I think that as long as the discharge holes are at that point you get the boost. As a matter of fact, if you look at outta the box carbs, as long as there's holes everything's fine...
take a look:



In both pix the top of the booster is way up the narrowest point, and the holes are around that area. IN fact, at the annular booster cutoff pic you can see what panic sez (and he's right on the money) that the annular boosters are only a tradeoff because of the thickness, and the discharge holes on that one are below the narrowest point in the booster, not even the narrowest point in the barrel... Now give that the annular booster pic may be a out-of-the box stuff, and they just didn´t care about lining up those pressure differentials for getting max performance because they just overkill settings so if would work anyway... I believe that the downleg booster pic is better aligned. Now if you can tailor all the things and line up the discharge holes in a downleg annular booster and since you're fabbing it you can go all theoretical and keep the discharge holes .030" downstrean the narrowest point (in the booster or in the barrel???) and keep a thin, nicely, smooth top and razor end annular booster...

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Air moves thru the carb (and venturis) because of pressure differentials

Say we have the following (I'm not using vacuum, but absolute pressure)

Outside pressure 15psi
Manfold pressure 14.5 psi

So air tries to get into the manifold because of the .5psi differential

Now let it flow thru a cylindrical venturi (no curves)
Essentially the pressure in the venturi would be about 14.75 psi where fuel exits are and you would get no fuel flow or very little (doesn't do you much good does it)..

That's why there's a venturi, to have a localized lower pressure...

Now let's have a venturi there.....

Air pressure above venturi 15psi, air then speeds up/pressure reduces

air pressure in middle of venturi 13 psi, air slows down, pressure increases

air pressure at end of venturi 14.5psi.

But that 2psi difference is enough for fuel to flow since the float bowl is at 15 psi, and the fuel exit is at 13psi

Now lets use a booster...... Put exit of booster in middle of venturi.

Air pressure start of inner venturi 15 psi
Air pressure exit inner venturi 13 psi

So the little venturi has a pressure differential of 2psi vs .5 for the no booster arrangement.....

Air wants to go this way because of the greater pressure differential, and then the booster is a venturi too, so at the middle of the booster you get 11psi..... Now you have a 4psi differential driving the fuel into the air stream instead of 2psi.............

So the booster acts like a multiplier of pressure differentials to get fuel flowing.....

The booster effect is most needed at low air flow to get the fuel flowing because you need some minimum pressure differential for the fuel to come out..... With the multiplying effect you need less air flow to do it....

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I'd go into more detail, but then the same people who were wrong before will just repeat the same errors.
Good luck, nice project.

_________________

now, openly snapping at panic post. And still, I'm a moron (not for the steam blow, just things are the way they are).... sooo tired.... (of me)

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Basically, you can put either the entrance of the booster at the narrowest point, or the exit (as it is in your pics there). I'm not sure which if either is better. If the narrowest point of the booster sits at the narrowest point of the main throat, then I think you will get no boost.

Lou

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Home of Slant6-powered fun machines since 1988

I am certainly no expert, and have not a flow bench, although I've used one, and I'm not certain at all where anyone has gone "wrong" here... but here is my thoughts anyway as we all attemp to learn something.

The main body in most carbs also has a restricting venturi effect designed into it...all carbs..., I'm not sure and don't think so...

The idea in my opinion is to accelerate the air through the whole body so that as the air moves thru the "actual" venturi, it is moving even faster and the localized affect would be a nice strong signal at the venturi fuel delivery nozzle/and or holes...as stated above...and based on the airfoil effect since the venturi is just an airplane wing formed in a circle...

I do believe that this is the main reason that this is known as a "booster" signal. The venturi is a booster of the pressure signal developed at the nozzle.

The above description is absolutely correct and the best way to think about carbs if you truly want to understand how they work...and engines in general for that matter...and that is that the whole operation is based on pressure differentials, with atmospheric pressure being the baseline for the differentials to develop. In the carb, the float bowl is vented to atmospheric pressure...so to deliver fuel anywhere else in the system there has to be lower pressure at the delivery point.

I too, would like to know what we all are missing...

Panic... please educate us...because your statement is an invitation or a smug pat on your own shoulder that only you can feel...one or the other.

I know alot...thru experience, education, luck, common sense...etc...but I don't know everything. And most important, I want to know when I am thinking wrongly, or don't know the apropriate facts but think I do...so that I don't make poor decisions from lack of knowledge, because it certainly isn't from lack of trying to learn.

