[ISTA] Cam, heads & boost control

[Barry Tisdale <btisdale@cybersol.com>]

The tuning continues.  As some of you know, I'm developing a chip for 40#
injectors, 96 ported heads, 20g x 10cm² turbo, a semi mild Isky cam and
5100 rpm shift points.

FWIW, here's what I'm finding -

Max boost obtainable in 1st and 2nd gear is limited by two things: first,
the light engine loading at higher rpms (3600-5100) and the exhaust
pressure blowing the WG open regardless of what the boost is set at.  This
effect is shown by logging a run w/ the WG actuator *disconnected*, i.e.,
no pressure to the diaphragm - boost will hit a peak of around 16# @ 3600
rpms or so, then fall to 12# or so by the 1-2 shift.  Ok, now wire the
actuator shut - WG will never open - same effect, but manifold pressures
are 3-4# higher, like a peak of 20# falling to 16.  DO NOT stay on the gas
like this - you'll spike out of sight on the 2-3 shift!

After trying a couple of 'high pressure' actuators, I'm back to the stocker
w/ an added, adjustable (HEAVY) spring.  The higher pressure actuators were
of smaller diameter, the stocker is 2 1/2" - you NEED that extra area.  A
threaded rod to the radiator support holds the front of the added spring,
can crank in a lot of tension.  The WG 'crack' pressure becomes meaningless
in this situation; the air pressure needed to unseat the WG is just one of
two factors at work - the exhaust pressure is also trying to blow the WG
open.  For a desired boost of say, 20#, your 'crack' pressure may measure
35# or better.  Using a full 3/16" ID line to the turbo & WG quickens the
response, also.

That's the mechanical setup so far.  Chip-wise, the boost needs to be
controlled in 3rd & 4th gears only because of the above stuff; its
self-limiting there already.  What I've got so far is the F70 table values
set just above the spike levels in 1st and 2nd, (around 90 =~ 19.5psi) so
there's *no* boost control there.  At the 2-3 spike, the boost regulation
kicks in (spike is 23# or better), dumping full manifold pressure to the WG
diaphragm (incremental changes up/down set to 100%).  From there up, boost
holds relatively steady @ 20psi after the spike.  Actual boost pressure is
then set w/ the spring pressure, not the modulation of the WG.

Yeah, a good external WG would probably go a long way towards fixing this
problem, but I thought I'd see what I could do w/ the integral WG in the
Mitsu turbo.  Larger turbine housing would probably help also, but then
spooling would suffer, then you need a high stall TC, etc, etc.  WINTER's
the time for dismantling the truck, not summer....}:}E

Just my results & testing - hope its of some value to someone.

Barry - Sy#26