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  1. #61
    Legendary Lancista 1,6 HF's Avatar
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    Re: Carb Upgrade - S3 Fulvia

    There’s an old joke that rules of thumb are useful if you’re measuring thumbs. And in that context I think Marnix's comments are very well taken, particularly his reminder that the carb barrel and venturi sizes are expressed as diameter, while their real flow effect is in circular area. In other words, not d, but pi r². So while the real delta may be 3mm, the smaller the diameter, the more % difference 3mm makes. Which means with a relatively small 29mm venturi, you might get by with a 32mm barrel – even if the ‘ideal’ rule is 1.25.
    Ed Levin
    Fulvia 1,6 HF

  2. #62
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    Re: Carb Upgrade - S3 Fulvia

    I had to read that a couple of times, think about it, have a cup of tea, read it again and think about it some more, but finally the penny dropped and I've got my head around what you and Marnix mean now.

    I had to Google "how to calculate the area of a circle"; here it is for anyone who is interested:

    To find the area of a circle with the radius, square the radius, or multiply it by itself. Then, multiply the squared radius by pi, or 3.14, to get the area. To find the area with the diameter, simply divide the diameter by 2, plug it into the radius formula, and solve as before.

    So just as an example, if we were talking in terms of inside diameters we'd say:

    The 29mm venturi is 3mm "bigger" than the 26mm venturi.

    If expressed as a percentage of the diameter of the throttle exit, the 29mm venturi is 81% the size of the throttle opening and the 29mm venturi is 90% it's size when calculating based on internal diameters.

    So, it would be said that the 29mm venturi is 9% 'bigger' than the 26mm venturi in terms of diameters.

    But, what you and Marnix are saying is that we should not look at it just in terms of diameters but also in terms of the area of the 'holes'. If we do that:

    The area of the 32 mm throttle exit is 804mm (sounds like too much, but imagine 804 little squares measuring 1mmx 1mm or about 8 squares each measuring 10mmx10mm; it helped me see it in my head)

    A 26mm venturi 'hole' has an area of 531mm which is 66% the area of the throttle exit

    A 29mm venturi 'hole' has an area of 660mm which is 82% the area of the throttle exit

    Therefore the 29mm venturi is 16% 'bigger' than the 26mm venturi in area when calculated as a percentage of the throttle exit area although it's only 9% 'bigger' when calculated in terms of inside diameters.

    Yes, you're right; I hadn't thought of it that way before and it's definitely worth working into the equation.

    Thanks Marnix and Ed for pointing that out; I wouldn't have thought of it.

    That was quite a mental workout...I think I need another cup of tea!
    Last edited by halejustin; 12th June 2019 at 10:14 PM.

  3. #63
    Senior Lancista Taddraughn's Avatar
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    Re: Carb Upgrade - S3 Fulvia

    This conversation is starting to give me flashbacks to fluid mechanics class sophomore year lol... You only need to know a couple more variables to start using Bernoulli's equation.

  4. #64
    Legendary Lancista bmarler's Avatar
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    Re: Carb Upgrade - S3 Fulvia

    Quote Originally Posted by Marnix View Post
    bmarler has been doing the same as I have been doing. He contacted me and we discussed the modifications outside the forum, as it was a bit too specialized. He is a nice guy, so you can contact him for sure about his experience.
    i'm always happy to provide whatever i can to the conversation, but i would defer to marnix vast amount of information regarding these carbs. without the help and lengthy discussion i don't know if i would have been so eager to tear into the carbs as i did.
    that said, it's very rewarding to go down that path, and gaining knowledge is never a bad thing.
    bmarler/
    1967 fulvia sport zagato
    1961 appia vignale convert.

  5. #65
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    Re: Carb Upgrade - S3 Fulvia

    "But, what you and Marnix are saying is that we should not look at it just in terms of diameters but also in terms of surface area of the 'holes'. If we do that..."


    Yes, though not "surface area" but "flow area".

    Surface area also figures into this, because the sides of the venturi – like any pipe – cause flow friction. And this is a bigger issue with smaller ‘pipes’ that larger ones, given that the flow area goes up on the square of the radius (pi r²), while surface friction is based on circumference, which goes up on double the radius (pi 2r, or pi d). So the smaller the pipe the greater the surface friction relative to the flow area. In this context, that’s not a major factor, but we should be clear about what we’re physically describing.

    And you thought your middle-school geometry classes would never have any practical value...

    Ed Levin
    Fulvia 1,6 HF

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    Re: Carb Upgrade - S3 Fulvia

    Thanks for that Ed. Yes, I remember sitting in class thinking "I'll never use any of this out in the real world!". How wrong I was. I wish I hadn't daydreamt so much now...

    After reading your reply I realized that the word 'surface' in 'surface area' in the way I was using it was confusing so I've edited my post and changed 'surface area' to just 'area' (of the apertures in question).

    I've thought about this overnight and to help me get it straight in my head I've made up a chart.

    It compares the different size venturis and the percentage of expansion they create expressed as a percentage of the throttle aperture area size in both the DHLB 32 and DHLB 35.

    What it shows is that the 27mm venturi when used in the DHLB32 most closely replicates the 'venturi-to-throttle expansion' of the 29mm venturi in the DHLB 35, which is the Dellorto factory setting for my car (818.302 engine). So Marnix was right when he suggested going up a size or two from the 26mm venturi, which was the initial natural choice.

    Hard to grasp when put into words, so here's the chart (click on it to view larger version).

    All the calculations were done using the aperture areas, not their diameters.

    Any observations welcome.

    Click image for larger version. 

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    Attached Thumbnails Attached Thumbnails Screen Shot 2019-06-13 at 10.34.27 am.png  

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