Resonance in 405 SRi Auto
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Thread: Resonance in 405 SRi Auto

  1. #1
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    Default Resonance in 405 SRi Auto

    As mentioned in my 405 Mi16 for sale ad I have been thinking that an Sri auto fits the purpose of an every day road car better than the Mi16.
    I got a three day Vic Roads permit for the SRi I have had sitting around for a while (the original one was suffering from 2 years of intensive hard work rally surveying, so went to the crusher) to give it a try.
    Everything is spot on except for a body resonance at around 2500 rpm.
    I knew this was there and always thought that it was the exhaust but no difference after completely changing it.
    I've checked mounts etc with no luck. The engine doesn't seem to have any abnormal vibration and its worst vibration is at a different rpm to the body resonance. Resonance is the same whether under load on the over run or out of gear.
    Also, there is a lot more road rumble than the Mi16.
    I can put up with it I think, not as bad as a VN Commodore, but it has me perplexed.

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  2. #2
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    It has to come from an engine/transmission mount or something rigid fixed to it, like exhaust, or a suspension mount. The last would usually depend on road speed.

    The alternative approach is to alter the resonant frequency of the body by glueing some mass (eg lead sheet, but perhaps even malthoid) onto it.

  3. #3
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    Or maybe sit a few people in the car?
    I have removed and checked the gearbox mount, not perfect but certainly not failed. This is a more elaborate thing than the 205, early 405 mount.
    With the gearbox resting on a floor jack the resonance is reduced but still there.

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    Fellow Frogger! racing405's Avatar
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    Any chance the front balancer on the auto engine is supposed to be different to a manual and you have the wrong one fitted? With a drive plate rather than a flywheel, I could imagine that possibility.
    racing 405
    1:59:09 last time at Phillip Island - less than standard Mi16.

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    1000+ Posts schlitzaugen's Avatar
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    I think the most accurate way to pinpoint the source of the noise is to use a stethoscope (or juts a plain hose) and feel your way around. It may be some heatshield that's come loose, or some sheet metal that lost its rigidity (weld spot broken) and so on. Way too many variables to go at it with a first principles approach.
    Last edited by schlitzaugen; 2nd February 2017 at 04:53 PM.
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    Thanks for the suggestions, may not be necessary as a low km car just came up on Ebay!

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    1000+ Posts cam85's Avatar
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    Have you tried different tyres? Almost always the culprit!
    94 205 Gti Classic #9
    91 205 Si
    87 205 GTi Race Car
    http://www.aussiefrogs.com/forum/res...-race-car.html

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    Won't be that, still does it with the car stopped and in neutral.
    There is a lot of clunking from the front end, bearings and strut tops needed, should be an easy fix.
    By the way S2 405 strut tops also work a treat in a 205, stop the incessant clunking they all seem to have.
    Last edited by GRAHAM WALLIS; 2nd February 2017 at 06:13 PM.

  9. #9
    1000+ Posts schlitzaugen's Avatar
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    Mine doesn't clunk. It does have new everything at the top, but doesn't clunk.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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