Thanks Rmac....is there any particular reason why they are bypassed?
In a nutshell: if your engine and transmission are bone stock, it's unlikely it will produce so much overboost that the overboost protection valve will ever be needed. And bypassing it gives you a simpler system, fewer joints, therefore fewer potential places for boost to leak out between the intake manifold and the ALDA.
And if you have modified your engine to be a racer, then keeping the overboost protection valve will defeat your purpose.
The average 27-35 year old stock turbo puts out only about 0.61 to 0.68 bar of boost.
The overboost protection valve takes the output of the boost sensor switch, located on the rear of the intake manifold. This switch closes when boost reaches 1.2 bar, thus making the overboost protection valve close, thus denying boost to the ALDA, which results in the ALDA shutting down fuel.
It's a complicated system with a lot of pressure hoses and joints. Sometimes the car loses get-up-and-go. The prime suspect is always the ALDA, and the easiest way to test the ALDA is to connect the boost outlet nipple at the rear of the intake manifold, directly to the ALDA input nipple, with a boost gauge tee'd in
Sometimes a shadetree mechanic will perform this test and then simply unhook the boost gauge and leave the manifold's boost nipple directly connected to the ALDA, bypassing the overboost protection valve. It leaves a system with fewer joints, thus fewer potential places for boost to leak out. The overboost protection valve in some cases may have failed and be leaking boost, so bypassing it is a convenient workaround.
The only way you are ever going to see 1.2 bar of boost, enough to make the overboost protection valve necessary, is if you either set your turbo's wastegate to allow a higher level of boost than the IP can deliver fuel to use, or if you can somehow induce the turbo into a severe surge condition (i.e. pushing boost into the intake manifold without the cylinders being able to intake it, to the point where flow is reversed, back out through the compressor blades).
About the only way I can think of to put the relatively wimpy stock turbo into surge would be to use a manual transmission to produce sudden large rpm drops between gears while at the same time having the wastegate wired shut. Just maybe. Perhaps this is the reason why MB sold all its USA 300D turbo diesels with an automatic transmission. You just might be able to induce surge in a bone stock 300D if your wastegate were set to produce 1 bar of boost and you accelerated in L to say 3000 rpm then lift your foot and drop it into D. Maybe. If you do this with the overboost protection valve hooked up, it will cause the ALDA to reduce fueling during the suddenly-lowered rpm region of operation, until exhaust gas volume rises again to the point where torque the turbo wheel produces catches up to the compressor wheel's tendency to resist it due to the high bar of what's in the intake manifold. So in this case the overboost protection valve will reduce stress on the turbo shaft.
There is no need for an overboost protection valve if the device it controls, the ALDA, has been removed. One reason to remove the ALDA is to improve off-the-line acceleration at the expense of emitting a cloud of dark smoke.
Also there is no need for the overboost protection valve if one increases the fueling by building larger elements into the IP and increases the boost by swapping-in a larger turbocharger than stock plus an intercooler. In other words, *intentionally* running the engine at >1.2 bar, i.e. more bar of boost than the overboost protection valve guards against.
Experimenters over at superturbodiesel have found that 0.75 to 0.82 bar of boost is all the engine can use, even if fueling is turned up to max, with an unmodified stock injector pump. Any wastegate adjustment beyond 0.82 bar simply puts needless pressure on your head gasket.
The unmodified stock IP delivers about 55-60 cc of fuel per cylinder every thousand injections. So, if you want to raise the engine's horsepower from the stock value of 120 hp by 50% up to 180, what you do is, increase fueling by 50% to about 90 cc/thousand (by building larger elements into the IP), and swap in a higher-capacity, more efficient modern turbo which is capable of blowing-in 50% more air to match the increased fueling. At this point you will be using right around 1.2 bar of boost, and if at this point you haven't already ripped out and thrown away the ALDA then you should at least get rid of, or simply bypass, that pesky overboost protection valve.
But if you attempt to give it enough fuel to match 1.2 bar of boost, and if that 1.2 bar is put through an intercooler...then you'd better equip your engine with an exhaust temperature gauge and very closely watch it. Lest you start melting piston crowns, valves et cetera.
Now you know everything I know, and that's the best explanation I can come up with. Anybody else care to try?