4x48mm = 2x65mm?
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  1. #1
    Fellow Frogger!
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    Icon3 4x48mm = 2x65mm?

    Ok, so I'm looking for 220NM+ from my currently stock 194NM 206 GTi. I'm going to be getting a stage 1 cam soon, and I'm thinking of using multiple thorttle boddies as well.

    I've worked out that 2 65mm throttle boddies have the same cross sectional area as 4 48mm throttle boddies. Also, I can get as many 65mm ones as I want for free, and the throttle linkage will be easier.

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    I'll have the manifold made up, and I already have the CAD drawings for the intake ports and bolt holes. Anybody know of somewhere that can do the cutting and welding for a reasonable cost?

  2. #2
    1000+ Posts alan moore's Avatar
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    The difference being is that with 4 throttle bodies, the run to the valve will be straighter, and that a quad kit is available that will fit straight up without the hassle and extra cost of trying to reinvent the wheel.

    Unless of course you have access to a TIG, lathe and milling machine and have the time to spare to do the work, and it still may not work as well as the quad set. If having to pay along the way, I would think that the quad set would only cost a little more and be more effective, with far less pain.
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  3. #3
    1000+ Posts Fordman's Avatar
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    Firstly, by my calculation, 2 x 65 is 8% less c.s.area than 4 x 48, assuming that's the diameter of the throttle plates.

    Secondly, I wondered if this would have the same effect as a large single carby compared to a progressive twin-throat carb. The large diameter is good at higher RPM but is like a giant flat spot at low speeds. A smaller diameter gives progressive acceleration at lower speeds. I dont know if that principle applies to fuel injection, however.
    I once replaced a single barrel carb on a then newish XD Fairmont Ghia, with the XE Weber progressive carby as the owner wanted the economy. I was amazed at the torque increase during acceleration which was due to the smaller venturi working at low speeds. A win-win job.



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  4. #4
    1000+ Posts PeterT's Avatar
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    It doesn't work that way. With a plenum chamber the throttle is shared, as only one cylinder ever sucks at any one time. Do a search on the web. There's formulas for plenum chamber volume, engine volume, throttle area etc.

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  5. #5
    Fellow Frogger! racing405's Avatar
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    If you are going to run quad throttle bodies, don't be sucked into bigger is better. I can't tell you what the optimum diameter is, but big diameter drops air speed, somewhere there is a sweet spot for torque and throttle response. My experience with the Jenvey set up has been good!
    racing 405
    1:59:09 last time at Phillip Island - less than standard Mi16.

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