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Weber Carb Venturis

Discussion in '308/328' started by 308tr6, Feb 15, 2008.

  1. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    Does anyone know if the venturi diameters on late carbed cars changed any? My 79 308 GTS has 32mm, and curious if any of the early carbed cars were different (possibly larger - 34s or so). Trying to find out if Ferrari "restricted" the late model carbs in any way, and thus if any benefit to going up a size or so?
     
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  3. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
    2,149
    way north california
    Full Name:
    chris morse
    Hi Rico,

    My 77 has 32's. I think that is the standard size. Russ Turner, (SNJ) has done extensive work with the carbs and jetting and posted some very helpfull info. I believe the earlier cars had bigger jets, different cams and ignition timing. These are the major things that account for the difference in the carbed cars power. In 78 Ferrari changed from the HEAVY thermal reactor to a pair of cats and a standard muffler. Comparatively this probably made a very small change in power.

    Chrck Russ's posts, IIRC, he gained a bit on the top end power, going to the 34s, but lost a bit of lower end and driveability. And this is on a 3.2 engine that already had the jetting optimized. So, probably not a good idea.

    hth,
    chris
     
  4. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    I am just trying to tweak whatever I can here and there. The guys at Norwood gave me some timing specs for the cams, etc., and I am planning on changing the ignition system out as well. Also, balancing the flywheel and clutch assembly, etc. In essence since I didn't mod my engine rebuild with the exception of compression, I am not so much trying to "build power" as much as I am trying to remove the restrictions to it. Also, favoring the lower end response over max HP.

    Anyway, sounds like the 32's will stay, appreciate your input. Thanks!
     
  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    20,309
    Texas
    Full Name:
    Steve Magnusson
    The problem is that the larger venturi size isn't so great at low RPM because the air velocity is lower (so the pressure difference pushing the fuel into intake is less so harder to control the amount of fuel entering and poorer atomization). If you go by the Weber sizing recommendations, a 308 is already way over-carburetted (only needs 3 according to Weber) so changes there aren't going to give you a big low RPM torque boost. Understand your desire, but not easy unless you add some displacement (like 40% ;)) -- have you considered a TR? ;)
     
  6. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    A TR, hmmmm....Oh no!, not this slippery slope!

    I have considered Keihin's however! (Blasphemy I know...)
     
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  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    I think in a three liter without head work 32mm venturis will cover most street-friendly hp upgrade tweaks. As Steve said, it's all about low end velocity and vacuum signal versus top end flow. That said, if you are going with ported heads and a cam, you may well reach up into the 34mm range. The 308s (and 246s) all had 32mm venturis - as an extreme example, there are specs out there on FChat on the NART LeMans 308 jetting and I think it may have had 34s, but will have to check. For some example dynos, check out www.carobu.com and they have some interesting combos.

    As to the cars being 'over-carburreted', there's a thread all by itself if you ask Mark E.! :)

    In my opinion, most 308s, especially the later ones are jetted quite lean - a situation worsened by modern fuel formulations which are very different than in the 1970s. If you have the original jetting still in your car, it may be factory correct but it is sub-optimal for use with today's gas. Numerous threads (and opinions on this).

    You do not really see this with 1970s K-jetronic injection if it has been maintained properly, as they require routine mixture re-setting using a gas analyzer which compensates the system for the different fuel formulations.

    Good luck and have fun!
     
  9. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    So optimal jetting for late 79 carbed for street use might be....???? Please don't make me search for this info...I hate searching.
     
  10. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    It might be, and this may not work on yours, around:

    140/F24/200/55

    Good luck!~
     
  11. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    Sounds great, thank you. I will start there and migrate as it appears fit - not even sure what I have yet, dissassembly in progress...
     
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  13. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    So, I got it all taken apart and my jet sizes seem to be as follows:

    Long ones: 125
    Medium: 60F5
    Little: 55 (extremely hard to read - maybe something else)
    Is there another set I missed?

    Obviously I am not a carburetor guy. Which is which here, and for the recommendations above could you indicate which is which (main, idle, etc.)
     
  14. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    See:
    http://www.ferrarichat.com/forum/showpost.php?p=134612244&postcount=1

    This explains how to change jets and where they are. You should put all back in and leave them except for the part that has the 125 jet; the referenced post will help explain.
    Good Luck!
     
  15. 308tr6

    308tr6 Formula Junior

    Dec 23, 2003
    466
    SDakota
    Full Name:
    Rico
    Thank you very much!!!
    I tried about 4 different searches and never did see that. I suck at search...
     

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