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In a lot of the cheap cars I have looked at their exhaust components, I always find the catalytic converter right after the exhaust manifold instead of being after the down/front pipe in a "typical" exhaust setup. Why do cheaply manufactured cars come like that, what kind of gain putting the cat right after the manifold can the manufacture make other than having shorter wires for the o2 sensors? Or am mistaken and those cars come with two cats like I have seen in some other cars?

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    Not being a car guy, when I saw this question in the Hot Network Question sidebar, it looked like a title off of Arqade, something to do with when a feline climbs up on the engine block. – hBy2Py Feb 2 '17 at 13:37
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The main purpose of putting it right after the manifold is heat. The cat functions best when hot. Placing it as close as possible to the manifold helps the cat heat faster and stay hotter, which produces better results in what it's designed to do: curb exhaust emissions.

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    Great answer! +1. However, I would have mentioned that some cars actually have two catalytic converters, one close to the exhaust manifold and another one a bit further away. – juhist Feb 1 '17 at 18:15
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    @juhist - Absolutely correct. Looks like you just did mention it, so thanks for the assist! – Pᴀᴜʟsᴛᴇʀ2 Feb 1 '17 at 18:16
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    @method - I believe the usual purpose is for the first to control CO and HC, while the second one controls NOx. At least I believe that's what it's for. Some manufacturers prefer to split the two out for some reason. – Pᴀᴜʟsᴛᴇʀ2 Feb 1 '17 at 19:15
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    This should be obvious. Everyone knows cats enjoy warmth. It makes them curl up and purr contentedly. – Mason Wheeler Feb 1 '17 at 20:32
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    @MasonWheeler - You know what they say about cats and the internet ... goes together hand in hand. – Pᴀᴜʟsᴛᴇʀ2 Feb 1 '17 at 20:34
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On cheap models the cat is part of the exhaust manifold - i.e. the manifold and cat are a single unit. This is done to save cost.

The answer about heat retention is valid but this isn't the factor that really affects cheap models. On some vehicles, you may find the turbo is integrated into the manifold or the manifold is part of the head itself. Its all down to cost.

Cheap cars like this are designed with different serviceability ideas in mind. In some cases, it actually means the cat is easier to replace although I don't believe that the designers consider this for disposable models, perhaps they anticipate the vehicle would be scrapped long before the cat needs replacing.

Have a look at Toyota Aygo cat images.

  • Wow never seen that before very interesting design, do you think it is worse performance wise to have the restrictions in the exhaust right after the manifold with no room for the gasses to expand? – method Feb 2 '17 at 17:24
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    @method Traditionally we have assumed that better flowing headers of the manifold with smooth bends and equal lengths are better for performance, however there is a Youtube video that uses an engine dyno to compare the output of an engine with steel performance exhuast manifolds. One of the comparisons they do is the manifold fitted right out of the box, then they take a hammer and bash in the collector part of the manifold and the results were about the same. I think it depends on the kind of engine and application but TRADITIONAL thinking says it would hurt performance. I'm not convinced. – DizzyFool Feb 2 '17 at 20:49
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Small catalytic converters mounted close to the exhaust manifold to warmup quickly and be active as soon as possible after engine start-up. The start-up mixture is usually rich and needs to be oxidized to reduce HC and CO emissions. An O2 sensor mounted ahead of the cc catalyst controls rich/lean mixture to operate the engine close to the chemically correct stoichiometric Air Fuel ratio for best cat operation. The second catalyst mounted under the seat is there to promote reduction in NOx after the HC and CO are eliminated upstream. The O2 sensor between cats monitors the exhaust mixture so that the fuel injection can be altered to be sure that very little O2 enters the second cat. The NOx needs to give up its Oxygen and the best way is for exhaust to be slightly rich. So. The cc cat needs a slightly lean mixture to oxidize the HC and CO. The UF cat needs a slightly rich mixture to reduce the NOx. The O2 sensors keep telling the engine control computer the status of the exhaust and thereby control the injected fuel. It is a delicate balancing act by the fuel management system to yield great results.

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