I've noticed that some cars (mainly modified hot hatches) emit a pneumatic sounding 'hiss' when the driver changes gear. Just curious as to what causes this - some sort of air assisted clutch / gear shift modification? Thanks for any responses.
Pneumatic 'hiss' when changing gear - what is it?
Jun 11, 2007
8 Replies
Atmospheric dump valve for the Turbo, aftermarket "bling" as opposed to the silent recirculating type fitted as standard.
Wastegate of a turbocharger.
Si
Some standard turbo wastegates can be heard when they open, but are certainly no where near as noticable as the aftermarket ones. Also given it's mainly 'modified' hot hatches, you'll probably find a speaker involved ;-)
The message from "Tony Bond" contains these words:
Or, in some cases, a loudspeaker that emits the "correct" noise for those that don't have the wossnames.
Or the high-pressure pump on VAG PD-TDI engines.
It actually tends to be a dump valve in the air inlet piping, rather than the wastegate itself, which is usually fairly silent.
You're not wrong. I stand corrected.
I wondered why rally cars 'chatter' rather than hiss and discovered that it's because they're not allowed BOVs for some reason and the chattering is blocked compressed air banging hell out of the compressor!
Si
I rather much prefer that chattering sound, as opposed to the chav like Psssshtt from a dump valve. I'm just not so keen on the amount of stress it'll be putting on the bearings, though.
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