2009 5:31 am
Location:

Re: How to select the right carb boosters?

Post by travis »

Interesting. And I don’t disagree with you…while I’ve never done dyno testing I have swapped multiple carbs in plenty of different applications and never felt any huge SOTP difference…even when using something like a 750 on a nearly stock 350 or 351 in a heavy truck.
Post Reply