Px script - automatic analyzing the cylinder pressure waveform
Px script
Contents
1. Purpose
2
2. Recording the waveforms and starting the script
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3. Analysis results
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3.1 The "Report" tab
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3.2 The "Quantity" tab
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3.3 The "Valve timing" tab
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3.4 The "Ignition timing" tab
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3.5 The "Inlet" tab
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3.6 The "Exhaust" tab
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Px script - automatic analyzing the cylinder pressure waveform
1. Purpose
The Px script automatically analyzes the cylinder pressure waveform and generates a print out or
report with a number of additional parameters and characteristics of the engine and the
associated control unit. The calculated values are pneumatic and geometric characteristics of the
cylinder; the list of found deviations is displayed in the form of text messages. To improve speed
and accuracy of valve timing research, the cylinder pressure waveform is converted into diagram
of the gas amount in the cylinder and is displayed in two different ways, using a script.
A detailed diagram of the cyclic filling of the cylinder during the intake stroke, which
characterizes the properties of the entire intake manifold of the engine is also provided.
A diagram showing the energy consumption for scavenging exhaust gases from the cylinder is
provided as well. Using the above diagrams and the the ignition timing signal, the ignition timing
diagram is built and can be displayed.
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Px script - automatic analyzing the cylinder pressure waveform
2. Recording the waveforms and starting the script
To display the waveform, we use a transducer that converts pressure in to voltage. The voltage
output can then be displayed as a trace on an oscilloscope screen.
Recording the waveforms for the Px script
● It is recommended to disconnect the fuel injector for the cylinder to be diagnosed
if the engine to be diagnosed is equipped with a fuel injection system. The engine control
computer may set a DTC (Diagnostic Trouble Code) for injector open circuit on the
disconnected cylinder. In some cases a
100 Ohm resistor can be connected to the
disconnected circuit to avoid setting trouble codes. Alternatively, a scan tool can be used to
erase the trouble code after the testing is concluded.
If fuel supply to the cylinder being tested is not interrupted, hot surfaces in the cylinder can
cause ignition of the air-fuel mix which could cause damage to the pressure transducer.
Additionally, the unburnt fuel can wash down the cylinder walls and cause wear and / or loss
of compression. The low tension piston rings used on late model engines are very susceptible
to loss of ring seat and compression when washed with fuel. The unburnt fuel entering
the exhaust system may also cause the catalytic converter to overheat.
If it is not possible to interrupt the fuel supply to the diagnoses cylinder, such as with
carburetors or throttle body injection, allow the combustion chamber to cool for
approximately 5 minutes. A good procedure is to remove the spark plug, wait 5 minutes, then
install the pressure transducer. To avoid damaging the catalytic converter due to the unburned
fuel entering it, it is recommended to reduce the duration of the measurements to a minimum.
In any case, the duration of the measurement should not exceed 3 minutes.
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Px script - automatic analyzing the cylinder pressure waveform
● Install pressure transducer Px or Px35 1 in place of the spark plug for the diagnosed
cylinder
and connect it to the input №3
of
the
USB Autoscope IV.
If using the USB Autoscope III, USB Autoscope II, USB Autoscope I, or USB Autoscope,
connect the pressure transducer to input №1.
The plug wire for the disconnected and removed spark plug must always be connected to a
spark tester with the spark gap set to about 5 mm.
The pressure transducer replaces the spark plug.
1
High pressure (in cylinder) transducer Px35 is compatible with USB Autoscope IV and USB Autoscope III
but, is not compatible with USB Autoscope II, USB Autoscope I and USB Autoscope.
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Px script - automatic analyzing the cylinder pressure waveform
If necessary, use the deep well adapter when installing the pressure transducer and a plug
wire to connect the spark tester to the ignition coil. This would be the case with DIS and / or
coil on plug type ignition systems.
If the spark plugs are recessed, necessitating using an extension, a deep well adapter
should be used.
● Connect the sync transducer to the plug wire that is connected to the spark tester and connect
to the input In Synchro.
● Start the engine and allow it to idle.
● In the USB Oscilloscope window select mode "Px => Px" or "Px => Px35" (depending on
the pressure transducer type) or, if the deep well adapter is used, select "Px => Px+Longer"
or "Px => Px35+Longer".
● Turn on "Record".
● After 3…5 seconds slowly raise the engine speed to 3000…5000 RPM with minimum
opening of the throttle and then close the throttle.
