Blow off valve or not?
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Joined: Oct 2007
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From: Mooresville, NC
I swapped to a Greddy bov that I got as a gift. My vortech bov used to bleed off air the same way when idling or light partial throttle. Would that be an issue that it now does not bleed off the pressure at idle and light throttle? It does blow off when shifting in boost and all.
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Joined: Apr 2009
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Not quite. The valve is open during high vacuum situations (idle, decel, cruise) because the SC is supplying more air than the engine needs. The valve is open to reduce the workload on the SC. With the valve open, the SC isn't compressing any air and therefore creates less heat and less parasitic drag on the engine. The valve is relieving pressure from the spinning SC at idle so the engine will not be sucking in air from the open valve. If the SC was spinning so slowly that it's delivering less air than the engine is inhaling with a closed or nearly closed throttle plate, then it could suck air through the BOV but I've never seen that circumstance. My valve is blowing massive amounts of air out during idle.
Using a BOV or diverter valve/bypass valve (same thing except one vents to atmosphere and one vents back into the pre-compressor intake) will improve engine efficiency at less than WOT and will reduce the wear and stress on the compressor. Similar to a turbo, when the throttle is shut, a large pressure wave is created that loads the compressor and can cause damage or excess wear unless it is vented via BOV.
Using a BOV or diverter valve/bypass valve (same thing except one vents to atmosphere and one vents back into the pre-compressor intake) will improve engine efficiency at less than WOT and will reduce the wear and stress on the compressor. Similar to a turbo, when the throttle is shut, a large pressure wave is created that loads the compressor and can cause damage or excess wear unless it is vented via BOV.
I am aware that having the valve open at idle is common on a sc set up but in the case of the video examples (turbo set up) its not correct. I am aware of the engines that produce healthy vacum that will combat the spring in a bov this is why almost every bov manufacture gives you adjustability in the tension of the spring to combat this issue. People have not been instructed in the correct method to set up their bov or diverter valves and simply think the adjustment is for sound......If you want sound get a MP3 of a BOV from Fast and the furious and play that on repeat. the higher a valve flows the less sound it will make. sound is partly restriction in flow like when you clinch ur lips to whistle.
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Joined: Jun 2007
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I think venting to atmosphere rather then recirculating that hot air adds some performance. Like several of us, I just re plumbed the stock bosch recirc valve to blow off. It works good, it’s a nice alternative if you prefer not having that pronounced BOV sound as well. I hardly hear mine.
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I have a bov on mines and I will tell you right now, on a single bov for a supercharger setup, you will get compressor surge. The SC is tied onto the crank and keeps pushing massive amounts of air even when the BOV opens unlike the turbo where pressure drops substantially as soon as it opens. The only way I could rid of the surge was to install a second bov.
Joined: Sep 2008
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From: Bay Area, CA
I recently reconfigured my recirc valve (not Bosch, but Turbosmart) to vent atmosphere. I noticed some minor decrease in AIT when cruising downhill with throttle plate closed. Also, I get a nice whoosh sound.
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Joined: Feb 2008
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From: LaVista, NE
i got a turbosmart dual port. when at WOT it closes. any other time not WOT it vents to atmosphere and intake (hence dual) that way you get the swooosh sound and get some air back. best of both dont you think?