Why does my TD300 (all diesels?) need a vacuum pump for the servo?

Think about it, there's no restriction to air entering the inlet manifold therefore no chance of vacuum build up. To make matters worse in that respect, as you say, the turbo actually pressurises the inlet but when the accelerator is released on the petrol, the butterfly valve strangles the inlet causing a vacuum build up with no need for an exhauster pump.

Thanks for that Oily, that answers my question however I'm now wondering how the air into a diesel engine is regulated if there's no restriction/butterfly as in a petrol engine.

Cheers

Tony

The air isn't regulated, the fuel is.

Thanks for that Oily, that answers my question however I'm now wondering how the air into a diesel engine is regulated if there's no restriction/butterfly as in a petrol engine.

Cheers

Tony

There is no restriction, generally the engine speed/ power is controlled by the amount of fuel injected.

Cheers Oily

It all makes sense now

Thanks for your sharing knowledge

Tony