Why would the exhaust gas for this be obtained from separate outlets
>in each arm of the exhaust manifold, and not from one opening after
>the manifold has reduced to one exhaust pipe? The four separate pipes
>in mine (1.3 litre Mazda I-4) then reduce to two and then these two
>separately enter what appears to be an alloy plenum chamber bolted to
>the side of the air cleaner housing. Any ideas? I've done lots of
>reading on this, but can't figure it out, except perhaps this setup is
>needed on say a turbo-charged model, and it is easier to have them all
>the same, whether T/C or not.
I've just pulled what I thought was a plenum chamber apart, and find that there is a small passage to the outside of the airfilter (inside the housing), and between this and the two exhaust connected pipes, a set of reed valves that appear to be one way TO the exhaust ports, and not TO the carburettor. So it appears to be air injection into the exhaust stream? Between these reed valves and the air cleaner (upstream from the apparent flow) there are filter pads. It is a mystery to me, unless some silly bugger has put the reed valves in backwards.