Strange goings on with exhaust
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  1. #1
    Tadpole supersi's Avatar
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    Apr 2004
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    Icon5 Strange goings on with exhaust

    Having dropped a 309GTi 1.9 engine into a 205 CTi 1.6 body, do I need to change the exhaust sytem over too? I'm wondering if I'm not getting good performance because the exhaust is in fact too small for the engine. Also how easy is it to swap them over? Does the 309 have a similar underbelly layout to the 205?
    Any ideas?

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  2. #2
    Tadpole
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    Wiseman's Ferry, NSW
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    Default Probably...

    Hi,

    I don't really know - sounds like an interesting mod, though. Here's a few thoughts on what might be going on...

    The size of the exhaust system needs to match the volume and velocity of the exhaust coming out of the motor. The exhaust manifold on the 1.9 will have been designed with this basic idea in mind. The job of the manifold is to merge the pulses coming from each of the cylinders down into a single pipe (this is called an interference system) - I believe that peugeot usually do a pretty good job of this design where the pipe diameters and lengths are specified to maximise the scavenging of exhaust gases from the cylinders. There's a lot of malarky involving pressure waves and so on which I won't go into at the moment.

    What happens after the manifold is also crucial. The manifold can only work properly if the pressure is right at the cylinders (the manifold entry points) and at the manifold exit. In other words as long as the motor isn't producing more gas (by increasing the capacity, upping the usual operating rev range or mucking about with the exhaust valves) _and_ the rest of the exhaust system can cope with the outflow from the manifold all is hoopy.

    The first question is - what is the internal diameter of the exit pipe on the 1.9 manifold compared to the same diameter on the 1.6 manifold? If they're the same then the diameter of the 1.6 piping should cope with the 1.9 flow rate. If the 1.6 is smaller then you'll probably want to increase the pipe size in the system to match the 1.9.

    If the 1.6 piping is the same you might still have a problem. The next constriction in flow is introduced by the catalytic converter. If the 1.6 cat has a lower throughput than the 1.9 cat then using the 1.6 cat will produce a reflected pressure wave back up the exhaust pipe towards the manifold - this will prevent the manifold from working properly and you'll get less efficiency and power. The same applies to the muffler.

    But wait - it gets worse The relative positions of the exhaust system elements are also important, i.e. the distance from the manifold to the cat and from the cat to the muffler. Any cat will produce a reflected pressure wave back at the manifold because they all constrict flow. The size and position of the cat on the 1.9 will be designed to both reduce the constriction and have a less detrimental back pressure effect.

    Anyway - enough food for thought.


    Cheers,

    Tony
    --

    "Aaaaarrgghhhhhhhh..."

    205 GTi 1.9

  3. #3
    Tadpole supersi's Avatar
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    Default Thanks for your thoughts Tony

    The 309 which the 1.9 came out of apparently had a coby exhaust on it with no catalytic converter. I was unable to get my hands on the exhaust as the wreckers sold it to someone else. I'm thinking I should just find another 309GTi exhaust and bolt that on. I'm wondering how much modification I'll have to do for it to fit a 205 though.

  4. #4
    Fellow Frogger!
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    Default

    Quote Originally Posted by supersi
    The 309 which the 1.9 came out of apparently had a coby exhaust on it with no catalytic converter. I was unable to get my hands on the exhaust as the wreckers sold it to someone else. I'm thinking I should just find another 309GTi exhaust and bolt that on. I'm wondering how much modification I'll have to do for it to fit a 205 though.
    Hi

    Was the donor engine NZ new or a Jap. import? If it was sold new here it would be British spec. without a cat. If it was an import it would have been a poor strangled thing with a cat.

    Cheers

    Tony
    95 MI16
    71 404 U10 ute
    66 404 coupe
    55 203C (gently warmed) gone... sold...

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