Most likely a pressure relief on the intake side of a (very) turbocharged engine. A pressure relief controls boost by bleeding off air pressure, as opposed to a wastegate, which throttles the turbo's speed. With a pressure relief, the turbo stays spinning at high speed, which translates to less throttle lag.
-Carl
It's a dump valve working on a turbo car. It is a throttle activated valve which releases (dumps) any pressure in the inlet manifold when the throttle is reduced (ie during gear changes). Dumping this excess pressure reduces turbo lag when changing gears, as the turbo isn't slowed down by the excess pressure which would normally build up in the inlet manifold during the decelleration of a turboed engine. For maximum sound effect, the dump valve is set up so that this excess pressure is released directly to atmosphere. The dump valve is seperate from the turbo wastegate which is used to limit the ammount of pressure that the turbo produces, although some modern electronically controlled engines, have a vacuum controlled waste gate, which the ecu activates to dump pressure during gearchanges, to reduce turbo lag. Although this approach doesn't produce the same sound effects.
Moray Cuthill moray_dot snipped-for-privacy@btopenworld.com change _dot_ to reply.
**************************************************************************** **************A couple of times lately I have followed a certain type of car, whose make was not obvious (night time, and I never saw the front ) I would say upper-class Japanese (Toyota, Mazda, Nissan) sports or maybe a Porsche.
Anyhow....Every time the driver changes gears upwards (once again I have never been close enough to hear them chage down ), the car gives out a most unsociable PSHSHSH! that lasts maybe 1 second. It sounds not unlike the air escape from a pneumatically-braked truck, only less restricted (trucks have that sort of strained feeling about the air getting out, if you know the feeling) and is about as loud.
Air-assisted gearchange? Waste gate from a turbo?
Whatever, it would be lovely to have one drive around the neighbourhood at 3am. I can hear this noise way above the other car's exhaust note, and it's plainly audible at 50 metres, in heavy traffic.
Any knowledge anyone?
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On Fri, 8 Aug 2003 17:12:24 +0000 (UTC), "Moray Cuthill" wrote something ......and in reply I say!:
Thanks for that. It certainly did not sound like a normal waste gate.
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