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308 intake runners and port matching

Discussion in 'Technical Q&A' started by ants2au, Dec 9, 2003.

  1. ants2au

    ants2au Formula Junior

    Nov 19, 2003
    690
    Sydney Australia
    Full Name:
    Anthony
    Didn't want to hijack another thread, so here goes a new one.

    I am actually doing my carbs at the moment on my 308, and I have found the following:

    The intake runners were too smooth for proper fuel mixing (maybe that's original, or maybe someone got to it before me).

    The port matching from carb to manifold, and manifold to head, were off only on one side of the port diameter (how odd, they match on one half of the circle, and off on the other).

    So my plan is to match the port on the carb/manifold side, but taking some metal of the manifold opening.
    As to the manifold/head side, I am abit conserned taking metal of the head side, as I want the port flow to go from large to small as it gets to the valve opening.

    Does anyone have any opinion on this?
    I want to keep the flow diameter the same (ie large to small to keep the air velocity increasing), but want to get rid of the ridge in the flow path that is the manifold/head interface.

    Also I will be finishing off the port walls with 40 grit, as the intake runners are so short, it needs all the mixing it can get
     
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  3. mk e

    mk e F1 World Champ

    Oct 31, 2003
    11,965
    The twilight zone
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    The Butcher
    Were the manifold meets the head, if they match on one side of the hole but not the other, the manifold is bigger then the head right? So opening up the head to match makes them the same at that point, you will still have whatever taper ferrari designed in. The other option is JB Weld or similar in the manifold, just get it good and clean first. I guess you could slot the mounting holes a little and slipt the difference too.

    As for the surface finish promoting fuel mixting, well I've never seen that work. Normally a rough surface finish does 2 things. Roughening the inside wall of a bend can help the flow make the turn. Or if the head is already over ported, roughening the surface will reduce the effective flow area a little andimpover low end torque. Unlesss you have the head on a flow bench I don't tink I would get in there with a sander and mess with the finish, JMO.
     
  4. BigTex

    BigTex Seven Time F1 World Champ
    Rossa Subscribed Owner

    Dec 6, 2002
    74,351
    Houston, Texas
    Full Name:
    Bubba
    Recent Forza article on 308 heads noted that the match on the exhaust side was even worse!
     
  5. Dave

    Dave F1 Rookie

    Apr 15, 2001
    2,722
    Little Rock
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    David Jones
    I would also get new gaskets to factor into the mix before you start grinding...
     
  6. Matt Morgan, "Kermit"

    Matt Morgan, "Kermit" Formula Junior

    Nov 12, 2003
    405
    Ferndale, WA
    I would not recommend the coarse finish you are suggesting either.
    On a longer intake tract, it used to be believed that a coarser finish would minimize "wicking" of fuel to the port walls, thus causeing fuel mixture problems, and unburned fuel. My experiences have shown that a smoother finish on shorter runner motors (especially Carbed Ferrari) that there is not enough exposure to the walls to cause this to occur.
    What I do find however, is a rougher finish gives carbon an easier surface to adhere to.
    On the actual port matching, definately remove any areas that the flow would "run into", as in the head opening protrudeing into the air flow. I have found that the flow hitting the surface causes a turbulent tumbling action that will still be occuring as the mixture enters the cylinder, thus causeing erratic mixture flow patterns.
    Minimal "drop off" areaas are not nearly as disruptive to the flow, as they tend to simply become a dead air area, or more neutral, thus less effect.
     
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  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
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    Russ Turner
    Would second Dave's comment about gaskets. When we matched the new carb manifolds to the qv heads the stock factory gaskets had noticible slop into the air passage. Easy enough to fix, but have to know to look for it.

    best
    rt
     
  9. carguy

    carguy F1 Rookie

    Oct 30, 2002
    3,335
    Alabama (was Mich.)
    Full Name:
    Jeff
    When doing the major service on my TR, inspection of the head intake ports showed them to be very smooth, practically polished. The match between the intake tube and head was very good with hardly any discrepancies. The intake tubes were also very smooth inside, but as shiny smooth as the heads. It was very nice work by the factory.
     
  10. ants2au

    ants2au Formula Junior

    Nov 19, 2003
    690
    Sydney Australia
    Full Name:
    Anthony

    Thanks guys for the response.

    Yep was going to factor in the gaskets for sure.

    As fo the TR, that's fuel injected, that's a different ball game, in that the runners have to be supper smooth. No fuel running in them, just air (well 90% of the length.).

    As to roughing the runners, since it is very short, thinking about it now, it makes sense what you guys have said. Not enough time for the fuel to stop on the sides. But roughing the runners is applied to bikes, and they have very short runs, that's why I thought of it.
     

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