Manifold Pressure: How to Measure It?

Has anyone ever measured the intake manifold pressure?

I'm not referring to the mg/R value, as in the comparison between fabric and paper air filters, but specifically to the pressure in millibars (mB) at full boost pressure and engine speed, or the pressure difference compared to the ambient pressure.


The underlying thought is as follows:

The VNT15 is perfectly adequate for standard performance. The slight restriction of the intake air results in a much quieter intake noise, which is likely the reason for this.

If the intake manifold design causes a pressure drop of only 50 mbar, it reduces the maximum flow rate from approximately 9 kg/min to 8.5 kg/min. (Inspired by Ulf's thread about the VNT15 performance.)
Many tuned TDI engines, including mine, operate near the maximum boost limit of the VNT15 turbocharger.

According to some people who know, for example, 5 MB is definitely not enough to push 9 kg/min of air from the front of the intake manifold to the turbocharger.
So, how much is it?

So, if I could potentially gain something by modifying the intake system, I would definitely do it.

Certainly, the VNT 17 is the better solution, but aside from the fact that it costs money, it's a real hassle to replace.

Hello Garth,

Measurements taken on a test stand using a standard AFN(VNT15) unit showed that at the rated power point – i.e., full load of 81 kW at 4150 rpm – a pressure drop of approximately 80 mbar occurs. Depending on the degree of contamination of the air filter, this pressure drop can quickly increase to 100 mbar. The measurement point was located directly upstream of the compressor inlet. However, due to the curved piping, it cannot be completely ruled out that a small dynamic pressure component slightly distorted the result. I believe, however, that this is negligible for your considerations.
Optimizing the intake path is certainly a suitable measure for the already heavily stressed VNT15 turbochargers in the chipped AFN engines. Reducing the compressor pressure ratio – while maintaining the same airflow – reduces the already critical turbocharger speed. Examining the compressor map can be helpful. To reiterate for clarity: The compressor pressure ratio is the ratio of absolute pressures after and before the compressor (the ordinate axis of the compressor map). If the intake path is optimized or the intake manifold vacuum is reduced, the compressor pressure ratio decreases while the boost pressure remains the same. Incidentally, the EDC (Electronic Diesel Control) with its boost pressure control does not measure the intake manifold vacuum. If the ambient pressure decreases due to altitude (e.g., 2500m mountain pass), the EDC will still regulate to the target boost pressure, but with a significantly higher pressure ratio and therefore higher turbocharger speeds.
Given that a turbine with a variable geometry turbine (VGT) is also involved in this whole process, the optimization of the intake manifold has the following effect: the lower compressor pressure ratio results in less turbine power required, which means the VGT vanes are opened further, leading to a lower exhaust back pressure, a lower exhaust temperature, and less pumping work due to a better scavenging pressure gradient. In short, it's mostly advantages. I believe that in practice, it should be possible to achieve around 20 mbar of improvement, perhaps even more, depending on the effort and resources available. However, before you start implementing any changes, I would recommend installing a measurement point for a before-and-after comparison. Vacuum gauges are available for a relatively low price.

Regards,

Sepp.

Hi,
I've measured it before.
With my ALH, I took measurements near the KGE (presumably a specific component or location). There, a maximum vacuum pressure of approximately 50 mbar was observed.




If the intake manifold creates a pressure drop of only 50 mbar, it reduces the maximum flow rate from approximately 9 kg/min to 8.5 kg/min. (Inspired by Ulf's thread about the VNT15 performance.)
Many tuned TDI engines, including mine, operate near the performance limit of the VNT15 turbocharger.

According to some people who know, for example, 5 MB is definitely not enough to push 9 kg/min of air from the front of the intake manifold to the turbocharger.
So, how much is it?
.

Ultimately, isn't the pressure difference between the inlet and outlet the most important factor when considering maximum throughput?
When I'm at full throttle and 3800 RPM, if I briefly release the accelerator and immediately reapply full throttle, the boost pressure quickly jumps to around 1.3 bar and then drops back down to 0.9 bar.

According to this, he's still far from reaching his performance limit. This doesn't necessarily mean that certain parameters aren't being exceeded, but the reserve capacity is likely much greater than the 50 mbar vacuum in the intake area. Mine isn't tuned, but then the question remains: what exactly does 'tuned' mean for a TDI engine? It's likely that even a 17-size turbocharger could be pushed to its limit.