I have spent alot of valuable time on this forum trying to help, not to mention learn...I could've spent it making money, keeping to myself, or chasing girls...but instead I was here typing as fast as I could, willing to embarass myself with spelling errors from typos and lack of forethought, for the benefit of others and the desire for them to critique my work and help me learn more...It would be nice if you would do the same...or keep out of the thread.

A good spirited debate is in the best interest of everyone, no matter which perspective a given person takes...decisions based on knowledge and science are the only smart decisions...and I wish politicians would learn this more than automotive enthusiasts!

We all suffer a life that must be lived, and our cars and interests are only secondary to food, family, and responsibilty. So time is a luxury that few(even in the internet enabled world) bless us with when it comes to passing on knowledge at the same time as the reasoning for that knowledge. Much pain, suffering, and expense would be saved if people only passed on the reasons why they believe something, and people only listened when those reasons were based on rational proccess...and the decisions made in the same like mindedness...


Anyway, that is enough of a rant.

Gearhead

_________________
64 GTO...10.80's@122 on street radials
Destroked 455, Qjet, stock ign, 2400 stall

64 Valiant
Old 225, 4spd, 2.92-8.75, 2bbl, headers
dual 2.25"

An exit of a booster should be at the narrow portion..

Found a diagram of a 4bbl Mazda Rotary carb. (even has nested boosters)
Also looks like the top part of the exit for the fuel is where the venture starts to expand again......




BTW, on the cutaway pics of the booster you can see some of the problems with stock carbs........... notice the casting lines, and how multiple pieces don't line up....... Just little cleaning up little details like that can add up and help you

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I want to step up and say sorry to Panic.

I have always tried to avoid getting "into" anything with people on forums. My personal belief is that it just isn't worth the time and effort to be madd at someone you don't know, especially given the easy way that what seems harmless when you are writing something, can be taken very differently by those reading it.

I have read many posts on this forum, many of which were written by Panic. I have never held an opinion that anything he said was inapropriate or wrong. But tonight I wrote what I wrote with conviction, only to discover another thread where I had appearantely insulted his comments without meaning too.

Also, I read another thread where I made a comment about staying out of ignition related threads for lack of time to write, even though it is my expertise. But one of my comments in that thread, was really no different from Panics comment in this thread...so I am as guilty. I also replied to his comments in a pointed manner, because I became emotional over the direction of my thoughts after reading...for that I am ashamed.

Sorry Panic...hope you will accept...truly I am here just to learn, and pass on what little I know, hopefully to the benefit of others.

Gearhead

_________________
64 GTO...10.80's@122 on street radials
Destroked 455, Qjet, stock ign, 2400 stall

64 Valiant
Old 225, 4spd, 2.92-8.75, 2bbl, headers
dual 2.25"

Hey guys

Ed, thanks for the diagram. Now I get the 2 booster arrangement that we were talking about before. Yup, seems that exit is to be on narrowest point of the main venturi, and discharge holes on the booster right below the narrowest point in the booster. Looking at that diagram makes you wonder how come american 4bbl carb manufacturers hasn't used that primary booster / secondary booster diffrence...

As far as for alignment issues, I don't need downleg if I'm keeping the exit on the narrowest section of my venturi so my fabrication would be a lot easier as I can get along with modifying stock pieces and I skip the welding, brazing and bending tube part. I'll get to work and post pics of my progress (If I have any....)

Karl, thanks for your input, both the emotional and the rational part.

We need way more people like you guys
and now I am. Sorry, panic. My bad. I shouldn't mix up my $#!+ mood with reading tech stuff.

... blah blah I have run across many people whom knows a lot about their ways around this, theoretical and practical and both, and they never asnwer to a question that comes from plain old lack of knowledge demeaning the one who asks in honest ignorance and is willing to learn. And for many "how if...." slilly questions I may have ask in my entire life, say a dozen of them have turn into great working ideas... once someone was patient and kind enough to educate me. I really hate the "naw, I'm keeping my knowledge to myself" attitude. I rather prefer the rough: "hey, you're a dumbass cuz xxx and yyy and zzz and you're missing this and this and you should do ,,,,," kinda patronisingly rough enlitement. Other than that, I respect very much his dry, acid, short remarks and comments and most of the time he's right or very near... In fact I believe that the tech part is always right on his comments, is just that... You know what, he's right hehehe. The one that's wrong before would be wrong again, I'm wasting my time on this $#!+ when I wanna be wasting my time learinng stuff.

