Loss of bottem end torque
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  1. #1
    Fellow Frogger!
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    Loss of bottem end torque

    If you have been reading my past posts, i had a few questions on installing an airfilter for a 98 306 Xsi. Well i just installed one and it sounds great but i seem to have lost some bottem end torque. I made a new induction hose to fit the filter (a simota filter) and its a 3 inch straight pipe. There is a 1/2 inch outlet pipe that connects to a hose that goes back into the engine. The inside diameter of this outlet pipe is smaller than the original one so could this be effecting the power? or did peugeot make the original induction hose an S shape for a reason? any advice is greatly appreciated

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  2. #2
    Fellow Frogger!
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    Sounds like you are talking about a oil-vapor recirculation from you're head or crank case. (the 1/2 inch pipe)
    When I put carbs on my 405 I had to completely redesign the recirculation of oil vapor (normally fed through the air box to a pathetic little solex.)
    This vapor now runs to an alloy breather bottle with a K&N filter atop. This also doubles as the oil filler.
    I have seen other applications where this "Hose" is sitting inside a plastic drink bottle, with a few perforations.
    The end point is if this is the oil vapor recirculation pipe; it doesn't need to be connected to the intake arrangement.
    Regarding the 'S' shape original induction; a combination of reasons.
    1. Limited space
    2. To feed air into the bottom of the air box before the filter.
    3. To ensure water did not enter the induction.
    There could be other reasons that only a French man could explain ?
    Also, have you considered that it is not that you have lost bottom end ? More likely that you're mid range to top end is that much more improved ! While the power at lower revs has remained the same !!

  3. #3
    Fellow Frogger!
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    Well the with the standard airbox, the engine was more than happy to rev to 6k, but now it doesn't seem to reach 6k as quickly as before, it could be just my imagination though. I'm going to fit a 2" pipe instead of the 3" to see if that changes anything.

  4. #4
    1000+ Posts PeterT's Avatar
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    I don't know. You young blokes never learn! The air intake runner length is designed to match a harmonic at peak torque. Fitting an after market filter shortens the length, resulting in a resonance at a higher RPM. The length is measured from the valve to the outlet of the filter. I have seen a 10% increase in torque just by getting the length right. The only way to have your cake and eat it too, is to use a dual length manifold like on the S16 and 2L Mi16. Your peak HP will also be down because you are now drawing in hot air from the engine bay, particularly on hot days. Oh well, at least it sounds good.

    '92 205 Mi16
    '90 Mi16x4

  5. #5
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    So is it the length or the volume of the induction hose that matters? How do i find the ideal length/volume?


  6. #6
    1000+ Posts PeterT's Avatar
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    Length tunes the torque peak. You'd have to look up a book. But from memory, 750mm = 2000RPM, 4000RPM etc. Measure the original if in doubt.

    '92 205 Mi16
    '90 Mi16x4

  7. #7
    Fellow Frogger!
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    I'm sure you're theory is correct Peter !
    But I'm a little puzzled ! If Peugeot in there wisdom designed these intake runners as you say to match a harmonic at peak torque. Surely there measurements would be correct ?

    An excerpt from you're quote:

    "I have seen a 10% increase in torque just by getting the length right."

    In which instance have you seen this increase ? By improving a factory set up ?

    C'mon Peter you are just a boy racer in disguise !

  8. #8
    Fellow Frogger!
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    I've put the original air filter back in and it seems to pull better at low rpm! pity the induction note is gone, but also if you've read my other 'pinging' post, the pinging has seemed to go away also. Looks like i'm sticking to the oem box for the moment, im glad i've only spent $100 on the filter and parts.

  9. #9
    1000+ Posts PeterT's Avatar
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    The 10% appeared on our dyno whilst tuning a custom made 34cc twin cam injected single cylinder engine. It's hard to believe that something so simple can make so much difference. I can assure most manufacturers now get the lengths right these days. Even Ford has dual length runners on 6 cyl. engines.

    '92 205 Mi16
    '90 Mi16x4

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