Thread: BOV orientation
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mark houghton mark houghton is offline
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Join Date: Jul 2008
Location: Central Washington State
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BOV orientation

Hey kids, I'm about to embark on dissecting my stock boost recirculation piston manifold assembly, and using the various parts to link up with my shortneck IC. I have a nice BOV/.bypass valve (popular piston type, not diaphram) and whilst researching various installation options, I came across the following post regarding vertical vs. horizontal mounting of the BOV and supposed benefits of feeding the boost to the bottom of the BOV vs. the side port. Keith has been preaching this for a long time and I finally understand what he was trying to tell us. See the link:

Inlet Overrun Valve Modification? (BOV, lag)

Benefits include allowing the turbo to free-wheel (i.e., spin freely without producing any boost) when at idle as well as at steady state no-load cruise...the theory being that a turbo spinning fast under no load will spool up quicker when the throttle is opened and the BOV valve closes.

I've ran this around in my mind, recalling the old physics classes from too many years ago, and a good healthy dose of logic, and it would appear that this makes sense. But the spring tension in the BOV would have to be something considerably less than the max boost you're running, in order to allow the valve to open under the relatively lesser amount of vacuum seen when at partial throttle vs. a completely closed throttle valve. The premise is that with a weak spring and very little boost pushing on the piston, vacuum alone will overcome the spring tension and open it enough to redirect (to the intake side of the turbo) the small amount of boost generated under cruise conditions. Thus, when the pressure on both the in and out sides of the compressor are identical, it will spin as fast as it can with no load.

Also, you need to visualize four forces controlling the BOV....the boost pressure trying to push the piston open, the spring pressure trying to keep it closed, the vacuum (when present) at the top of the piston trying to open it, and the vacuum-line-turned-boost source when the throttle plate is opened fully and that small vacuum hose is now pressurized as boost builds in the intake manifold. With all these forces, the key lies in the fact that whatever boost pressure is applied to the bottom of the piston to force it open will be exactly the same boost pressure that's being applied to the top of the piston..pushing down. Net-net, the piston does not move under boost, even with a weak spring in place. But as soon as the differential pressure between the top and bottom of the piston changes (creating negative air pressure above the piston), it will open (as in when the throttle plate is suddenly closed at shifting, or when the throttle is held at partial steady-state cruise and a small amount of vacuum is created).

With this understanding, one need only determine how soft the spring needs to be. My BOV came with a stiff spring to hold against about 1.0 bar, and shims to make it even stiffer if wanted....because most people depend on the spring tension only to keep it closed. Theoretically, the spring tension in the above configuration would need to be only slightly less (inversely) as the amount of vacuum available from the throttle body at steady state cruise...so that small amount of vaccum can overcome the tension and open the valve.

Just thought I would spend the evening rambling. Does anyone know how many inches of mercury vacuum is generated in closed throttle plate situations vs. partially open steady cruise throttle positions? Or does anyone think this is all bogus and not worth the effort of fine-tuning?
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 12-07-2009, 07:40 PM
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