Paul,
By the springs in the wastegate diaphragm, I would say there is at least 5
psi. no hard data, but I think high strung diesel systems and race turbo's
are even more.
Larry


That is a great idea. I have some of those 2nd gen waste gate servos up at
the hangar.
All you need do is pull on the waste gate flap with a small spring scale
and then calculate  the area of the flap.

I just have a TO4 here in the office.
--
Paul Lamar ...No rotor no motor.

Larry,
Tests at Mistral of Turbo charger backpressure, show an average of 2 bar, so
that must be 29 psi.
George ( down under).

I don't believe it. Something wrong. That would be about 40 psi boost
Or 55 psi absolute 110 inches of Hg. A two rotor would be putting out 800 HP or more
and I don't think Mistral is interested in building engines like that.

If it were intake manifold pressure it would be somewhat more believable at 2 bar or
15 psi boost.  Able Ibarra gets 800 HP on alcohol, 18 psi boost and NOS.

You have a decimal point off there somewhere.

Paul Lamar ...No rotor no motor.

Paul,
I was asking about pressure in the combustion chamber just before exhaust.
Gordon mentioned the back pressure on the Turbo they were testing, as
backpressure affects exhaust flow. If atmosphere is 14.71 psi 29 (2 bar) is
only double that at full power.
If that doesn't make sense, I'm sorry I said anything!!!
I'm afraid I'm out of my depth with turbo's.
George (down under)


You did not say absolute. Maybe Europeans assume it is absolute when they
use the term "bar".

Still 15 psi gage in the exhaust manifold is a lot. The boost must
then have been around 25 psi. That is still a lot of boost.

Well it could also have been Gordon's mistake on how he was measuring it.
He may have been measuring  total pressure in the exhaust manifold.
If the pressure sensor was facing the flow it will read dynamic pressure
as well as static pressure. In that case I can believe it was two bar absolute
as the flow is near supersonic at times.

----------------------------------------------------------

I measured a 2nd gen turbo waste gate pressure today.
There are two .71" diameter holes with an area of .4 square inch each.
Total area is .8 square inches. The spring force on the gate is
about 10 pounds so that is 12 psi exhaust pressure maximum.  The
gate starts to open at 12 psi with no help from the intake manifold pressure pushing
on the diaphragm and spring. In the real world the gate would start to  open long before
that as the servo diaphragm is about 3.5  inches in diameter. Area is
about 9.6 square inches.  5 psi boost would then generate about 48
pounds tending to open the  gate. In other words the gate starts to open
at one or two psi boost long before the exhaust manifold pressure gets
to 12.5 psi. At 5 psi boost the gate is wide open. Wow!!! I am amazed.

The stock built in waste gate becomes totally useless for our purposes.
The 2nd gen turbo weighs 22 pounds with waste gate while a TO4 turbo
weighs 16 pounds without a waste gate. That is a six pound penalty for a device
that does not work all that well. The amount of exhaust gas flow is overwhelming
those two tiny holes.