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Discussion Starter · #1 ·
Hi all...

Looking down the throats in a Weber 45 you can see two strangulating items...

1 The Aux Venturi and its winglets and centre lump

2 The Butterfly and its spindle and screws

I am think its possible to increase flow if you modifye these items, as well as i belive that its possible to remove the choke and AV all together and substitute them with a homemade choke/venturi, all in one item... Is there anybody in the group who has any hands on experiance in this field ?

Olle B
 

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Richard Jemison - Alfar7, has posted pictures of how to modify them. You should be able to find them with the search function.
 

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Richard has a lot more experience with 45's than 40's. He won an SCCA championship with an Alfa V6 with 45 or 48 Webers and he has 45's on his Giulietta race car. I'm surprised that he has spent much time working on the small Webers. But whatever he does, the principle is the same for both sizes.
 

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Richard Jemison
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Carb Mods

Pics of some internal mods are on my thread in the Motorsports area under Pics from So.West division. Using Laptop so have no pics in this computer.

Look at my post #114 at:
http://www.alfabb.com/bb/forums/motorsports/146464-old-alfa-racer-pics-so-west-div.html
Other post that are revelent:

http://www.alfabb.com/bb/forums/carburetors-fuel-injection/151959-weber-40s-45s.html

Venturi size - Page 2 - Alfa Romeo Bulletin Board & Forums.
45mm Webers can be bored to use 48mm butterflys, and 48mm Webers can be bored to 50mm which is what I used on the GTV6 GT2 car. This increases flow through the butterfly area which is restrictive if high RPM power is target.
If the Aux Venturi and chokes are shaped correctly (as in the pics in the thread above) the internals will flow as much as can pass over the butterflies...
 

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Discussion Starter · #6 ·
Thanks Richard...

...for adding the apropriate links to these threads in the Forum, there is so much information so its difficult to find what you are actually looking for... However, as you understand, I am searching for high RPM power. I run 45 DCOE9´s on my 1600 cc engine and my peak power is 8000, (engine revs 9000+) I use 40 mm chokes (venturies) and 5.0 Aux venturies. I can not ovebore my 45´s because of FIA regulations, and I dont like to be too far in the "grey zone"... I have done some work on both the chokes and aux venturi, which is close to the work I have seen on yours and others pictures, so now... where is the restriction? I belive it is both in the AV and the butterfly area and BTW, my screws for the butterflies are already countersunk.
Now... I want to get rid of the aux venturi, the compleat assembly...... have you done anything in this area ??? ...and yes, I have a few ideas...

Olle B
 

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Richard Jemison
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ux venturi

I want to get rid of the aux venturi, the compleat assembly
I don`t think you will have much to gain and doubt that fuel delivery will be satisfactory. The venturi affect is what delivers fuel. On a 1600 motor 40mm venturis reshaped and shortened (adequate airfoil shape to get adequate pressure differential to atomize the fuel delivered to the intake air) should be more than large enough for the 1600 at 8k. Any larger won`t result in adequate atomization, 45mm carbs are pretty much maxed out flow wise if modified as I have done, at that choke size. Nobody checks for carb size. Use 48 DCOE`s and 45mm covers...
 

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Discussion Starter · #8 ·
My new test will show what happens with my homemade aux venturies... It crossed my mind to use 48 with 45 lids...but there is two great BUT´s... I want to have a good nights sleep and I have refused to cheat so far, so why should I start now ? If my competitors cheat, fine... and it is actually impossible because I can not open up the inlet ports in my inlet tracts/cyl head... Making my 45 flow better is legal and within the regulations...
My hope was that somebody within the BB groupe had done some similar work...

Olle B
 

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Discussion Starter · #9 ·
For the attention of Richard Jemison...

What exactly do you mean by...

"adequate airfoil shape to get adequate pressure differential to atomize the fuel delivered to the intake air" ???

Are you talking about both Venturi choke) and aux venturi ?

Olle B
 

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Richard Jemison
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Venturi shape

I am on my laptop so pics are not available....

I am refering to the venturies, aka choke tubes.

Choke tubes are cast in a common length which is adequate for the typical use up to about 34mm. Their cast shape is absolutely horrible as a "venturi" design.

For the venturi to work as a venturi is intended the airfoil shape must be maintained, and if the section of the venturi (airfoil) is reduced (opened up) then the chord lenth must be shortened to get any real increase in speed/pressure reduction.
As well the intrance shape must be improved to reduce the "stagnation" at the leading edge which both reduces flow and reduces atomization.
 

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Discussion Starter · #11 ·
Hi Richard...

...I really apriciate that you care to answer my questions... but do you work to a formula or an equation when you redesign the Weber venturies or do you work by trial and error ? Have you proven your results with a flowbench ? Does your improved design increase or decrease the created "vacuum" for any given flowrate and/or venturi diameter?

When you shorten the venturis, do you also modify the exit of the venturi in the housing? As you know, there is a quite steep step in the machining of the hosing, stepping up in diameter from the "venturi" diameter To the "butterfly" diameter ...?

Lets say I would like to modify a set of 40 mm venturies for my 45´s, they should be shortened by some 4 mm, am I correct ?

Olle B
 

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Richard Jemison
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Short Venturis

Yes they imporvrd flow and were played with on my Superflow flow bench, but only for comparison. Did not attempt to measure vaccuum as that is really obvious...

The tube should be shorteded to just past the opening for the accel pump squirter.
You can taper the step, but that in itself wont change flow.
 

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Discussion Starter · #13 ·
????????

I thought that increased flow meant decreased vacuum!!!!!!

I will try to shorten a set of 40 mm chokes/venturies according to your recommendation, however, can you indicate were the "waistline" should be located, measuring from the leading edge...

Olle B
 
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