K24 (was "is there such a thing as a K25?")

[Eric_R_Kissell at email.whirlpool.com]

on Fri, 25 Jan 2002 Ken Keith <auditude at neta.com> wrote:
[snip]  Some on this list have swapped between K24 and K26, to try to
>find a good balance between (either) top end power and a early boost
>onset.  [snip]
Ken,
I cannot criticize the top end of the K24 relative to the K26. I have two
items to offer about this.
1. My biggest complaint about my 1.8 bar K24 equipped 1986 5000cstq is
   that I accidently bounce it off of the rev limiter all of the time.
   I just never can feel the supposed drop in performance as the rpms go
   up because they go up so quickly. What I mean is that in normal street
   driving, I have experienced no noticable loss of top end. Maybe the top
   end performance of a K24 is a complaint for those who race or for
   someone who spends a lot of time above 4000 rpm, but I do not share it.
2. According to the turbo maps at www.sjmautotechnik.com the K24 is more
   efficient than the K26 in general. I estimated curves on the map for
   behavior flowing both 1.4 and 1.8 bar and the K24 appears more efficient
   without getting into a overspin condition, unless my estimated curves
   are really wrong.
Furthermore, I believe the settings of adjustable wastegates need to be
evaluated again for pushing 1.8 bar. The reason I looked at a K24 map in
the first place was that I was experiencing a strange new type of lag
sensation when I tightened down my adjustable wastegate. Even though the
turbo was spooled quickly to 1.8 bar, you could feel it kick in as the
engine rpm increased. From my estimate of the curve on the turbo map I
believe that my turbo was spooling to 1.8 bar too fast which drove it into
a less efficient region. Then as the engine sped up the turbo became more
efficient, still at 1.8 bar, and flowed a cooler charge which I believe I
could sense as a lag. What I want is the required mass flow with the
coolest charge I can get, so I decided to adjust my wastegate to build
boost slower so that the pressure curve followed the more efficient zones
on the turbo map. When I did this the strange lag sensation went away. I
have no HP or other performance comparisons to offer, though I could
produce some sort of acceleration numbers sometime since the wastegate is
easily adjustable. I did not check, but the K26 may not spool up fast
enough to cause this behavior.

Has anyone else with an adjustable wastegate experienced a lag sensation
with a spooled-up turbo pushing 1.8 bar?

Eric R. Kissell
1986 5000cstq, 1.8 bar (SJM), K24, Fuchs
1989 200q Avant, 1.5 bar, K24, Bilsteins
1987 VW GTI 16V, 1984cc, Bilsteins
Evansville, Indiana, USA