Greetings.

Eike.

eike wrote:

Hi, I've measured it before. With my ALH, I took measurements near the KGE.

There, a maximum vacuum pressure of approximately 50 mbar was observed.

Quote:

That already includes the pressure loss of the mass airflow sensor (yes, it also consumes a few MB).

Isn't the pressure difference between the inlet and outlet ultimately the deciding factor at maximum flow rate?
When I'm at full throttle and 3800 RPM, if I briefly release the accelerator and immediately reapply full throttle, the boost pressure quickly jumps to around 1.3 bar and then drops back down to 0.9 bar.

According to this, he is still far from reaching his performance limit. This doesn't mean that certain parameters aren't being exceeded,

Quote:

During the adjustment process, the "actual" target pressure is often exceeded (overshoot). However, these short-term "overloads" are hardly significant over the entire operating period and will therefore likely be simply accepted.

However, the reserve will likely be significantly larger than the 50 mbar vacuum pressure in the intake area.
As Sepp wrote: The load/speed results from the pressure ratio} of the input/output.

If you double the pressure using a charger, any pressure losses at the inlet will also be doubled at the outlet.
1 bar -> 2 bar
0.95 bar -> 1.9 bar at the same turbocharger speed.

To compensate for the missing 100 MB, the loader has to work much harder, which in turn leads to a more aggressive control of the VTG (Variable Turbine Geometry) -> increased exhaust backpressure -> power loss.

On the other hand, a pressure difference of, for example, 50 millibars is quickly reached in everyday life: for instance, between high and low-pressure weather systems, or due to a height difference of approximately 400 meters.

I once measured the differential pressure directly in the air filter housing (to determine if the filter was already clogged after 37,000 km).

'In that case, I was getting a low pressure of 7 mbar at both filters (new and used) at full load with an engine speed of 4200 rpm.'

'50 mbar is definitely achievable, because it's coming from the air chamber through the pipe.'
The LMM already creates a slope, and then every bend in the pipe, etc., adds to it.


CU Gremlin.

Hi everyone.

Thank you for the quick responses, especially for the information regarding millibars (mB).

While the aspect ratio is correct, when I consider it from the perspective of the mass ratio, which is ultimately what matters, it is not.

Here's a striking example: At an altitude of 5500 meters (equivalent to a pressure of half atmospheric pressure), the loader can still achieve a pressure ratio of 1.5 bar, but not at a flow rate of 9 kg/minute.

However, what is important for ventilation is not the pressure ratio, but the mass flow rate.

This means that a decreasing pressure at the inlet of the charging device has a disproportionately large effect on the mass flow.

In addition, the increasing back pressure from the exhaust gases leads to increased compressor work while the turbine efficiency decreases.

The occasional over-boosting is partly because he prefers to do it at lower engine speeds, where the mass airflow of the turbocharger is well within its operating limits. Alternatively, it might also be due to the turbocharger's speed momentarily exceeding its limits, which probably isn't a problem.

The latter is not suitable as a working point.

Okay, I see potential here – let's see what I can come up with...

reduces resistance in the intake system.
Here's the translation:

'Regarding the stainless steel intake manifold, etc. (it's a bit louder), always keep the filter clean. Everything working together reduces resistance and prevents the turbocharger from working too hard, which results in more torque and makes the TDI engine happy!'

Alternatively, you could leave everything as it is and add a compressor to the intake, which should then address the issue of negative pressure. However, I wouldn't call that tuning; I would consider it exploiting possibilities.

das hat wohl VAG im sinn gehabt, als die die luftwege direkt an den kühlergrill gebaut haben. RAM-Air

before someone comes up with this idea: not good for the LMM and therefore not allowed by Bosch .

CU Gremlin.

Hello.

I once rerouted the air supply connection from the air filter housing to my LDA (with a 0.1 bar scale) for the pneumatic system.
To prevent the air consumption of the pneumatic components from affecting the results, I disconnected them from the main line and temporarily connected them to a gasoline line filter.

According to my LDA measurements, the pressure inside the air filter box at full throttle was practically equal to the outside pressure up to approximately 3000 rpm. Above that, the pressure decreased to about -40 mbar at 4500 rpm.

