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I think Richard will agree that the days of the long (6 inch max or less) stacks, was also the days of the large ports. Much has been learned with flow benches on shaping intake tracts. I have worked with old engines with large ports and Weber modifications and to get 1600's to pull below 3500 rpm with a 37 mm or larger intact tracts is no fun, IF you want top end power. The dyno sheets on these heads tell most of the story. Gordon Raymond
 

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Here below are CFM`s for Weber 45 DCOE`s
with various chokes:
30 - 142 cfm
32 - 164 cfm
34 - 186 cfm
36 - 207 cfm
38 - 218 cfm
40 - 222 cfm
At what pressure drop? I've never seen Weber flow data before. You are a real gold mine. Any flow data for the 40 DCOE's?

Thanks,
Mike R

PS: The Weber Tuning Manual is published by Weber. Most Weber parts sources either have it or can get it.
 

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Discussion Starter · #23 ·
Let me stir the pot some more : #1- Why do we want or need venturis that flow more cfm than the head does ? #2- If a large venturi produces a weaker signal at the AV , why can't a richer main jet make up for this at low speeds ?
 

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Richard Jemison
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Simple Qustion.....

In fact it is so simple Jim or Rossogtv4me could answer it!!:rolleyes::p

A richer mainjet would cause too rich a mixture for all of the REV range. There are 5 jets in the carbs. And you need a balance of all to have a drivable car.

The other thing I forgot to bring up is the accelerator curcuit.
Plus the other 2 factors that affect the accelerator output other than it`s jet size.

We didn`t even get into the 3 items that control the amount of fuel that is added as the throttle plates open.
 

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Richard:
Do you have a table of cfm vs venturi size for 40DCOE's?
 

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28"?

Does ice count? I,ve about 28 " of water surrounding my house now from a recent flood. 14 degrees and its frozen now. :DGordon Raymond
 

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Richard Jemison
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Stuff.

It is common to do flow testing at 28" of water. Just wondering if that is what was actually used.
Mike, yes. Same as 1 PSI.....

Gordon:

I`ve already got a sunburn! But I forgot to call your wife re the $800,000.00 Ferrari to tell her that It looks good, but the sludged up motor reduces the value substantially, and with the same problem in the Alfa she should just insist on both being sold!:p
 

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I would like to see one of the modified venturi's in picture form as a comparison of before and after.

I have some 45DCOE's with 32mm venturi's that are going into the 2 liter engine I am building. I was thinking of stepping up to 34mm venturi's for more air flow and a better overall driveability. I would have never thought about modifying the venturi for a better flow.

This thread is interesting reading.
 

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Vent modifications.

OK Chris, (velocedoc), here's some pictures. These are from a pair of 40DCOE-2's modified by Ron Neal at Ausca some 40 years ago. The dyno sheet with this engine a 1600 single plug with 10 mm cams and 10.2 compression shows 146 corrected hp on a "Go Power" dyno. You can see that these show what Richard Jemison has discussed. The main choke has a long tapered airfoil shape. The other vent shows the thin taper without the pressed in insert. The big one with air bubbles:)rolleyes:) in the casting is a 34.1, the other a 4.5. Hope these pictures help. :DGordon Raymond
 

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More vent pictures

Here's the rest. :DGordon Raymond
 

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Raymond, being that I am a visual person, these pictures explain alot to me. I can see what you are talking about, the long radius, when you compare this to a stock venturi with a short radius. How does someone modify the venturi's for the long radius? Is the a profile that you use? What improvements do you notice when you are driving with these venturis?
 

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As Richard explained, the stock venturi's are a compromise for manufacturing ease and functional drivability. His position is based in racing, and from a developmental perspective. In actual function, sometimes visible on a dyno, but more often in the winners circle, the elimination of manufacturing compromises, blueprinting, or just smoothing the induction tract by porting, valve seat angles, and bowl dimensions, particularly if done with a flow bench, along with good cam design, different intake and exhaust cams, will yield subtle, but noticeable changes in performance.
Any airfoil shape is a compromise relative to an expected, constant airflow. A good example might be the comparison of the airfoil shape of a C47 (DC -2/3) wing to an F16. the former, thicker and rounder, the latter sharper and flatter.
The compromise with Weber vents, is that they are expected to operate at low and high air speeds. Efficiency is lost in the extremes. The F16 airfoil shape does not work well on take off. Lots of thrust is required. The C47, likewise, is very inefficient at high speeds due to drag.
Back to Alfa's. The long vent shape becomes more functional as air speed increases, (racing or performance) the rounder factory compromise, lower air speeds, (street drivability). A good compromise, is a smaller vent size, with better high air speed profile. Thus the development of the flatter profile, as well as odd, non factory sizes. The whole process is a compromise, based on expected gains in particular areas.
As I believe was mentioned earlier, these are modified most easily by measured lathe cuts progressing either down (or up) the vent. I cut both ways. This yields a series of "steps" not varying much in height, that are smoothed with a flap wheel on a die grinder, just as in porting. (A messy job, learned in my more foolish youth!)
For the perfectionist, results can be checked on a flow bench, but from experience, unnecessary as once the technique is mastered, any small differences between one vent and another is unimportant. (Read, close enough;).) I hope this helps Chris. :DGordon Raymond
 

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Yes, again. Thank You Excellent Post!
quick side note. When porting aluminum bits of anything with flapper wheels and such Its a good idea to use a breathing mask I have been told for many years now
 

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Discussion Starter · #37 ·
Mixture

Answeres still evade me ! If initial low end mixture needs to be richer without hurting mid & upper rpm , can't the emulsion tube be changed to one that leans out mid & upper ranges ? How about a larger air corrector , a ball bearing limiting the accelarator pump stroke , or more weight on the pump check valve limiting the high speed acc. pump bleed ? Maybe the mixture is just not atomizing properly with a large venturi at lower rpm ? Also , the need for venturis that flow more than the cylinder head still doesn't make sense to me . I need a chassis dyno in my garage . My butt is getting tired of trying to feel the differance between 125 & 135 mains ! An o2 sensor tells me too rich , but my butt says otherwise !
 

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Answeres still evade me ! If initial low end mixture needs to be richer without hurting mid & upper rpm , can't the emulsion tube be changed to one that leans out mid & upper ranges ?
A couple of things to keep in mind about the main circuit if you want to lean out the top end:

Main jets affect the entire main circuit rpm range. Larger jets = more fuel throughout the range.

Air correctors have much more effect at the top end of the range. Obviously, the larger the correctors, the weaker the mixture.

For emulsion tubes, top of tube = bottom of range--holes at the top of the emulsion tubes affect lower rpm. More holes at the top weakens mixture at lower rpm, fewer holes at the top richens the mixture.

Therefore, as a gross generalization, the emulsion tubes tend to have more control over richening or leaning out the low end, while the air correctors tend to have more effect at richening or leaning out the top end.

So, when your O2 sensor says you're too rich, the question is, at what point in the rpm range is it telling you that? If you're too rich all the way up and down, that tells you one thing. If you're too rich only at the top, that tells you another. And if you're too rich only at the low end, that tells you a third thing about what jet, tube, and corrector changes make sense.
 
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