VNT- Fact VS Fiction
Vanes Sticking:
The VNT never really suffered from this issue. Although, what does occur is closely related and produces the same effect.
If the vanes were to stick, the result would be reduced cross-sectional area, between the blades. That reduction in area increases back pressure, EGT's, and exhaust gas velocity. Although, carbon and foreign matter (i.e. oil) build up on the blades and in the turbine housing produces the same result. The sweeping action of the blades are constantly being exercised, regardless of the type of driving. The patented three solenoid system switches between a "boost-mode" and a "vacuum mode". The resting position of the vanes are normally closed. Vacuum must be applied to the lower nipple (actuator) to allow for exhaust exit, at idle, P/T, etc.. Pressure is applied to the upper nipple (actuator) while in boost. Both tasks accomplish the same thing. The thought was that if the turbo never saw boost then the vanes would not be exercised and this would limit travel (i.e. "coke-up".) Normal P/T driving can produce EGT's as high as 1500. The build-up will inevitably happen but boost can put enough PROLONGED heat into the turbo and help keep it clean. It is the build-up process, not travel limitation, that causes the rise in "minimum attainable boost".
"Minimum Attainable Boost"
Minimum attainable boost is the lowest boost pressure a turbo can achieve. This was the other problem, with the VNT. The M.A.B. was all over the map. Running this test on three VNT's could produce three M.A.B. levels (8-11psi). The VNT was calibrated to 12psi. If a turbo can't make less than 11psi.. How much room does that leave for system control? This can be checked by placing a hose directly from the intake to the upper actuator nipple. Do not remove the lower! Removing the lower will close the vanes (notice the exhaust noise reduction) causing over-sensitive response. If you MUST try this, BE CAREFUL!
"Adjusting Minimum Attainable Boost"
There are two pins on the center section, of the turbo. They are the physical stops for the actuator arm. Depending on where the arm was welded on, the M.A.B. could be raised or lowered. Other tolerances come into play but are not easily adjusted. Get a baseline number! The lowest "typical" M.A.B. number is 9psi. Take a grinder (dremel) and grind away some of the arm, where it contacts the pin (Bottom pin). LITTLE BY LITTLE ! This may take a while because it will be difficult to work with a hot turbo. After each grind, run the vehicle with the hose connected to the intake (see above). This will obviously produce better results when the starting PSI is high (10-11psi). The end result is less over-boost, more system control, improved drive ability, and less turbo bearing stresses.
Why was the VNT so good yet so bad?
The VNT was excellent at resolving the major turbo issue. Lag ! The concept allowed for great throttle response and mid-range torque. But much like everything, there was a trade off. The VNT generates a huge back-pressure increase. 30% to be exact! Along with it being designed for an engine displacement of 1.5-1.9 max it doesn't breath. The compressor is so small that when in over-boost it cavitates, the vanes close to raise the boost, and cavitation gets worse. All this is going on while the turbo generates excessive heat while it beats the air. Eventually, in some cases, this finishes the turbo bearings off and BOOM. The average operating shaft speed, of a VNT, is 179,000 RPM. This is approx. 40,000 more than a T-II, at the same boost level. The high RPM's and over-boost conditions can make for short lived bearings. A larger compressor can help reduce this phenomenon but this does not address the back pressure issue.
There are other issues that surface when high HP is desired.
VNT- Fact VS Fiction ( Revisited )
"But, where a standard turbo would continue to choke off exhaust or dump it out a wastegate, the VNT opens the vanes and reduces back-pressure. Normal position at idle or part-throttle cruise is something like 40% - 50% according to a Motor Trend article on the LeBaron GTC. "
Opening the vanes does reduce the back pressure, but not enough. This would not be an issue for a properly sized turbo. If you would, think of it this way. Take a T-II unit and weld the wastegate hold shut. The only other passage around the turbine hole has now been blocked (B.P. goes up). Add 12 intregal vanes, reducing the available exhaust path area (B.P. goes up again). Place a 90 degree elbow on the turbine housing ( and again ). Now make everything too small for the application. This is where that 30% comes from. Opening the vanes does reduce the back pressure but not enough.
