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Discussion Starter · #1 ·
I am curious to know if anyone has measured the exhaust pressure between the head and the turbo. I am interested in the ratio between boost pressure and exhaust pressure.

Discussion Starter · #4 ·
You have to remember that everything in our world is inefficent.

IE: We have to put in more to get out a certian ammount. But that
ammount will never be equal (at least with current technology)
to how much work we put in to begin with.

a generator cannot power an electric motor that turns the
generator itself because of it's lack of efficency.

just as equal, if we were to measure the pressure in the
exhaust manifold pre-turbo and the pressure in the intake
manifold post-turbo. the pressure in the exhaust manifold
will always be higher for a given pressure in the intake manifold.

Your ratio is going to largely depend on the ratio of your exhaust
turbine to the ratio of your compressor turbine. equal size
turbines and equal A/R's will carry a pressure ratio that are -CLOSE-
to one another:

IE if you have A/R's of .64/.60 then you could have pressure
ratios of 14PSI/17PSI intake/exhaust respectively.

I wouldn't know the exact ratios.

What you should really be interested in is just porting and
cleaning up the exhaust ports and the manifold for free flow, not
to reduce backpressure; because it's going to be there to begin

However, everything after the turbo needs to be free flowing as
possible; backpressure here only works against the turbo, along
with the compressed intake air which is trying to force the turbo
to stall and spin backwards.

what alot of people don't understand is that any turbo has a
compressor stall point; a point to where it doesn't matter how
much exhaust you're flowing, the design of the blades cannot
keep the intake air from stalling the compressor and making it
flow backwards.

If you try to run a stock CT-26 at 20PSI you'll stall it. the action of
it stalling and running backwards to relieve intake pressure is
what ruins it; as now it has two forces acting upon it only
backwards. the compressor is trying to relieve pressure while
the exhaust is trying to pack pressure into the manifold; of course
the pressure there will build so much that it will break the blades
or the point at where the blades mount to the shaft; rendering
the turbo useless.

if you've ever heard of someone complaining that they ran their
stock CT26 at 20PSI for a month and then it stopped working;
they can probably take off the manifold and wiggle the turbine
on the exhaust side.

It's very dangerous to run a turbo to it's limit and beyond; the
blades could break from the shaft and rip completly through
the turbo housing and go through your car; and you.

always remember, some are designed to spin to 200,000 rpm;
most spin up to 60,000 and beyond. if you were to accelerate
a piece of aluminum that fast, it would pierce steel.

if you've ever picked up a turbine blade (I have) you already know.
them things aren't dull, they're extremly sharp and will cut you

Discussion Starter · #5 · (Edited)
Oh, I'd like to reccomend anyone who is serious about getting
the most information out of their vehicle;

some companies make lasers which read turbine RPM's. i'm sure
they're not cheap, but they come with the laser, wiring and
electric analog gauge.

I believe the one I was looking at started reading at 40K rpm
and went up to 200K.

Ah, yes; AVL makes one.

TS350 turbo speed sensor.

Optical measurement principle
Maximum speed: 200,000 revolutions per minute
Speed-proportional analog output signal
(0 V ... 10 V corresponds to 0 rpm ... 200,000 rpm)
Wavelength: 645 nm (laser)
Digital output (counter)

it doesn't have a price tag, you'll probably have to try to find it
through another outlet; I wouldn't mind paying $500 for one.

Ah, more information.

the datasheet reads slightly different than the quote above;
The AVL TS350 Turbo-Speed Sensor is specially designed for contactless
measurement of a turbocharger's speed up to 200.000 rpm. The sensor is installed
by means of an adapter at the turbocharger's air inlet. A trigger box is used for power
supply and for converting the analogue output signal to TTL signal.
The measurement is carried out optically, based on the laser beam (645 nm)
reflection and detection principle by means of photo-diodes at the turbocharger’s air
inlet. No modification of the turbocharger is necessary.
The speed range is 6000 rpm to 200,000 rpm (100 Hz to 3300 Hz) and the signal
range is 0 V to +5 V.

583 Posts
I'm glad to see you trying, but unfortunately you aren't correct. Your first point about the EBP always being higher than the MAP, would be true except you forget that we are adding alot of energy to the system by releasing the potential energy of the fuel. It would be nice if all that energy went into turning the crank, but like you said all things are inefficient. Second the turbo never ever spins backwards (maybe if you had a big intake backfire at idle and thent he engine died) If at one point the turbo is making 19psi and spinning XX,XXX RPM to do it it cannot instantly reverse direction at 20 Psi, there is no way that little shaft could handle that.

It is possible to get the MAP higher than the EBP but not very likely on any kind of turbo setup any of us would use.


Discussion Starter · #7 · (Edited)
well at least I tried :)

There is such a thing as compressor stall. I'm probably describing
it wrong.

ah yes, I had my terms confused; I was corrected, there is no
actual reverse rotation on the turbo if you surge it far enough;
it's "impossible" as one has said.

yet the effects of compressor SURGE alone is what kills turbos

stall is at the opposite end.
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