● After the idle speed has stabilized, quickly snap the throttle wide open. Then immediately
close the throttle. Alternatively, instead of closing the throttle, you can turn off the ignition
while keeping the throttle open until the engine comes to a complete stop. If keeping the
throttle open while shutting off the ignition, additional information will be recorded for the
script tabs "Inlet" and "Exhaust".
● Turn off the waveform recording.
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Px script - automatic analyzing the cylinder pressure waveform
● Save the recorded waveforms using the menu "File => File save as…".
● Analysis of recorded signal by the Px script is performed by selecting
"Analysis => Execute script".
Note that the script analyzes the entire file of recorded waveforms. It is also possible to select a
part of the recording, the script will then analyze only the selected part.
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Px script - automatic analyzing the cylinder pressure waveform
3. Analysis results
3.1 The "Report" tab
The conventional or classic tool for assessing the state of an engine cylinder and piston
is a compression gauge. It is designed to measure the compression or peak pressure in the
cylinder obtained while cranking the engine. The measurement is a complex value and depends
on losses through cylinder leakage, the compression ratio, the valve timing, the cranking speed,
and the state of the intake and exhaust ports or manifold. A reduction of compression pressure in
a cylinder is usually thought of as being caused by cylinder leakage or valve timing. However,
the reason can also be reduced geometric compression ratio, from for example, a bent piston rod,
due to hydro lock. Hydro lock occurs when a piston tries to compress something
non-compressible, such as a liquid. The Px script, can distinguish cylinder leakage from low
compression ratio, because it independently calculates gas losses and the compression value.
The "Report" tab from the Px script.
The compression ratio can usually be found in the service information, under general engine
data, and depends on the engine's design.
Normal pressure or gas loss for an engine in good condition is in the range 10…18 %. A loss of
more than 20 % could indicate excessive leakage in a cylinder. The algorithm for calculating
cylinder losses is complex, with some variables that are difficult to account for. A typical
problem is the heat loss of the gas in the cylinder. The heat loss arises from the fact that the gas
temperature in the cylinder during compression, even without ignition, is rising above the
temperature of the cylinder walls. Consequently, part of the heat energy of the gas in the cylinder
is transferred to the piston, cylinder and cylinder head. The loss of heat causes a loss of pressure.
In practice, the calculated cylinder pressure loss of an engine in good condition is about 10 %.
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Px script - automatic analyzing the cylinder pressure waveform
3.2 The "Quantity" tab
Shows a diagram of gas in the cylinder depending on the piston position and the stroke.
The "Quantity" tab from the Px script report, this engine is in good condition. The graph
indicates the amount of gas in the cylinder relative to the position of the piston in the cylinder
and the stroke.
This is the characteristic shape of the left part of the red and green traces from an engine in
good condition.
When plotting a diagram of the amount of gas in the cylinder 4 colors are used that reflects the
working strokes. The piston is at TDC on the left side of the diagram and at BDC on the
right side. The volume of gas in the cylinder is represented by how high the trace rises in the
vertical direction.
As the piston move farther away from TDC on the intake stroke, given as the green trace
on the diagram, read from left to right, the volume in the cylinder is increased, the pressure is
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Px script - automatic analyzing the cylinder pressure waveform
lowered, and so ambient air flows from the intake manifold and in to the cylinder. This causes
the green trace to rise.
At BDC the piston changes direction and the volume in the cylinder begins to decrease, but the
amount of gas in the cylinder continues to increase as evidenced by the blue trace on the
diagram. The increase in gas volume occurs because the gas has weight and thus have inertia,
causing the flow to continue even after the piston has changed direction on BDC. After the gas
flow has stopped, gas may start to flow back in to the intake manifold due to the piston action.
This back flow depends on the timing of the intake valve. When the intake valve is closed, no
flow will exist, and the blue trace becomes essentially flat. In this particular case, the filling of
the cylinder is maximized at
155° before TDC, and the intake valve closed approximately
140° before TDC.
After the piston passes TDC, previously compressed gas in the cylinder begins to «decompress»,
but since the valves are closed, the amount in the cylinder is still almost unchanged, so the graph
looks almost straight line (yellow trace of the diagram, the left side, read from left to right).
However, the clearly visible gradual spread between the straight yellow diagram trace and the
blue trace, indicates the quantitative and heat loss of the gas in the cylinder. The greatest amount
of loss is observed near TDC when the gas pressure and temperature are at their maximum.
The exhaust valve begins to open before the piston reaches BDC, in this particular case the
opening starts at 140° after TDC. Since the pressure measurements are made without a source of
ignition so no combustion, the cylinder pressure at this point is almost identical to the pressure in
the intake manifold. which is well below atmospheric pressure. The pressure in the exhaust
manifold is close to the atmospheric pressure, and exceeds the pressure in the cylinder.
