Quote:
Originally Posted by glend
Closing the throttle body plate, is typically a device used to prevent diesel runaway, and completely separate to most EGR valve setups, although the throttle body plate can be used to generate suction into the manifold for EGR gases when the valve itself is opened. The throttle body plate in a diesel, is not a throttle per re as in a petrol car, it is a runaway safety device primarily.
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I always assumed they would be used to help in case of a runwaway (And I know all three oilers I have had had one, and all three closed them on shutdown) but two Nissan manuals (Different cars) stated different purposes, one specified it was used to reduce compression thump on shutdown (An old D22 Navara with the ZD30 hand grenade and when I tried disconnecting the actuator, it really did stop with a car shaking thud) and the other (D40 STX-550 with the V6 TDI) states it is used to increase EGR volume, given it would have no manifold vacuum to help it along otherwise and in some conditions the inlet pressure might be higher than the exhaust, so flow would go the wrong way.
Regards the variable vanes, they are on the hot (Turbine) side anyway. Without whacking a camera in there it is hard to say how they behave on getting off the throttle, but unless the throttle body is snapped shut (And I understand it not to be, but I would need to try some OBD or CANBUS sniffing on the D40 to see if it's position is reported) it won't cause any any boost pressure spike anyway.
Funny enough on my race car I have always run without any form of blow off as they are a bit much of a blunt instrument for my liking and made the car harder to drive fast, on that car I just let the compressor stall and sound like a demented pigeon, and if it died, it died. The turbine housing cracked around the wastegate flap from the heat before the compressor gave any trouble. That might change with my new ECU, the maker of that is fond of using a small electronic throttle body as a blow off valve, controlled by the ECU. A scalpel instead of the traditional BOV meataxe.