I have had a little bit of experience with BOSCH valves runinng on 180's with upto 18psi... Never seen any problems. They are designed for a stock application but are farily well built (just because they are not shiny Aluminium doesn't mean they are poo) and as long as they are used within those limits you should have trouble free motoring... As Gavin said a lot of the time the diaphram splits or seat wears causing leakage... Mots of the time you will notice if your BOV has gone as it will start to leak boost causing preformance loss...
If it does seal shut - it will start to get laggy as the air returning from the throttle body will cause the exhaust wheel to spin in the reverse direction which is definatly not good. Also notice some cars that sort of pulse with BOV discharge - this is due to compressor surging and is also not good. If you have this sounds look at it asap...
The reson most change BOV is becuase the stock spring preload can no longer hold the diaphram shut under load and the BOV is forced open under the boost pressure in the piping - but i think we are talking upwards big boost here
Don't get me wrong if you want the sound then its not a waste of money... to me it is thou... but if you get one make sure it is a dual stage with plumb back and trumpet... Took me ages to find that problem on a 180 when it kept stalling at the lights.... I used to have to sit @ 2000rpm... everyone thought I was up for a TGP![]()
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