Therefore, once the exhaust valve starts to open, the exhaust gases from the exhaust manifold
begins to flow into the cylinder. This flow equalizes the pressure in the cylinder with the
atmospheric pressure. This equalization is reflected in the diagram as a sharp rise of the
yellow trace.
After passing the BDC point, the piston starts to push gas from the cylinder into the exhaust
manifold (red trace on the diagram, read from right to left). When approaching TDC, the exhaust
valve begins to close and the intake starts to open. At this point, the pressure in the cylinder is
still close to atmospheric, as the cylinder is still open to the exhaust manifold. After passing
through the TDC point, when the exhaust valve is fully closed and the intake is opening, part of
the remaining gas in the cylinder flows into the intake manifold, since there is low pressure or
vacuum in the intake manifold. Thus, the amount of gas in the cylinder is not minimum at TDC,
but later. In this case, the minimum amount is reached approximately 20° after TDC, as shown
on the diagram as a drop in the green trace. Further, because the volume in the cylinder is
increasing, the gas flows from the intake manifold again.
Thus, using the graph of the gas quantity in the cylinder we can detect and measure where the
intake valve closes and where the exhaust valve opens. If the nominal values for valve opening
and closing are not given, deviations will have to be detected based on cylinder to cylinder
variation (or comparing to a known good engine).
Different manifold designs will show different relative timing with respect to intake and exhaust
phases. However, the width of the intake phase is always substantially the same as the exhaust
phase. The phases are always substantially symmetrical relative to TDC as well. In practice this
means that when the intake valve closes at 140° before TDC, the exhaust valve must be
opened approximately the same 140° after TDC. In other words, in the same relative position of
the piston. Because of this symmetry, on the diagram of the amount of gas in the cylinder the
same characteristic points are located above one another. This is true for engines with narrow
valve timing and with wide valve timing - phase asymmetry usually does not go beyond ±10°.
This rule does not necessarily apply to engines equipped with Variable Valve Timing, however.
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Px script - automatic analyzing the cylinder pressure waveform
If the timing belt or chain is installed one tooth late on an engine with a single overhead
camshaft, the error generally cause about a 15° delay. In figure of the amount of gas in the
cylinder is reflected as an offset of the closing of the intake valve at about 15° to the left, and the
opening of the intake is about 15° to the right. In this case it turns out that the characteristic
points are away from each other by approximately 30°.
Valve timing is set incorrectly so the valves open and close late.
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Px script - automatic analyzing the cylinder pressure waveform
With late cam timing, the red and the green traces, showing exhaust and intake gas volume,
respectively, overlap for the first approximately 20…30° of crankshaft rotation away from TDC.
In this figure, can be seen that the red and green traces displaying the amount of gas in the
cylinder are superimposed on each other for the first part of the crankshaft rotation.
In such case, the green trace rises up, superimposing on the red trace and after deviates down.
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Px script - automatic analyzing the cylinder pressure waveform
If the camshaft is installed 1 tooth early, the valve timing is advanced. This is the same as early
opening and closing of the valves. In the diagram that is shown as an offset of the closing of the
intake valve to the right, and opening of the intake valve to the left. Again, the characteristic
points are again moving away from each other approximately 30°.
Valve timing set incorrectly - the valves open and close early.
Typical waveform distortion seen when the valve timing is advanced.
The waveform distortion shown in figure where the valves are overlapping is typical for
advanced valve timing. The red and green traces do not overlap each other as they did with late
valve timing and they have characteristic angles.
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Px script - automatic analyzing the cylinder pressure waveform
3.3 The "Valve timing" tab
This diagram gives the same information about the volume of gas in the cylinder as the
"Quantity" tab does, but shown in relation to the angle of the crankshaft. The amount of gas in
the cylinder is expressed as the distance from the center of the diagram. The trace's distance from
the center is a visual representation of the amount of gas in the cylinder.
Typical diagram from the "Valve timing" tab. This is an engine in good working order.
This diagram shows the amount of gas in the cylinder depending on the angle of the crankshaft
and the stroke of the tested cylinder. The A marker shows where the intake valve is closed,
and the B marker shows when the exhaust valve begins to open. Note that the marker locations
are symmetrical relative to the vertical line.
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Px script - automatic analyzing the cylinder pressure waveform
Zoomed figure in capture showing more detail on the center of the diagram from a typical engine
in good working order.
