manifold dimensions for twin weber 40 dcoe's
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  1. #1
    Fellow Frogger!
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    Default manifold dimensions for twin weber 40 dcoe's

    I am in the process of building a 130 hp version of an XN engine for a 504 rally car. This will be the first iteration of what I hope will become a 150-170 hp version sometime next year.

    I have located an XN6 head, which will receive the standard flow improvements and will be fitted with twin Weber 40 DCOE's. I am trying to gather as much information as possible about the intake manifold. What is the best length (including plates and tubing) from the head to the mounting point of the Webers?

    I am after enough low end torque, but I also need power at high revs. The following are part of the set-up in case this information is needed to determine the optimum length:

    Stock valve size
    Around 9.5 to 1 CR, hopefully even 10.5 to 1 depending on the pistons I can source
    Wade 240 cam
    4-2-1 extractors
    Free-flow two inch exhaust
    4.11 LSD
    Stock BA7/4 (to be replaced by BA7/5 next year)

    All suggestions will be greatly appreciated.

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    Thanos

  2. #2
    Budding Architect ???? pugrambo's Avatar
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    Quote Originally Posted by Thanos
    I am in the process of building a 130 hp version of an XN engine for a 504 rally car. This will be the first iteration of what I hope will become a 150-170 hp version sometime next year.

    I have located an XN6 head, which will receive the standard flow improvements and will be fitted with twin Weber 40 DCOE's. I am trying to gather as much information as possible about the intake manifold. What is the best length (including plates and tubing) from the head to the mounting point of the Webers?

    I am after enough low end torque, but I also need power at high revs. The following are part of the set-up in case this information is needed to determine the optimum length:

    Stock valve size
    Around 9.5 to 1 CR, hopefully even 10.5 to 1 depending on the pistons I can source
    Wade 240 cam
    4-2-1 extractors
    Free-flow two inch exhaust
    4.11 LSD
    Stock BA7/4 (to be replaced by BA7/5 next year)

    All suggestions will be greatly appreciated.

    Thanos
    7" from the butterfly of the carb to the inlet valve is about the best all round dimension so you need to make the manifold to fit into that
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  3. #3
    Fellow Frogger!
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    Quote Originally Posted by pugrambo
    7" from the butterfly of the carb to the inlet valve is about the best all round dimension so you need to make the manifold to fit into that
    Thank you, Rambo

    To be sure I understand (engine newbie, that I am):

    Which inlet valve you are refering to?

    What yould happen if it was longer, say 10".

    Thanos
    Last edited by Thanos; 24th September 2005 at 09:02 PM.

  4. #4
    Fellow Frogger! PCOATES505's Avatar
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    Hey Thanos,
    Whilst there may be optimum lengths for the manifold runners, you will find you are governed by 2 factors;
    1. Length required to clear the distributor (from face of head along top of runner to clear dissy is 95mm)
    2. Length required to allow fitment of air cleaner witout cutting holes in mudguard.
    A short manifold is more suited to high RPM whereas a longer one will give better low end torque.
    I am sure you will find on the 504 that a manifold that clears the dissy and allows fitment of air cleaners will give you very little scope for playing with intake runner length, but will give performance suited to your needs.
    Regards Pete
    Last edited by PCOATES505; 25th September 2005 at 12:26 PM.

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  5. #5
    Fellow Frogger!
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    Quote Originally Posted by PCOATES505
    Hey Thanos,
    Whilst there may be optimum lengths for the manifold runners, you will find you are governed by 2 factors;
    1. Length required to clear the distributor (from face of head along top of runner to clear dissy is 95mm)
    2. Length required to allow fitment of air cleaner witout cutting holes in mudguard.
    A short manifold is more suited to high RPM whereas a longer one will give better low end torque.
    I am sure you will find on the 504 that a manifold that clears the dissy and allows fitment of air cleaners will give you very little scope for playing with intake runner length, but will give performance suited to your needs.
    Regards Pete
    Thank you Pete for yet another tutorial. According to my measuring tape I need a minimum runner length of approximately four inches to clear the distributor (95 mm as you said), so the minimum is fixed. I was looking at different air filter elements that fit the webers and it appears that I have a couple of inches of flexibility regarding the maximum length. Torque pig that I am I was thinking of how I can use that to maximum advantage. I have seen photos of 504 engine bays with twin webers and the lengths are all over the place. The one with the shortest length also has Devil 4-1 extractors (which seem to help with high rpm output only) and a close ratio box. Based on what you say and the fact that I have super long 4-2-1 extractors (increasing output throughout the rpm range) and a standard BA7/4, I will split the difference and go with 140 mm intake runner length. Feel free to throw a veto if you see a problem with that.

    Moving right along, slowly. Front end almost done, decided not to wait for the 28 mm sway bar. The P-S ram would not fit between the P-S rack and the early 504 cross member. I saw that even if I drill a hole to anchor the ram, I would have to file down the ridges above the holes the P-S rack bolts on to get the ram to fit. Then I had this moment of clarity and figured why screw with it, wait for a 505 cross member and aluminum motor mounts, this way I gave the entire front end. So I be will using the old cross member and the P-S rack without the pump for the interim and install the 28 mm sway bar and power steering at a later time. I will measure F/R ride heights when finished, hopefully on Tuesday, and let you know.

    Regards

    Thanos

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