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

[Trevor Frank]

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]

This was basically my thought Scott, I have a ricier than you, purple
adjustable Gretty type S as my primary valve and a Gretty type R that I
was thinking of placing on the t-body side.  Although I haven't had luck
with the bosch valves I do like diaphragm valves for two reasons.  1 it
takes a little more r and d and cost to manufacture.  2. I suspect that
like the bosch the actual mass of the moving components is less on a
similarly sized diaphragm valve.  My personal favorite is the idea
turbonetics has with there trap door style valve.  Hap mentioned the
pressure wave issue and so did Scott.  Many bov manufacturers recommend
the bov be placed close to the t-body, indy car and F1 have it mounted
on the plenum.  I suspect with such a large volume there is quite a
disconnect and possible disruption that would occur after the bpv opens.
My lead in was mentioning pressure drop be it 1psi or 3 either way it is
a tortured path back to the bpv from the t-body.



I mentioned two reason's for the second valve, as a disscution point,
one was a simple mechanical safety valve to save the motor incase of
some issue with the bpv/wastegate.  The other would be a second bpv/bov
to vent some standing wave that may occur in this volume, in this case
controlled by the vacuum.



Not to start the bosch debate again but let's just say I have scrambled
one, and the ball bearing turbo's don't like injesting plastic/rubber.
Even though I am positive due to inertial mass that there is very little
chance that any other bpv would react quicker.







On to Ned's comment, I suspect on a race motor that keeping the turbo
spinning would be more important than issues of trying to stall the
compressor.  The slight annoyance of the noise and damage that may be
done would be secondary.  I suspect you would also have to be careful
coming to a complete stop without keeping your foot in it as to not
stall the turbo, and the motor completely.  A higher idle speed may also
make this possible without stalling the motor.  Of course I could be
completely off.



-----Original Message-----
From: QSHIPQ at aol.com [mailto:QSHIPQ at aol.com]
Sent: Wednesday, September 24, 2003 2:39 AM
To: CaptMagu at aol.com; Trevor Frank; airs41994 at hotmail.com;
s-car-list at audifans.com
Subject: Re: [s-cars] DahlBack Racing BYpass Valve, why not two.



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 located 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



=======
Notice: This e-mail message, together with any attachments, contains
information of Symyx Technologies, Inc. that may be confidential,
proprietary, copyrighted, privileged and/or protected work product,
and is meant solely for the intended recipient. If you are not the
intended recipient, and have received this message in error, please
contact the sender immediately, permanently delete the original and
any copies of this email and any attachments thereto.