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Jim2 Jim2 is offline
7.0:1 > 11.3:1 > 7.0:1
Join Date: Oct 2003
Location: Calgary
Posts: 2,146
This is my take, and I'm not saying BOVs are bad.

Stock 930s never came with BOVs. Never any damage.

Lots of other early turbo cars also came without BOVs. I know of a dozen or so performance street turbos and turbo race cars which sustained constant use/abuse without failure and no BOV. Garrett, Rayjay, Schwitzer, KKK etc. A good handfull were my own. Note that all were under 20 PSI.

There is a minimal boost spike when the trottle plate is closed. Stock 930 has a fast reacting boost gauge sender mounted upstream of the throttle and I don't think you can see any increase.

Non BOV throttle closure while at boost:
The compressor requires a minimum charge velocity across it's vanes to sustain a given boost pressure, if the charge velocity is not sustained the pressure bleeds off in the reverse direction through the vanes, hence, centrifical compressor. If the throttle closes under boost, the charge velocity instantly stagnates, causing the compressor to cross over to "surge", the charge then reverses back out the compressor. Not pretty, but certainly no 60 psi spikes with our compressors. Maybe a few PSI for a split second before serge kicks in.

The oscilating serge noise resuting from lifting off the throttle can be detected on some installations. After the event described above there is momentarily low pressure downstream of the turbo, yet the turbo is still spinning due to inertia so it recompresses the air until it reaches it's maximum sustainable pressure relating to the velocity of the charge across the vanes, but since the throttle is still closed the charge stagnates again sending the compressor back into surge. Repeated cycles absorb the turbo's inertia.

BOVs alow the turbo to sustain some interia by reducing the charge pressure, but sustaining the charge velocity across the compressor vanes so as to avoid serge.
Old 01-29-2007, 10:02 PM
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