I live on a long, straight 30mph road with parked cars either side, many houses owned by families with young children. Obviously, this makes it perfect for cars to be thrashed down it at full chat.
Recently I've noticed two or three cars, all of the heavily modded variety (the ones that accelerate hard everywhere at all times), that emit a loud hiss of compressed air when they change gear. I have my theory about this (poseur gimmick?), but I'd be interested to know what it really is.
V
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A turbo dump valve - allows excess air to be released from the turbo during gearchanges and in turn keep the turbo spinning. Said to improve performance, but from talking to smart owners who bother to fit them all it results in is oil being sprayed everywhere.
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I'm not sure they're supposed to increase performance, I think it's more to reduce strain on the turbo.
Although, by fitting one, it allows you to set the turbo to a higher pressure, so indirectly it can allow higher performance.
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Dump valve, probably.
They relieve excessive back pressure on the compressor when the throttle is closed, reducing stress on the bearings and stalling of the turbine/compressor.
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And they can be just as effective if the excess air is dumped, silently, back into the intake, so the loud noises, as our friend initially proposed, are mainly for pose value.
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I've noticed some lorries make a similar noise when changing gear, presumably the same thing.
Cheers, Sofa Spud
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I think this hiss from lorries is from the gear change itself as the clutch is air operated and air might also be used to move the actual cogs
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I'm sure buses used to make a similar noise too (probably still do). As a cyclist, strange how they always did it just as they were overtaking you (about 2" from your handlebars, and just at the point when the driver can see you jumping out of your skin in his wing mirror).....still, I guess there's not too many perks to being a bus driver these days.
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