Re: Bypass valves and turbos

[QSHIPQ@aol.com]

In a message dated 9/19/99 8:56:51 AM Central Daylight Time, 
smillsx@earthlink.net writes:

> >Replacing the bypass valve will fix it, with either stock plastic, or metal
>  >rebuildable, available from TAP. I'm undecided, so I'm letting it ride 
for 
> a
>  >bit...

If you have a k24 turbo, don't let it "ride for a bit"... 1) The pressure 
spike from the throttle shutting sends a pressure wave back to the blades of 
the turbo, causing either a stall (best case scenario) or a broken shaft 
(probable scenario). 2)  If the bypass valve is leaking at all, you are 
making the turbo work harder to make the manifold boost the computer is 
looking for (technically, you are making a leak in the pressure hose).   I've 
replaced a couple k24 turbos on both 91200 and S cars that only had a failed 
bypass valve when looking for the failure cause.  The one from TAP is 
serviceable, and the improved one, adjustable.  

>  
>   An observation from Monterey- All of the factory race cars had what at
>  least appeared to be the stock _plastic_ bypass valve, as did Ned Richie's
>  "Yellow Hawk". This is a pretty simple part- if it works for the 700+ HP
>  GTO car, I don't think a stage III AAN will overstress it ;-)

Different part number.  Minimum it would use the Sport Q bypass valve which 
has a thicker diaphram membrane.  Secondly, even the sport q's bypasses fail 
regularly, the plastic ones I saw at MH99 were nice shiney new; replacing the 
valve every race is different than what most on this list might tolerate :).  
 Neither the stock or the SQ bypass valves aremeant for regular hits above 
18-20psi, so a stage III will way overstress it, in fact.  The first Sport Q 
bypass valve I used almost 6 years ago, failed within a year with less than 
20psi of peak boost.  I currently use the HKS sequential bypass valve in my 
10vt, and it has performed well, but few basic issues with it (a whole post 
is dedicated to this valve in the archives - in summary):  1) it vents to 
atmosphere, so the "measured air" theory goes out the window 2) it's made of 
chrome plated plastic  and 3) for the street price of close to 300USD it 
should be made of some metal.  4)  It's secondary opening is only slightly 
larger than the stock audi bypass valves, meaning 5) 2 audi bypass valves 
would be more effective AND cheaper, 6) the "progressive" really doesn't 
apply to the audis, when they dump, progressive doesn't come to mind (it's 
built for small japanese turbos with little excess airflow).  I don't 
recommend the HKS unit (like you couldn't tell:).

I have yet to rebuild any of the TAP supplied units (couple dozen sales in 
the last 3years), it's a direct replacement for stock, and it's serviceable.  
No endorsement, but sure makes it's own sell after re-doing a stock valve in 
less than a year, btdt.  The adjustable feature (no experience with it yet) 
makes it even more desireable, especially for multiple bypass valve 
applications.

Bottom Line:  Make sure you have an operating bypass valve on your turbo car, 
and that it works properly.  Not doing so, can overspin your turbo from the 
leak at WOT, and/or break the shaft of the turbo from a pressure spike at 
closed throttle.  A simple effective toy, that all turbo cars should have.

HTH + my .02

Scott Justuson
QSHIPQ@aol.com