X-Mozilla-Keys:
Chuck wrote:
.............. I set throttle at 4,400 rpm (half prop pitch in) which was
about 2psi boost (34" of 40" avail),
snips.........
You lost me here Chuck. What do you mean (34" of 40" avail)?
BTW what TO-4 trim are you using?
Paul Lamar
charles.harbert@fmglobal.com wrote:
Paul, I have a twin engine manifold guage in the plane that I use to monitor
available boost (absolute manifold pressure (map)) (before throttle plates
(tp)), and actual engine boost (after throttle plates). I wanted to make sure
that the waste gate was operating properly, and later on while flying at higher
altitudes, I can adjust the waste gate to make more boost from inside the cabin
(overide the waste gate spring).
I gave you both readings, so you could see what boost was available (before tp),
and what the engine was seeing (after tp). Normal sea level pressure is
roughly 30" , or 0 psi (15 psia) boost, so 2 psi (or 17 psia) is 34" map. The
40" available is 5psi (20 psia) which is being limited by the 5 psi waste gate
spring.
This was what I wrote about last week as to why I thought it was a good idea to
use a waste gate (a big one) for the T-04 on the 13B. Otherwise, I think that
with a turbo 13B, the available boost could be too high, plus you'd be making
hotter air and wasting energy creating this higher pressure. I recognize that
the 13B will have 1/3 lower mass flow than the 20B, but a T-04 is a big turbo.
I'd also suggest that the waste gate exhaust be run back into the main turbo
exhaust pipe for noise suppression. These make a lot of noise when they open,
and I don't think you give up much doing it this way than having a seperate
pipe..
I haven't looked at the Turbonetics invoice in a yr, but I believe it was an A/R
ratio of 1.3. I'll check it when I'm next out at the plane and let you know.
Here is what Corky Bell says about the subject. Maximum Boost Page 144.
"One of the key items in integrating the wastegate into the system is
the loca-
tion of the exhaust gas bleed-off from the exhaust manifold. This
feature is
critical, because it determines such things as load balance between
cylinders,
accurate and quick response of the gate, and, in part, turbine inlet
pressure.
The bleed-off must vent from a location where the pulses from all
cylinders
have been collected. This virtually always means the manifold, close to
the tur-
bine mounting flange. Symmetry and easy flow paths are ideals for laying
out
a wastegate system.
It is vital that exhaust gases be given an easyjob of changing
direction from
the route toward the turbine to the bypass through the wastegate. If
flow has
any difficulty whatsoever changing direction to exit through the
wastegate,
the ability to control boost in the higher rev ranges may simply
disappear.
Exhaust gas return from the wastegate to the tailpipe after the
turbine
should receive the same forethought as gas entering the wastegate.
Theprin-
ciple here is to avoid interfering with exhaust gas flow exiting the
turbine. In-
terference will raise exhaust back pressure, thus reducing power. An
integral
wastegate will usually channel bypassed exhaust gas back into the system
im-
mediately aft of the turbine wheel. This is acceptable for economic
reasons but
is not in the best interest of power. A few integral wastegate designs,
like some
models of the Japanese IHI, have provided a separate exhaust pipe for
the by-
passed gases. When this separate pipe is available, it should be taken
advan-
tage of and routed some distance down the tailpipe before being dumped
back
into the main exhaust system. A minimum distance would be 18 inches.
As discussed in Chapter 11, the ideal circumstance for bypassed gases
from
the wastegate is a completely separate tailpipe; This offers the most
positive
wastegate response, lowest back pressure, and no interference with flow
through the turbine. Perhaps not cheap or easy, but a maximum-effort
turbo
system will have a separate tailpipe for the wastegate, with suitable
consider-
ation given to thermal expansion of the tube without any cracking of the
joint
between tube and tailpipe"
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