Cracking Manifolds... er Wastegates

[QSHIPQ@aol.com]

Some discussion on where to put the wastegate pickup and the FV vs the spring
pressure confirms my earlier concerns (QLCC posts earlier this year)
regarding the proper setup for the turbo cars in this respect:

*  New Documented turbo terminology:  Wastegate "Cracking" - Corky Bell's
definition of the baseline spring pressure at which the WG valve first moves
off closed position.  (I referred to this in earlier posts as WG "bleed,"
describing the same concept).  This is due to a combination of two things,
very fast turbo spoolup speeds and/or pulsations in the turbo MAP surge line
causing non linear boost levels at the WG feed line (turbo surging or surge
line).  The concept really boils down to this: A WG with a higher "cracking"
point tends to make the turbo work less hard to achieve boost, creates less
heat, and adds more bottom end torque.  Those 3 items sound good, good and
better to me.

My intepretation and concern earlier voiced, was that turbos, especially
modified ones (btdt x alot) tend to cause a greater amount of cracking due to
a relatively fast boost efficiency curve on the turbo MAP.  A heavy wastegate
spring tends to put the cracking higher in the boost range, since there,
additional pressure at the manifold is available to counteract cracking.
 This method would require consideration of safeguards: 1) The addition of a
Adj wastegate spring perch 2)  The desire for a HKS boost controller, since
this toy can create a counter-pressure tunable to rpm range (i.e. keep the
turbo below the surge line, and maintain high turbo MAP efficiency) 3) the
desire for an absolute boost pressure ground switch to the FPR, precluding
(read cheaper) the need for a true Absolute Pop-off Valve.

Given the non-ideal placement of the wastegate, combined with the relatively
small WG top pressure tap, the FV approach might have more downsides than
just the need for more baseline spring pressure.  At altitude (see my "Turbos
at Altitude" post 7-26-96), the differences probably offset:  The FV delay
and WG exhaust tap from #5 cylinder offset by the danger of more baseline
spring pressure on an absolute (altitude compensating) computer.  Certainly,
the common available tuners boxes include that spring, and many QLCC kids are
learning this as well.  

My suggestion on MC motors:  Get the heavier spring.  If you can afford it,
go with the HKS boost pressure regulator (expensive).  Beyond that, a larger
tap into the WG cap might be a better/faster way to address top pressure.
 Regardless, the tap for top pressure is BEST at the same tap as the lower
part of the WG.  Here it has the fastest response, and on the MC motor,
probably doesn't heat soak the charge air as fast as some would lead you to
believe.  Another consideration would be to just change the FV chip tables to
restrict boost at the bottom (ala 20vt cars) of the WG, and leaving the top
vented to atmosphere.  This seems like a relatively easy mod, since most QLCC
chips already modded that table. Given the above, probably worth
investigating.

On the S4/200/S6 the WG tap is at the Compressor Outlet, which offers the
slowest response, but also the least heat soak, a very touchy subject with
the k24 based turbines.  This k24 address is also accounted for in the WG
design by audi, the 20vt motors use WG FV regulation on the bottom side of
the turbine, which really negates the need for a huge anti-cracking spring.
 However, I do notice that just about all the stage III computers for the
20vt cars include a stiffer WG spring, so some cracking must still occur with
the really high boost levels.

That dumb spring just seems to look better and better in the grand scheme of
things when modding the computers on audi turbocars.

HTH

Scott 
QSHIPQ@aol.com
'87 5ktqwRS2
'84 Urq