Article on The Internet Mini Encyclopædia
What is it? (I)
The waste gate is set at a weedy 4psi (ish) the modulator
pulses (I think) to bleed off some of the boost to con the waste gate into
thinking the boost is lower than it really is so the waste gate doesn't open.
The trick here is that the modulator is used at HIGH revs to allow more boost
(effectively about 7.5psi I think) and therefore more peak power. The lower
boost in the midrange reduces peak torque, thus preventing the engine from
tearing the gearbox to bits. They wanted the 'box to last as long as it would in
a stock MG Metro, they were not entirely successful. Scrap Metro turbos usually
come with scrap gearboxes. That is how I remember it, could be wrong.
If I'm not wrong then dumping the modulator would mean you had LESS boost at the top end thus giving you less peak power, but the same peak torque.
Don't take this as gospel, read DV.
Oh, and as for the high compression ratio, this was to make the engine more responsive and more economical around town. The problem this causes is that the set-up is very close to knocking at high revs, any tuning would take you over the edge. DV suggests either lowering the CR. (expensive) or building a rudimentary water injection system (sounds like fun) to overcome this.
What is it? (II)
We're all getting very confused on the subject of turbocharging recently. I think that it's time that anybody who's interested reads a good book on the subject. One of the problems is the terminology, it's very confusing. Somebody stated recently that a waste-gate and a dump-valve are the same thing, they aren't.
The waste-gate is a valve which controls the speed of the turbine to stop boost getting too high, it is controlled by a diaphragm and spring arrangement (high tech ones are controlled by electronics).
The diaphragm is connected to the inlet manifold; when
boost on one side of the diaphragm exceeds the pressure exerted by the spring
the diaphragm moves thus opening a valve (the waste-gate) in the manifold
between the engine and the exhaust turbine, this valve allows exhaust gas to
bypass the turbine thus preventing the turbine from producing any more boost.
A dump-valve is only really necessary on high boost engines (1bar and up) what this does is release boost pressure in the inlet manifold when the throttle is closed. If you shut the throttle from high revs the turbine will keep spinning, this causes boost to stay high, because the throttle is closed and the engine is not drawing much air so the pressure builds up between the throttle(s) and the compressor causing two unwanted effects:
the engine getting boost when it doesn't want it
this pressure slows the turbo down producing unnecessary lag when the throttle is opened again.
A dump valve dumps boost that you don't need on the
overrun. A waste gate controls the level of boost with the throttle open. By the
way, dump valves shouldn't be used on suck through systems since you could be
dumping fuel/air mixture into the engine bay. Scary!
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