- My model of MX-5 is: mk3 2007 1.8
What is this bit on the intake pipe.
It’s a small air resonance chamber.
Whats that do?
Depending on the design, either quieten the intake, or provide a charge of air to ensure smooth throttle response. Or both, The technical term is Helmholtz Resonance Chamber.
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Thanks 
If the puzzling part is in anyway bothering you , chuck it away and stick one of these on 
Cobalt cold air intake
Doe’s it improve anything?
Direct cold air into filter of choice.
Performance ? Probably minimal , throttle response ? Depends on other factors , filter, etc.
But as it’s available I thought why not 
The colder the air the denser it is and that means the more fuel you can burn which gives a small increase in horsepower, note the lower one mounts ones cold intake the more one risks the engine ingesting water and consequent terminal hydrolock, so unless you are a sunny days only driver you might think carefully about how low you go with the cold air intake.
On the otherhand Mazda have spent untold research dollars optimising the positioning of the intake to operate in a range of temperatures and even provided a Helmholtz Resonance Chamber to optimise the airflow to your engine, note even a standard intake can ingest water and leave you with hydrolock at for example Rufford Ford when the water level is high.
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Thanks guys, question answered
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