In comparison: A 2.2 with a standard VNT will have less area under the HP curve than a T-II, at the same boost. Raise the boost, port the head, add 16V's, increase the engine displacement, etc... The restricted HP number climbs even higher. Note: Adding boost will make more power, TO A POINT.
Although I do agree that it is undersized, over-boosting a VNT is no different than over-boosting a standard turbo. Neither is particularly good for longevity.
The last sentence is true. Although over-boosting a VNT is different than O.B. a standard turbo. Why? The shaft speeds, for any given boost, are higher. As the boost is raised, on a VNT, the shaft speed goes up exponentially. In other words, this thing is at warp speed to make 14psi (175,000+). The turbo-II shaft speeds are closer to 140,000-150,000. The difference between the speeds will increase, as the boost increases.
Where did you get this from? The VNT compressor map doesn't even show 179,000 rpm. For the 175 hp at 12 psi engine the VNT was made for, that equates to about 135K rpm on the map. Or was it 14 psi? Even then it's still well below 145K rpm. Even overboosted to make about 225 hp or so, it's still only around 168K rpm at about 17 psi. Maybe by the time you get to 20 psi, it might hit 179K, but that cannot even remotely be considered it's average operating speed.
Where is the intercooler in this equation? The intercooler is just as important as the desired boost, when mapping a turbo for a specific application. This is a good time to mention that the 90' VNT intercooler is the worst flowing/thermal eff. intercooler that was ever packaged in these turbo cars.
This intercooler is longer/narrower than the T-II unit. It has 18 internal fins/in. which places it between the stock T-II unit and the S-60. The fins/inch were reduced to lower the pressure-drop, at the cost of thermal efficiency. Adding to this is the 1/3 area that actually sees "good-air". The other 2/3 is blocked from society by the A/C condenser and the core support. The VNT's heated, HEATED air is not being cooled ! Compressor maps are associated to the compressor housing psi not the intake psi.
"" Maybe by the time you get to 20 psi, it might hit 179K.."" I Couldn't have said it better myself!!!
Again, the back pressure issue is only an issue when the vanes are closed. The turbine on the VNT is positively huge compared to the compressor so even with half-closed vanes at cruise, back-pressure can't be that bad.
Back pressure at cruise is not the issue! The vanes are not full-open at cruise to keep the turbo awake. Tipping into the throttle closes the vanes even further, releasing boost response that was already "ready to go".
VNT
functionality:
"A
picture is worth a thousand words!" SO.... Three times the space would
have been used to explain this. This
data can be seen using a DRB. The flight recorder accessory will obviously
make it much easier. Drawing
a vertical line, through the graphs, will associate the events.
==================================================================
Throttle
100%
...............
.
.
........ .
.
. .
.
.
0%..............
.....
.......
==================================================================
Actuator (vanes)
100% (open)
..
..........
...............
.......... ...... .
. ....
. .
. .
. .
.
.....
. .
0% (closed)
.
==================================================================
Boost/Vacuum
.
14psi
............ .
..
.
.
-- '0'
. ..
.
. ..
..............
.......
.
. .
...........
..
30inHg
==================================================================
A turbo 2.5 with a Mitsu is real close to the response/torque/HP (at the same boost) of a 2.2VNT. The realistic HP for the 2.2VNT is 167. That number on a T-II is 175, for the same boost level. If you are looking to use the VNT because that is the intended power train for your car.. go for it.
The 2.5
is even farther from the intended engine displacement of choice, for the
VNT. I am not saying don't do it OR that it cannot be done. Anything can
be done with enough dedication. I also failed to indicate that the VNT
cars came with a muffler with 22" (of H20) back pressure muffler while
the T-II cars came with an 18" system.
This was done to help control the worst case M.A.B. turbos and O.B. during
colder climate. Your application may require some unique attention.