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Px script - automatic analyzing the cylinder pressure waveform
If the timing belt or chain is installed one tooth late, on engines equipped with a single camshaft,
the valve timing for both intake and exhaust will be late. On the polar diagram this shows as a
rotation of the valve phase diagram clockwise about 15° (if a timing belt, more if a chain).
The intake valve closing and the exhaust valve opening are no longer symmetrical relative
to the center line or TDC.
Polar diagram from an engine with late valve timing. The valves are opening and closing late.
The A marker shows where the intake valve is closed, and the B marker shows where the exhaust
valve starts to open.
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Px script - automatic analyzing the cylinder pressure waveform
Typical distortion of the valve timing diagram in the zoomed center. The distortion is due to late
valve timing.
The first phase of the distortion occurs because the piston, after passing TDC in the cylinder,
starts to create low pressure in the cylinder and gases will flow from the exhaust manifold
through the still open exhaust valve. The second part of the distortion occurs because the piston
is already on its way down in the cylinder when the intake valve opens.
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Px script - automatic analyzing the cylinder pressure waveform
If the timing belt or chain is installed a tooth early, the valve timing will be advanced and the
polar diagram will again be asymmetrical. Under this condition, the diagram will be rotated
counter clock wise. The valve phases will again be asymmetrical relatively to the horizontal
or TDC line.
The valve timing phases are asymmetrical showing an incorrect valve timing. In this example the
cam timing is advanced one tooth. The A marker shows the angle when the intake valve
is closed, and the B marker shows when the exhaust valve started to open.
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Px script - automatic analyzing the cylinder pressure waveform
This zoomed figure shows the distortion of the timing diagram in the center, due to the early
or advanced valve timing.
The distortion of the red trace on the diagram is due to gases from the cylinder flowing into the
intake manifold because the intake valve opens too early.
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Px script - automatic analyzing the cylinder pressure waveform
3.4 The "Ignition timing" tab
If a sync signal from a plug wire or similar was recorded along with the data from the pressure
transducer, the Px script will also construct an ignition timing diagram.
This is an ignition timing diagram from an engine in good working order. The data is taken from
two throttle openings. One sharp and one smooth opening.
In the diagram colors are used to signify the load on the engine with blue being the lowest engine
load and red the highest. The higher the load, the warmer the color.
Normal operation of the ignition timing is shown in the diagram. As the engine RPM increases,
so does the ignition timing. This is what used to be called centrifugal advance. On the diagram
this advance is shown as an increase in the graph height as we move towards higher RPM (to the
right). Normal operation also implies that the ignition timing will vary with load on the engine.
With increasing cylinder pressure, also known as decreasing manifold vacuum, the ignition
timing should delay, so the spark occurs later. The opposite is also true; with decreasing cylinder
pressure, also known as increased manifold vacuum, the ignition should occur earlier, or
advanced. Because the ignition is delayed with increased load, the red trace, which signifies high
load, is located lower in the diagram than the green. The shaded areas signify where the ignition
timing will normally occur. Events outside the shaded areas indicate malfunction.
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Px script - automatic analyzing the cylinder pressure waveform
When the engine in a modern vehicle is overrunning, as happens when you abruptly release the
the accelerator pedal, or when the vehicle is decelerating, for example going downhill, the fuel
supply is interrupted. Because there is no fuel supply in this mode, the ignition timing does not
affect to the engine performance, so the corresponding traces on the diagram in this tab are not
displayed by default. They can be turned on manually and will display as blue traces.
Blue signifies very little load.
Typical diagram from the "Ignition timing" tab. This engine is in good operating condition.
Here the low load or overrun trace is activated.
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Px script - automatic analyzing the cylinder pressure waveform
This is a timing diagram showing abnormal timing control.
If there is a control or adjustment problem, the timing traces will appear outside the shaded
areas. In this example, the ignition timing is very late and does not adjust with either RPM
or load. The engine will have very low power. This problem was caused by a faulty
engine control unit.
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Px script - automatic analyzing the cylinder pressure waveform
3.5 The "Inlet" tab
This tab displays a diagram of cylinder fill, depending on the engine speed and load. The height
of the graph represents the amount of gas remaining in the cylinder after the intake valve closed.
The colors of the diagram traces reflect the engine load, the load value is being calculated
by the vacuum in the cylinder during the intake stroke.
This graph from the Inlet tab was recorded using two throttle snaps, fast and slow.
The red trace of the diagram allows us to estimate the influence of all the components of the
intake system in filling the cylinder. The red trace is a measure of VE (Volumetric Efficiency).
