As requested by chip......
The Amal Valve.........
the Amal valve otherwise known as a Wastegate control solenoid is a control method for boost used on the weber marelli system as used on Cossie as well as other cars.
The Amal valve is basically a body with 3 ports.
1 port is an inlet
the other 2 are outlets. 1 goes to atmosphere and the other to the wastegate.
The valve works in a way whereby the inlet is connected only to the wastegate until the solenoid is energised at which point, the atmosphere port is also opened.
Inside is a small plunger and a spring housed around the solenoid itself.
With no power, the spring closes the plunger and covers an internal port (to atmos) and thus directs the air to the WG.
This works as a fail safe and means should you lose power to the amal the spring will close the plunger and air will be sent out of the WG port to wastegate to reduce boost.
Hence the 'click' you hear when you put power to the amal, its the plunger being pulled of its seat by the solenoid.
The solenoid part of the valve is pulsed on and off by the ecu itself.
this opens and closes the atmosphere port, thus introducing an air leak to atmosphere (boost increase as bleed is away from WG) The frequency is fixed however the ecu varies the duration of the pulse to effect the boost bleed normally at around 50%.
The boost level is sensed by the Map sensor and this in turn can allow the ecu to adjust the output signal to the amal so as to maintain the correct boost level.
this is also how the ecu can lower boost as the revs rise, as by the time the revs get to 6000rpm, the ecu is reducing boost to save the engine, and also does this by reference to the ACT, again to reduce boost.
Bleed valve......
Easy to follow on from the above.
So what is it?
basically its a very simple manual bleed control similar to the amal.
It consists of a body to which a hose can be connected to each end and has a variable orifice inline inside which bleeds some air away to atmosphere.
When fitted between the compressor and the wastegate, the orifice can be opened up bit by bit to decrease the boost in the line being sent to the WG.
Put simply the bigger the orifice the more air bleeds away to atmosphere, the less boost is felt by the WG hence higher boost pressure created.
Whilst this is simple in terms of usage, it does not provide any form of ecu control like the amal, and hence has no option to change boost pressure should the engine develope a potentially fatal fault or due to high acts.