There were warranty concerns but the 2.5 was one big reason. With good response and torque, it was a huge threat to the continuation of the VNT program. The VNT suffered from so many things including: turbo over-speed, over-boost, turbo failure, TSB's, actuator failure, high unit cost, low market-ability, etc... A great deal of time and energy went into that system and then the 2.5 Turbo-I comes along, coming very close in the performance/drive- ability arena. This is all happening when the 2.5 has also received the use of the common block, which was a substantial cost REDUCTION.
Has time shown that concern to have been justified ?
It came down to an unjustifiable/high cost package; Whether warranty OR unit cost was considered.
Were all the VNT's produced in the beginning of the year?
Nearly all of them were produced in the '89 model year. The Shelby units were typically completed within the first 3-4 months, of '89; While the 1990 vehicles were primarily made during the last three months, of '89.
made to order or just produced and sent out to some dealers ?
Most of them were Chrysler lease vehicles. Some were even leased, with the intent to purchase, and put away as investments. A good number of these vehicles exist this way today, with practically no road time. The vehicles that were driven exposed A LOT of the problems that would soon be realized by the customers. This raised serious concerns and received a large amount of attention. This attention would lead to the cancellation of the program. The dealers received a low number of vehicles, just enough to expose the package; But the cancellation happened before the dealer stock could ever really get off the ground.
How do these VNT's differ from the CSX?
Some noticeable differences are: the radiator, intercooler, air-box, induction hoses, and metal lower turbo tube (over the trans).
A closer look exposes the '89s use of a 555 trans while the '90 received the 568.
Engine
control system refinements and turbo changes allowed for a lower back pressure
muffler to be used, on the 1990 vehicles. The CSX was rated at 22inH2O@5200;
While the '90 unit was rated at 18inH2O,
at the same RPM.
This is a valid concern! Intercoolers can be very useful and are a large contributing factor, to making good/safe power. BUT, if the intercooler in improperly matched to the system, it can cause other problems to arise.
If the I.C. is too restrictive: The intercooler plays in the role of many things. This can include wastegate duty cycles, compressor heat generation, turbo shaft speeds, intake temps, exhaust manifold back pressures, and the list goes on.
For example, if a poor choice is made: The turbo can be required to make 3-6psi more psi, at the compressor, to make the same boost at the intake. This higher demand will cause excess heat at the compressor, which places a higher demand on the I.C. The return on investment is greatly diminishing, at this point.
In addition, the turbo thrust plate and bearings can be placed beyond an acceptable range of operation.
On the exhaust side, the wastegate will be required to close down to increase the boost. Now all of the above is operating with the addition of increased exhaust manifold back pressure.
Is it possible to place an intercooler on a vehicle and go in the wrong direction/break even/cause durability issues? YES. The intake temps may drop but potential issues are being generated, in other locations.
In summary, an inefficient I.C. may come with a high back-pressure price tag.
I want to place it down in the nose, behind one air dam and behind the other will be a K&N cone filter.
This can work very well! Please remember that the amount of turns should be kept to a minimum. This can make the I.C. positioning (top to bottom VS side to side) an important contributing factor, to a successful package.
Think this will work??
Functionally, it could work. From an efficiency point of view, consideration could be given for a better suited intercooler. The SVO/Thunderbird intercoolers can yield excellent temp. reduction (high fin per inch count on the internal transfer tubes) but come with a high backpressure price tag.
Thought:
A large amount of intercoolers use an epoxy end plate sealant. This epoxy
is added after the end tank welding process and dries extremely hard. It
was used to fill the "pin-holes" generated/missed by the end-plate welding
process. This epoxy can with stand the turbo air temps just fine BUT can
be damaged/destroyed during an end-tank welding modification. The result
can be small holes/leaks in the intercooler. This not only can result in psi
loss, but also becomes an un-filtered air inlet. Be careful when welding
and pressure test the end product, under water.
The consumed
space (air) remains the same. What does change, is the air density (oxygenated
air).
Example:
When discussing or comparing air characteristics above or below sea level,
the discussion pertains to air density. The air density is not the question
of how much volume of air is available. It is.. Of that volume, how dense/oxygenated
is THAT air.
"A picture is worth a thousand words"
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* ** ** * *
* * *
* ****** * *
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********** **********
1 cubic ?? of air: Same volume, different density.
