K24 vs RS2 Spool-up. RS2 Results is now turbo comparison...

[Robert Myers]

At 06:42 PM 8/5/02, auditude at cox.net wrote:
>I thought a Schrapnelknobben was different than a simple bleed, in that it
>applied regulated boost pressure to the upper WG chamber.
>
>Is a Schrapnelknobben (the original Timmerman mod) a bleed, or an upper WG
>mod, or either?


You are exactly correct.  The schrapnelknob consists of an inexpensive
surplus gas pressure regulator which admits a specific (but adjustable)
maximum pressure into the top of the wastegate.  The pressure is derived by
tapping into the pressurized intake system.  This acts as if the wastegate
spring is stiffer actually than it is.  Use of the schrapnelknob by itself
in the older systems for which it was used will lead to the dreaded
overboost cutout.  Paul Timmerman also devised a resistor network
arrangement which essentially told the ECU a lie.  The lie was that the
pressure is lower than it actually is.  This allowed greatly increased
boost levels (such as 18 psig vs. 4 or 5 psig) and significantly increased
performance as a result.  It also could lead to overly lean mixtures at
elevated boost levels which could result in overly high combustion
temperatures and burnt valves and pistons.

BTW, the lie prevented the in-dash boost gauge from indicating true boost
pressure since the pressure transducer output was electrically clamped at a
lower than actual value and the gauge displayed this artificial value
rather than the true one.

Essentially, the schrapnelknob plus the resistor network provided the
boost/performance gains of a modern chip mod but did so on the cheap and
without some of the fuel mixture safeguards which are more desirable.

I used Paul's schrapnelknob along with an alternative zener/resistor mod on
two '89 200 tq's for over 60K miles each.  I saw no indication of damage
from lean mixtures.  YMMV.  BTW, it was a FUN ride.  :-)  AFAIK, one of
these cars is still being run by another list member.  I have no idea of
the current mileage or if these mods are still in effect.



Bob
*****
  Robert L. Myers   304-574-2372
  Rt. 4, Box 57,  Fayetteville, WV 25840 USA   WV tag Q SHIP
  '95 urS6  Cashmere Grey - der Wunderwagen    ICQ 22170244
  http://www.cob-net.org/church/pvcob.htm
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