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  1. #61
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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

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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.

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    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.

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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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  7. #67
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    Re: Carb Upgrade - S3 Fulvia

    OK, so it sounds like most are in agreement that from a venturi and throttle barrel size point of view, a 32 DHLB can theoretically work sufficiently in a 1300 engine, but will probably lose some top end performance yet gain some low end performance.

    I'm reading Marnix's piece on carburettors in the download section which points out that the 29mm venturi in the 302 engine and and 31 venturi in the 303 engine are actually oversize for a 1.3 litre engine. Apparently this was done to achieve certain performance results while keeping the engine a small CC capacity because of Italian motor taxation reasons (correct me if any of that is wrong).

    Here's a chart showing the correlation between engine sizes, peak RPM and the ideal venturi size (which in turn leads to the ideal choice of throttle barrel size using the 1:1.25 formula)

    (Click for larger image)


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    What it shows us is that for a 1.3 litre engine that hits peak RPM at 6000 RPM (which the 302 engine does), the ideal venturi size is about 27.75, so just shy of 28mm.

    That means that the 29mm venturi of the 302 engine is one size bigger than the ideal, and the 31mm venturi of the 303 engine is 3 sizes bigger than the ideal.

    The Lancia factory torque-curve chart below, from taken from Marnix's 'Part II, twin horizontal carburetor operation' in the downloads section, shows the effect different venturi sizes have on similar 1.3 litre engines.

    Although there are slight differences between the 302 and 303 engines, the 302 (29mm venturi) has more power at low revs, the 303 (31mm venturi) has more power at high revs. Marnix attributes this largely to the difference in venturi size.


    (Click for larger image)

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    So I guess this indicates to us that if we further reduce the size of the venturi from 29mm to 28mm for the 302 engine, we can expect this same trend to continue.

    The difference between the 29mm and 28mm venturi (1mm difference) is 1/3 the difference between the 29mm and the 31mm venturi (3mm difference). Using that as a rough scale, I've plotted a rough estimate of where a 28mm venturi may fall on the chart (just a rough guess). All I've done is tilt the 302 curve 1/3 of the difference between the 302 and 303 engine following the direction of the trend (the light blue line)

    (Click for larger image)

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    So I guess it's fair to say that all this strengthens the case that we could go down a venturi size or two (to 28mm or 27mm) in the DHLB 32's to get the required 'venturi-to-throttle barrel expansion' to facilitate enough fuel vapourization to make them perform satisfactorily with the 1300 engine (theoretically...).

    With my car being the S2 Berlina which is more sedate in nature than the coupe, and the fact I do most of my driving through small mountain towns and on winding, steep mountain roads, losing some top end but gaining some low down performance may not be a totally unwelcome thing.

  8. #68
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    Re: Carb Upgrade - S3 Fulvia

    The venturi size v max revs chart is in the Downloads under "Weber DCOE set-up" or something similar. Particularly if you're speaking of a berlina (which I didn't understand until now), you should be fine with either 28mm or 27mm
    Ed Levin
    Fulvia 1,6 HF

  9. #69
    Legendary Lancista Marnix's Avatar
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    Re: Carb Upgrade - S3 Fulvia

    While I didn't actually try this out in practice, I can agree with practically all of the conclusions.

    Keep in mind that the part I wrote is a simplification, I wanted to avoid it being too complex for the reader that has little knowledge on carburetors. When comparing the 302 and 303 engine, there are other factors to consider, like the difference in CR (9:1 for the 302 and 9.5:1 for the 303), difference in valve size (giving a different inlet/outlet gas flow efficiency), etc.

    But I do expect that torque curve will be flattened a little bit in the way you show (still need to be confirmed in real if all this works out as predicted). Particulary for a Berlina and that is not driven much on highways at high RPM, sounds like it may even be beneficial.

    Ed has been doing something similar on his C.45 racing carburetors if I am correct: the race version had 38mm venturies, making the engine quite hard to manage for normal street use. He changed them to 36mm, and got better low RPM performance at the cost of some power loss at high RPM. It is a nice showcase of what the effect of the venturi size is.

    I also think that with the power curve will come down slightly with the 32mm DHLBs instead of 35mm, but how and how much is anybodies guess. I guess it will be more or less compensated by a slightly higher torque at low RPM, but the car may feel "out-of-breath" when driving at high speed with higher RPM.

    Carbs are tricky in the sense that many variables are involved.
    Lancia Fulvia 2C 1964
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  10. #70
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    Re: Carb Upgrade - S3 Fulvia

    Quote Originally Posted by Marnix View Post
    Ed has been doing something similar on his C.45 racing carburetors if I am correct: the race version had 38mm venturies, making the engine quite hard to manage for normal street use. He changed them to 36mm, and got better low RPM performance at the cost of some power loss at high RPM. It is a nice showcase of what the effect of the venturi size is.
    That's right. With the engine rebuilt to street compression (10.5:1), the 38mm venturis weren't ideal, so I'm running the standard street 36mm. But I'm still experimenting, so I may try 37s...
    Ed Levin
    Fulvia 1,6 HF

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