Review: TAP Adjustable Bypass Valve (rough draft, VERY long)

[Jimmy Pribble <jpribble@trammellcrow.com>]

Listers,

By all means, please correct me, enlighten me, and/or comment on this review
before I upload it to my page (along with photos) later this week.  I would
be especially interested in other replacement options that I don't have
mentioned.  I keep hearing one about Demon Tweeks, but I need more details.
If you want more details in the review, please let me know and I will try to
oblige.  I have no affiliation with TAP whatsoever.

Best regards,

Jimmy Pribble
1993 /S4 quattro

Jimmy's Audi S4 quattro Pages
http://pweb.netcom.com/~jkp.mail/index.html
  


Review: TAP Adjustable Bypass Valve

Pros: quality construction, adjustable, rebuildable
Cons: expensive, not enough documentation, annoying fruit colors

Note: The bypass valve to which this article refers, goes by many different
names.  Audi calls it a cut-off valve, TAP calls it a dump valve, and others
call it (incorrectly) a blow-off valve.  For the purposes of this article, I
will call it a bypass valve, since that best describes its function.

Background on Bypass Valves
     The bypass valve diverts air back into the turbo inlet during closed
throttle conditions (esp. while shifting), in order to keep the turbo
spinning, therefore reducing turbo lag between shifts.

Bypass Valve Failure
     Symptoms of bypass valve failure are a "chuffing" or "fluttering" sound
when coming off the throttle.  Also, a low boost condition might indicate a
failed or improperly installed bypass valve (backward installs are
possible).

Replacement/Upgrade Options
OEM Audi Bosch Unit - (Audi 034 145 710 C) or (Bosch 0 280 142 108)
OEM Audi Sport Quattro Bosch Unit - (Audi 035 145 710) or (Bosch 0 280 142
102)
TAP aluminum bypass valve (non-adjustable)
TAP adjustable bypass valve

     The OEM Bosch bypass valve should be available from a discount supplier
for less than $50 and is most suitable for cars running stock boost
pressures.  The Sport Quattro Bosch unit (also used on the Porsche 944
Turbo) has a thicker and differently constructed diaphragm, but is otherwise
identical to the stock unit.  The original TAP aluminum bypass valve (which
may be discontinued, being superceded by the new adjustable unit), was
available for $145 and has the advantage of being rebuildable.  The TAP
adjustable bypass valve is reviewed below.    

Review
     The bypass valve was $181 delivered (3rd day ground) and arrived on
9/23/99.  It came packaged with a sheet of instructions and a TAP catalog.
The instructions (reprinted here), did not include installation
instructions, but were simply instructions for adjusting the valve.  Because
of the ease and frequency of backward installations, I thought this was an
unusual omission.  The bypass valve itself appears to be very nicely
constructed. It is about twice the size and weight of the OEM unit.  The
boost pressure adjustment (spring preload?) is a 10mm square nut located at
the vacuum nipple that leads to the intake manifold (IM).  The nut rotates
in 2psi incremental "clicks" to give between 12 and 36psi of adjustment.
The nut locks in place with a clip.  Rather than using a rubber diaphragm,
the TAP unit uses a polyurethane plunger.  The bypass valve came in red
anodized aluminum, which didn't look as bad as I thought it would (though I
still would have preferred the raw aluminum finish).  It is also available
in blueberry, er...I mean blue. 
     Installation is a snap and requires only a flathead screwdriver.  The
bypass valve is suspended by three hoses.  Loosen the three hose clamps that
are holding the hoses onto the bypass valve, and pull the hoses off the old
unit.  When installing the new bypass valve, make sure it ends up looking
like the photo below; do not assume your previous bypass valve was installed
correctly and simply hook-up the new one the same way.  *note: I had to cut
the small hose (IM) off of the OEM bypass valve and reuse it on the new one.
This is no problem, except that you will want to have a small hose clamp
ready, since the hose was either a part of the OEM unit, or it was glued
onto it, so there is no clamp to reuse.  Also, because the TAP bypass valve
is longer than the Bosch unit, the hose to the IM needs to be shorter
anyway, or else it could bend into the fan that is nearby.  

     Since my car is otherwise stock and I don't have a boost gauge, I left
the TAP bypass valve set at the factory default of 12psi.  It is my
understanding that the factory Bosch unit opens at about 6-7psi and the
Quattro Sport unit opens at about 7-8psi.  On the road, my car lost the
"fluttering" sound off-throttle, and replaced it with a noticeably louder
boost sound on-throttle.  Before, I simply heard a "whoosh" on boost.  Now I
am hearing that sound along with a distinctive whistle or high-pitched
turbine sound.  The car doesn't feel any faster, so I don't think I regained
any lost boost (as some people do when they replace their failed bypass
valves), but I did several 30-90mph runs, which involves a 2nd to 3rd shift,
and I know I was getting there faster.  My conclusion is that the TAP bypass
valve is operating correctly and my boost is staying up between shifts.
     I am very pleased with this product.  It seems to be sturdily built and
I don't expect that I will ever have to worry about it, even after I
eventually turn up the boost.  I don't know what good the adjustability will
provide, but perhaps I will find out after I get a boost gauge and
experiment with it.  Please note: It seems that one could create a false
failure with just the available adjustments (by setting the bypass pressure
too high, and thus preventing the valve from opening) , thereby creating a
dangerous condition for the turbo. I would advise caution when experimenting
with this device.  Anyway, despite the expense, the need for better
documentation, and the annoying iMac-like colors (hey kids, collect 'em
all!), I think the TAP unit was well worth the cost for me (but hey, I like
over-engineering as much as the next guy).

TAP (Total Audi Performance)
www.tap1.com <http://www.tap1.com>
Contact: perform@tap1.com
6241 S. Tex Point
Rooks Ind. Park
Homosassa, FL 34448
Phone: (352) 628-2834
Fax: (352) 628-6661

Universal Piston-Type Turbo Dump Valve - $175