When the intake charge is cooler, the charge has become more dense. The charge density increase makes the air more difficult to compress. This results in higher combustion chamber pressure, for any given boost level. It should also be stated that the higher density comes with the benefit of a higher oxygen level (diagram above). It is this increase in charge density that results in higher engine output, not because of an increase in the volume of air.
It should be stated that, of the elements found in air, we are obviously focusing on Oxygen.
If you're already running your turbo at max, what you may perceive to be a pressure drop across your intercooler could be nothing more than a temperature drop.
Removing the intercooler, in this case, may prove differently. An increase of delivered psi, to the intake, should be realized.
More air is necessary to maintain the same pressure at a lower temperature, and if your turbo is pumping all the air it can pump already, your pressure won't be as high.
There are two separate things occurring here. The air necessity statement may be the other way around and shifts the conversation to the compressor requirements as opposed to the inter-cooler discussion. It takes more air (cfm flow) to produce the same pressure at high temperatures. Why? Air density (thinner air/lower Oxygen content). With high ambient temps, the turbo will work much harder to make 14psi than it will during cold ambients. Why? The hot air compress' easier than the dense/cool air. This results in higher shaft speeds to accomplish the same boost level and elevated compressor outlet temps.
Shaft speeds, at the turbo, can decline as much as 5-15,000 RPM during cold weather.
If a turbo was run open-loop (no boost control) during the hot ambients and made 14psi; Then it would be possible to realize 15-16psi, during cold ambient operation (open loop). VNT's amplify this scenario!
To fully test the overall *effectiveness of an intercooler*, the incoming AND outgoing temperature AND pressure would have to be measured.
Absolutely correct! But the variable of temperature across the OUTSIDE of the I.C. should be added, to the above statement. (i.e. higher efficiency in cold ambients) The temperature across (ambient) the I.C. is as important as the differential temperature through (boost in/out) the I.C.
There is then a formula that would then be able to tell you the actual VOLUME of air coming through the intercooler, then the volume would have to be weighed against backpressure and so forth to determine the most *beneficial intercooler* for your needs.
This is where things can get tricky. The volume of the air does not change. If (for conversation purposes only) 350cfm is delivered to the intercooler, then 350cfm will flow out. But even with the volume the same, something has changed and that is the air density(mass) and pressure.
One of the largest contributors to the pressure loss is the friction placed on the air, as it travels on the surface of the transfer tubes/fins/etc.. and the restriction generated by turbulance/poor tube entrance/transfer tube geometry (fin clearance, tube size, etc..).
The formula (gas law) that I believe you are referring to, is as follows: PV=nRT where R is the universal gas constant.
You can't just go by a bench test of pressure drop across an intercooler....
Although, the continuation of your sentence says it VERY well.
even though this is a good method due to the smaller effects of temperature, when on a bench.
As you mentioned earlier, it would take test data at various pressures, temps. (both through and across the I.C.), and volumes to make an accurate calculation. But the psi drop across the intercooler can be a good reference point.
Likewise, you can't just go by fins per inch, as some air tubes can be wider, fatter, thinner, longer, shorter, and have thinner walls, thicker walls, be painted or not, etc....
Great
point! The reference to the fins per inch was made because the transfer
tubes are close in size (SVO VS Chrysler). It is the fins per inch that
are much higher on the SVO unit. When these differences occur (from your
list above), they would be included or added as variables to determining
the best unit, for the application.
But, ultimately it comes back to the back pressure issue,
When? Idle, mid-throttle, high-throttle/high RPM???
Idle
= 15 minimum
Cruising
P/T (low RPM's) = 20-35
High
RPM/High load = 35+
I should
add that pressure is a factor BUT volume flow is indeed, as important.
I do not nor have I ever run an increased pressure pump on any of my applications.
Porting the oil pump and reducing the rotor to plate clearance is about
the best that can be done. The old MP increased pressure pump is a "no-no"
in my book.
COMPRESSOR
SURGE:
This
can be achieved two ways, but should be considered separate issues. One
way, as posted earlier, is the non-bypass/closed throttle in boost scenario.