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

actually made the 4bbl carbs for Mazda. I believe the earlier carbs are better and it's possible they were no longer the supplier at some point...

I think this would be a GREAT carb for a slant six...

4 bbls (think it's rated at about 325-350cfm), tiny primaries (for economy), slightly bigger secondaries (although the size difference isn't like a spreadbores), everything is adjustable (replaceable mains, all the air-bleeds, the boosters bolt in, vacuum secondaries, the fuel pump pressure was 3-5psi just like what a slant-six has.......

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Yes, Zenith.... didn't know it was Stromberg also. I like Strombergs, thery're somewhat more complicated than others but fully tuneable.... I had one Hillman Avenger 1500 (Marketed down here wearing the Dodge 1500 brand) and it was a sporty version rated 100 hp, with dual 1bbl strombergs. Pretty peppy for a small car of that era down here.
never thought of that but makes great sense... you get to have a non overkill 4 barrels, with great tunability and the boosters on the primary side enhancing low end response, then the secondary side with larger non restrictive units....
hmmm... I'm gonna start looking at junkyards....

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Ok, here's some more info:

Primary venturi(s)
20mm with 13mm/6.5mm booster

Secondary venturi(s)
28mm with 10mm booster


So using the largest venturi to figure out relative flow primary/secondary

20^2/(20^2+28^2) = 34% of total flow rating is primary

So with 325cfm you'd get 111cfm primary plus 214cfm secondary. With the 111cfm you'll get better atomization and part throttle response as this is smaller than event the stock 1bbl......


And remember the 325cfm is an 4bbl / 1.5inhg drop measurement which would equate to 460cfm 2bbl / 3.0inhg........ and 157cfm on the primaries.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

You guys are really getting into some great stuff, I didn't have time for more than an overview...cause for once I'm going to bed early. I'll read this whole thread again this weekend and put my brain to work to catch up with you guys.

I will however, add one quick comment and picture...Qjets have a tripple boost ring design, and I wish I was home to cut one in half. I think I have more pictures buried in my website somewhere...I'll look. But here is one showing the enlarged outer boost ring on the larger 800cfm Qjet.



Ignore the circle, it was placed there for a thread where I was describing how to identify the 800cfm models by the circled bulge on the outer wall of the primary casting. 750 cfm models have a larger boost ring and so the bulge dissappears with the smaller main throat diameter.

The Qjet on a slant thread has a link to a Pontiac forum thread I wrote a few years ago where we discuss the strong boost signals and the problems it can sometimes cause...as well as the cures of course.

Keep up the good work guys!

Gearhead

_________________
64 GTO...10.80's@122 on street radials
Destroked 455, Qjet, stock ign, 2400 stall

64 Valiant
Old 225, 4spd, 2.92-8.75, 2bbl, headers
dual 2.25"

Hey Ed and Karl...

as I posted before, we're talking bout serious $#!+ here
I followed the q-jet thread and I found your comments on rich idling due to extremely easy dripping boosters very interesting...

I have stumbled into lots of good information about boosters in this days since I started to digging up on information, and so far almost everything has been instructive

As a matter of fact, I've learned tons of good carburetion stuff in a very few days... now I know why my car won't die if I close the 2 needles (butterflies will be perforated as soon as I take it away from the manifold), I'm able to tune the rich/lean bogs in my 2 barrels w/o doing more than changing my idle and high speed air bleeds (wich I made replaceable last time I took the 2300 appart) and I have increased my comprehension about carbs in general a great deal...

Getting back to this project, I now have much more understanding on boosters shape, behave and purpose. I think that I'll keep my first shot simple and maybe less elaborate and I'll go on from there... not that I don't dare to do a big mess, but I have only one car and I need it running everyday, so I have to plan things ahead really tight... I'll be building another carb(s) for tryouts, since I have another metering block just like the one I'm using now. Once I get away with one succesful booster fiddling I may try the double and triple booster arrangement...

Keep it coming!!!!

J

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Somehow, gearhead gave me a funny idea. Place dimples along the wall of the carb like dimples on a golf ball. This would obviously be another challenge that's results might not really be worth the effort. You should hear some of my other ideas.