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Restricting pods to help tuning...

Discussion in 'XJ Technical Chat' started by rnice, Feb 20, 2009.

  1. rnice

    rnice Member

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    Hey. Sorry for the multiple postings. This is stuff I've been thinking of all winter and now its crunch time.

    My PO put on air pods. I think the PPO put on new pipes, presumably less restricted.

    Now I've read all of the good work on this forum on tuning and carbs and the problems with deviating from OEM design.

    My question is if you're stuck with what you got and if your not looking for max hp out of your bike (who is with a 550?) - has anyone experimented with restricting air flow on pods to closer simulate the originally designed air box flow - with the objective of getting back more reliable carburation?

    I'm pondering taping sections of each pod and watching effects on performance.
     
  2. 82XJ

    82XJ Member

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  3. sushi_biker

    sushi_biker Member

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    I just read that thread. It's friggin' brilliant. With all the talk of pods and rejetting lately, I don't know why this wasn't brought up before. I'm dang sure going to try this when the weather warms up. It's nothing but a few dollars in parts.
     
  4. rnice

    rnice Member

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    exactly -
    a cheap fix to avoid the POD nightmare,
    and done right looks ok,
     
  5. fuferman

    fuferman Member

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    i've been thinking about doing something like this for a while and i think i have a way you could test the air flow. if you block off 3 of the carb ports on the manifold and hook a vacuum gauge and shop vac to the 4th. do this on the factory air box and the new one and compare. the shop vac would give you an even vacuum unlike the pulsing of the running engine. if you didn't have a vacuum gauge you could even use carb sinc bottles block one side and mark how far the water goes up from even on the other side. get them back to even and test the other air box.

    i'll probably give it a try next winter. i don't want to have my bike unridable for even one day right now.
     
  6. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Close, but no cigar. The PVC pipe manifold, on the surface, appears to be a solution but holds a couple of pitfalls:

    With the intake off to one side (or even in the center) the flow will NOT be the same in terms of VELOCITY for each carb because the VOLUME is strictly limited by the diameter of the pipe. The same amount of air traveling a longer distance in the same amount of time will HAVE to travel faster. In the stock airbox, all four carbs are pulling from one big "bucket" not being individually fed.
    This is why high-performance (carbureted) automotive intake manifolds have very complex shapes to their ports and passages, to equalize and smooth the airflow to feed all the cylinders evenly. The PVC pipes ain't gonna do that. You MAY be able to "tune" some of the imbalance out with carb adjustments but I'm afraid the resultant imbalance in cylinder tuning would rob power.
    Remember the "CV" stands for "constant velocity." While the PVC manifold will physically work, it will NOT provide the SAME velocity of intake air to each carb. I suspect a tuning nightmare will ensue.
     
  7. PainterD

    PainterD Active Member

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    I'd have to agree with Bigfitz about the idea. At first I sreiously considered the idea for mine, but when you sit back and think about it, he's right. You will never get equal intake for all four carbs with that PVC setup. The stock airbox is engineered that way for a reason.
    I figure being mine runs like a cat on fire now, why mess with it!
     
  8. MaximumX

    MaximumX Member

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    I've kicked around the idea of a more restrictive pod setup and I always thought that twin pods would be the way to go to provide more even volumes of air to each carb. Something like this:

    [​IMG]
     
  9. rnice

    rnice Member

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    I'm an electrical guy by trade, so I was thinking in terms of a parallel circuit, all current is the same when ignoring line loss.
    So in that regard I can see that the resistance to flow will look different for each carb, but I've seen higher end V twins with very asymetrical air routes.

    The picture drawn is a good idea. But it would work with PVC unless there is a 'Y' shaped piece that I'm not away of.
    You could do a 90, 4 way, 4 way, 90, with two pods in the middle. That would cut the flow in half and reduce the difference in air paths.

    My understanding from power plant piping is that the amount of turns is much more important the straight lengths when calculating resistance to flow. Maybe that doesn't apply with high-sensitivity applications like this.

    Might still be worth a try since investment is so low...
     
  10. sushi_biker

    sushi_biker Member

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    YOU shouldn't mess with it. This discussion is mainly pointed at those of us who have inherited the pod nightmare and don't want to remove our engines to install an airbox.

    Fitz is probably right about the whole velocity/shaping flow issue. I like the dual pod idea, also a PVC fittinig for 4 SMALLER pods could be fabricated to restrict air flow, yet keep it even for all 4 carbs.
     
  11. MaximumX

    MaximumX Member

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    Admittedly, my little design may require a bit more than just glueing some elbows together but it shouldn't be too hard to assemble with a little creativity, a hole saw and some PVC cement.
     
  12. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    No offense MaximumX, but since the firing order is 1-3-4-2, then 1 would be pulling immediately after 2, then a break while 3-4 do their thing. 1 would always be pulling in the residual vacuum of 2, and 4 would also be pulling in the vac of 3. This would tend to make 1 and 4 run rich, or 2 and 3 run leaner. They would not be symmetrical IMHO.

    So my suggestion would be 4 longer runners meeting in a square with one pod, if I were to do this.
     
  13. HalfCentury

    HalfCentury Member

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    IF, and only if, you REALLY wanted a stock airbox, it would seem not too difficult to cut one up and glue on some flanges so that you could assemble the pieces inside the airbox space in the frame. After, you could seal it up good with whatever goop floats your boat.
     
  14. MaximumX

    MaximumX Member

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    Good point, TtR.

    That's beginning to sound an awful lot like the stock airbox, isn't it?
     
  15. PainterD

    PainterD Active Member

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    Just a thought...
    Now how would it work with the 4-into-1 pvc idea with the airbox under the carbs? There seems to be enough room for one there. Probably use a U-shaped intake to the carbs from the airbox to keep equal pressure to each carb. I would think something along those lines should work OK.
    Just thinking outloud.
     
  16. Polock

    Polock Well-Known Member

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    just get out the electrical tape and have at it
    i think i saw a foam sock that goes over them once
     
  17. sushi_biker

    sushi_biker Member

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    I'm strongly considering it. Do you think it could be done by cutting it into no more than 2 pieces?
     
  18. Ace_Frehley

    Ace_Frehley Member

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    I've never had the air box out of my bike, but the PO had at one time, its only cut in 2 pieces. Basically straight across between the filter box, and the longer section of the box that the intakes fit into.

    I've never tried to get it out, but it looks like with the carbs off, I could remove the front of the box, then pull the filter section straight towards the motor and out the side.

    Hope that's clear enough to get what I'm trying to say.
     
  19. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    The problem is "sealing it up with whatever goop" afterwards. The airbox on my '81 is splitting apart at the seams, and no matter what "goop" I use it either "bounces off" immediately or fails after not long. If I could figure out an adhesive that actually BITES INTO whatever the heck bizarre plastic they made the airbox out of, it might work.

    The best way would be to cut it and then glue (the challenge) flanges onto the inside so it would mate back up correctly. In MY case, if I could figure out an effective glue, I wouldnn't need to replace it.
     
  20. flash1259

    flash1259 Member

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    lol wouldn't restricting the pods remove the effects you were seeking when you got them? why not just re jet the carbs?
     

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