[s-cars] DahlBack Racing BYpass Valve, why not two.

[QSHIPQ at aol.com]

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Several years ago, I proposed that those with monster turbos might explore 2
bypass valves and stagger them so that the first opens under light lift
(vacuum) the second opens under full lift.  The bigger/heavier the valve components,
the longer the transition back to boost (valve shut).  I've played
extensively with the HKS sequential BPV, which to me is a great idea, but the light lift
wasn't enough bleedoff.

2 adjustable preload bypass valves could be just the ticket.

WRT MLP's comments on placement...  Had numerous (humorous?) discussions on
this very subject.  In *theory* you want to place the BPV as close to the tbody
as possible, since the reason for it is to bleed the pressure wave, which
starts at the throttle body and migrates back to the turbo blades.  Another
theory presents itself that postulates that you want the turbo to freewheel as soon
as you lift throttle (vacuum present) and it will freewheel the fastest when l
ocated close to the turbo.

In *practice*, IME, the BPV closer to the T-body is faster to react to
boost/vacuum transitions, but the choice of BPV probably makes a bigger difference
(IOW a stock valve is quicker than a big heavy valve).  That said, I believe my
posts of yesteryears have postulated that a quick light vacuum valve (stock
or "small") might be best at the T-body, and that mongo big dog with a lot of
preload, sit prior to the IC.  My *theory* being a quick shift doesn't need a
full boost dump, but a full lift most likely means you don't need to worry much
about transition back to boost.

I also believe in that 6 year old thread, that Graydon Stuckey said you
really don't need ANY bypass valve if you keep the throttle body open slightly
between shifts.  If you look at the decel delay valves audi used on turbo cars
(including S) since 1988, this appears to be a valid idea.  And in/with practice,
it works.

HTH

Scott dubbledummp Justusson

In a message dated 9/23/2003 10:48:30 PM Central Daylight Time, CaptMagu
writes:

Trevor

It seems to me that Dave Jones, Mike Pederson and I have had numerous
conversations on this very subject. Mike and I have experienced a boost pulse  that
we thought might be helped by this solution. I can't remember if Mike and Dave
tried this or not. The pulse is particularily acute when I'm really loading
the turbo as when I make the climb up I-70 on my way home to Everboost. It
manifests itself on throttle lift after serious boosting especially if I only
partially lift the throttle. Without total lift and resultant vacuum it might be
tough to get the BPV to open. HTH.

Hap, to boost or not to Everboost, Maguire