by HiruxHijinx on Thu, 17 Apr 2008 1:33 +0000
Not sure on our engines, but I used to have a few petrol turbo cars, and the blowoff is designed to allow the release of air between the compressor housing of the turbo, and the throttle body.
This is useful (and increases performance) when the throttle is closed or reduced suddenly when changing gears at full boost - the turbo rpm does not drop as much, and the turbo is closer to boost rpm in the next gear, I found it to make a big difference.
NOW,
This won't work on normal diesels, as there is no "throttle flap", the rpm is controlled by injection of diesel, which draws more air as a secondary effect.
A petrol increases rpm by allowing the engine more air, which is then matched by the computer adding more fuel to obtain the correct mixture.
So with no throttle flap you have no real obstruction requiring venting of intake piping air,
AND
with no throttle flap you have no vacumn created in the plenum chamber / intake manifold to enable the actuation of the valve..
(the blowoff valve plunger is sucked open by the vacumn in the intake when the throttle flap is closed at high rpm..ie. engine trying to draw in air, but throttle butterfly is closed, so none can get in, creating a strong vacumn..)
Also the reason diesels need a vacumn pump to operate brake boosters and other vacumn actuated equipment. A diesel has no natural vacumn, unlike a petrol engine.
That probably makes no sense, but is the explanation as to why you can't have a blowoff valve on a diesel.
Sam.