The other involves shaft speed VS flow VS PSI. When looking at a compressor
map - There is a line to the left which is labeled "Surge Limit". This
is the point (Shaft RPM VS flow VS psi) where the turbo is unstable.
What
is happening in the turbo?
The
turbo shaft speed will oscillate to maintain a given psi/flow. Basically,
this is the turbo functioning outside the efficiency range. Not in
all cases does this mean that the turbo is being over driven. An "opposite"
example of this would be running a larger turbo too slow OR below the efficiency
point. The turbo will spool, generate boost, the boost will rise and over
shoot the goal, the wastegate will open, the shaft speed will drop, boost
will fall, the waste gate will close down slightly, the boost will rise,
and the whole cycle starts all over again. The turbo is operating too slow
and cannot control the the low boost setting because it is below/outide/beyond
the surge limit. This puts tremendous torsional and end loads on the shaft.
This will cause premature thrust plate and bearing wear and/or failure.
In a worst case scenario this will may allow the blades (compressor or
turbine) to come in contact with the housing. Usually, prior to this, noticeable
oil loss would have been seen due to the destruction of the seals. They
cannot compensate for such loose tolerances and component wear.
Alright, here goes nothing. Ever since I got my head gasket replaced, my car won run more than 3 or 4 pounds of boost. I've verified this with a real boost gauge, so instrumentation isn't the problem. I also had a backpressure test done and everything there was fine, too (I thought my cat was clogged) so, just to see what happens, I crank the manual boost controller a little bit (which has always given me more boost) and here's the surprising part-- the boost still stays at 3 lbs! Anybody out there in turboland have any thoughts, comments, ideas?
Just a thought, Sounds like the actuator arm has fallen off. The Turbo-I (Mitsu) C'-clips were made from low grade materials and were known to rust and then break. Reach back to the actuator and with low effort pull up on the arm. Obviously it should not come upwards.
If this
is the case: Short term fix..
The
actuator mounting bracket is also a low grade material. By pushing, apply
a force on the wastegate that would angle the arm down OR closer to the
turbine housing. Then place the arm back onto the bypass lever. It only
takes a slight angle to accomplish this. The continuous downward angle
pressure will keep the arm from coming off. Long term.. Replace the
clip.
Other thoughts:
The solenoid
is stuck SHUT.
The hose/'T'/hardline to the solenoid is pinched/plugged.
NOTE:
If it were broken/leaking, default boost would be 10-11psi.
Failed
actuator spring.
Leaking
bypass valve.
There are a few tests, that can be done, to determine the origin of your problem.
FIRST: Is there another code? A-side/B-side/solenoid 1,2,3 etc....? Usually a solenoid failure (electrical fault) will issue another fault. So based on that, we will start with the plumbing and actuator.
1.) Check
the vacuum hoses. (hard-line to nipple ends especially)
a.) connections at the solenoid's (1,2,&3)
b.) connection at the compressor housing (if used)
c.) connections at the intake
d.) connections at the actuator (top & bottom)
2.) Check
the actuator connection.
a.) actuator arm to turbo swing arm
3.) Check
the actuator functionality
a.) Top nipple- apply pressure "does it open?"
b.) Bottom - apply vacuum "does it open?"
4.) A little more involved..
The selector for the vacuum to boost transition is the large black solenoid mounted on the passenger side strut tower. Unlike the others, this solenoid is not sealed from the elements. Moisture can make its way in and contaminate/corrode the thin copper winding, spring, and pintle.
Test: Apply battery voltage to the solenoid and listen for pintle (clicking) activity.
DO NOT HOLD THE WIRES ON THE SOLENOID ! If the solenoid is a shorted or open circuit, it would set a fault. BUT if the solenoid is stuck (dirt,rust,etc..) it will not set a fault and the system (pneumatically) will not work properly.
FYI: Apply MOMENTARY Batt. voltage to..
Injectors:
To test and un-stick the pintle,
Solenoids:
To test and un-stick the pintle.
This
works VERY WELL ! Reminder=Momentary!!!! (1 second ON, 1 second OFF, etc..)