The higher the trace, the larger is the maximum cylinder fill. In most circumstances, this directly
relates to the cylinder's efficiency. Some of the factors that affect the cylinder fill are:
Variable Valve Timing, lift, duration, and overlap, the intake manifold design and geometry,
the maximum cross section of the throttle body, the throttle angle, and the flow restriction
of the air filter and the induction system.
The traces show the cylinder fill with low RPM or idle to the left and higher RPM to the right.
The lower the trace is, the more efficient and economical the engine is operating. The height of
the trace depends on variables such as: Air-fuel mixture, ignition timing,
EGR (Exhaust Gas Recirculation), any restriction from the exhaust system and power required
from the engine. As the AFR (Air Fuel Ratio) changes away from optimum, the trace will move
higher. The more EGR is used, the higher the trace will be. The more exhaust restriction there is,
the higher the trace will move. As ignition timing changes, so will the trace. An finally, as
demand on the engine changes, so will the height of the trace. Power demand can vary due to a
number of reasons, even at idle; head lights on / off, air conditioning on / off and so on.
Next we will compare the inlet diagram from two engines. These two engines have significant
differences in their induction system design. Neither of the engines use turbo or super charging.
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Px script - automatic analyzing the cylinder pressure waveform
The first one is an engine equipped with a carburetor and one intake and one exhaust valve.
No Variable Valve Timing is utilized on this engine. The intake manifold design has to
accommodate fuel as well as air.
Cylinder filling diagram from an engine equipped with a carburetor and one intake and one
exhaust valve per cylinder.
It is can clearly be seen that this engine does not have increased cylinder fill with
increased RPM. Also, the red trace is only within what is considered the normal band
in the 1200…4600 RPM range, outside that RPM band the cylinder fill is below. Based on this,
it is clear that this is an engine designed to operate in a low RPM range.
It should be noted that with the increase in RPM, the color of the diagram gradually goes from
warm to cold. This is especially pronounced after 4500 RPM. This is because the induction
system represents a restriction in this RPM range and the cylinder fill is decreasing.
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Px script - automatic analyzing the cylinder pressure waveform
Let's see the diagram from an engine with a different design. This engine is equipped with
Variable Valve Timing and is using
4 valves per cylinder,
2 intake and
2 exhaust valves.
The induction system is designed very differently, the engine uses port fuel injection and
the intake manifold is designed for air only.
Cylinder filling diagram from an engine with a 4 valves per cylinder and a different induction
system design.
The red trace clearly shows that this engine has much better cylinder fill throughout the RPM
range compared to the previous example. This engine is designed to operate well at higher RPM.
Even though the engine is designed for higher RPM, the cylinder fill at low RPM is comparable
to the diagram from the low RPM engine.
The color of the diagram is hotter than in the previous example, and starts to change
towards warm only after 5500 RPM. This means that the induction system is causing little or no
resistance to air flow into the cylinder.
Also noticeable is that the cylinder filling of this engine at RPM higher 4000 RPM significantly
exceeds 100 %. This is achieved without forced air injection. The over unity cylinder fill is
achieved through the use of Variable Valve Timing and a tuned induction system. The intake
valve is left open for some degrees of crankshaft rotation after BDC on the intake stroke.
The velocity of the incoming air is high and will continue to fill the cylinder even after the piston
has passed BDC, thus creating a slight over pressure in the cylinder.
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Px script - automatic analyzing the cylinder pressure waveform
3.6 The "Exhaust" tab
The "Exhaust" tab displays a diagram showing the amount of work spent on expelling
the exhaust gases from the engine.
The "Exhaust" tab. This diagram taken from an engine with normal exhaust system restriction.
The lower the trace, the less exhaust restriction exists.
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Px script - automatic analyzing the cylinder pressure waveform
The sloping red line signifies the allowable exhaust restriction. This trace was determined
empirically by collecting data from several engines, some in good working order, some not.
This diagram shows a fairly severe exhaust system restriction. In this case the problem was
a plugged catalytic converter.
The diagrams in this tab are calculated based on the effect the exhaust system back pressure has
on the piston movement. If the peak back pressure occurs when the piston is close to TDC, the
effect of the back pressure is minimal. This is because the piston speed is minimal at that
moment. On the other hand, if the peak back pressure occurs when the piston is approximately
midway through the stroke, the effect is much more pronounced. For this reason, the actual
exhaust system design does not have as much of an effect on the trace as does unintended
restrictions. Because of the dynamic manner in which the restriction is measured and calculated,
smaller restrictions can be detected before they significantly affects the engine performance.
Andrew Shulgin
v4.3.6.1
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