Since I am also interested in the underlying theory, I would like to present my (unlearned! ) calculations regarding pressure loss for discussion.

a) 4500 rpm result in 2,250 engine cycles per minute, each with a volume of 1.9 liters, which equals 4,275 liters per minute, or 0.07125 cubic meters per second.

b) Increasing the turbocharger's boost pressure to 1.3 bar increases the mass flow by a factor of 2.3, meaning that, theoretically, 2.3 times the volume of air is drawn in at normal pressure, resulting in 0.1639 m³/sec.

c) An assumed filling level (or delivery rate?) of 90% results in a suction-side volumetric flow rate of approximately 0.1475 m³/sec.

d) The cross-sectional area of the intake opening is approximately 20 cm², which equals 0.002 m². This results in a flow velocity of 0.1475 m³/sec / 0.002 m² = approximately 74 m/sec.

e) 0.1475 m³ of air weighs approximately 0.187 kg (air density = 1.27 kg/m³ under standard conditions).
This mass is accelerated to the aforementioned speed of 74 m/s every second.
The required "thrust" is calculated as F = m*a = 0.187 * 74 = 13.8 N.

f) A force of 13.8 N distributed over an intake cross-sectional area of 0.002 m² results in approximately 6,900 N/m², which is roughly equivalent to a pressure requirement of 70 mbar for accelerating the intake air or due to pressure loss through the narrow intake opening.

I attribute the actually lower pressure loss to a partial "energy recovery": the air flows relatively slowly within the large air filter housing, and the deceleration behind the narrow intake creates a certain back pressure, which should be subtracted from the calculated pressure loss of 70 mbar.

At least, the calculated order of magnitude is approximately equal to the measured value, which suggests that the calculation is fundamentally correct.

If you want to get an idea of the proportions of your car, you can use the following calculation method:

Intake manifold pressure loss = (RPM * Absolute boost pressure * Displacement)² / (Intake port cross-sectional area² * 381.2)
Inputs in units of 1/sec, bar, m³, and m², with the result in mbar, excluding any assumed energy recovery; the air density is assumed to be 1.27 kg/m³.

My observations confirm the quadratic relationship between pressure loss and rotational speed: at 3000 rpm instead of 4500 rpm, the calculated pressure loss is only 44% of 70 mbar, which equals 31 mbar.
When the estimated energy recovery is factored in proportionally, the result falls into the range of approximately 18 mbar, which is barely discernible on the LDA (Low-Density Arc) measurement.

By the way, I also did some additional measurements without the intake boot connected to the air filter box (using a hot air intake...).
The inlet opening on the casing is approximately 45 cm² in size, which, according to calculations, should result in a maximum vacuum pressure of about 14 mbar.
At 4500 rpm and full throttle, no vacuum could be detected on the LDA.

Based on my experience and the formula, it also becomes clear why the intake air path is often widened in heavily tuned engines.
The speed and boost pressure each contribute to the pressure loss in proportion to their squares.
Therefore, if the tuner simultaneously increases the maximum engine speed (Pmax) and the maximum boost pressure, the pressure loss at the intake manifold increases dramatically.

While this could theoretically be compensated for by increasing the turbocharger's speed, the current limited reserves of the turbocharger (needed for rapid pressure build-up at low engine speeds) would quickly push it into the area of its performance curve just before overload. In this region, its efficiency drops off, and the required turbocharger speeds would rapidly increase, similar to the additional power required from the engine.

The latter must, in turn, be generated by the engine as exhaust pressure and is therefore lost as useful power -> the tuning essentially feeds back into itself, resulting in relatively little additional power reaching the wheels.

Okay, but now let's hear from the educated mechanical engineers and other engine specialists: Please correct/supplement my calculation or summarize it in a more user-friendly format .

Hello,

Here's another update on my calculations based on the logged LMM values:

a) At 4500 rpm, the system achieves approximately 950 mg per dispense, resulting in 9,000 fills per minute, or 150 fills per second, with a mass flow rate of 0.1425 kg/second.

b) With an air density on the suction side of 1.29 kg/m³, the calculated suction volumetric flow rate is 0.11 m³/sec.

c) The cross-sectional area of the intake opening, which is approximately 20 cm², or 0.002 m², results in a flow velocity of 0.11 m³/sec / 0.002 m² = approximately 55 m/sec.

d) The air mass of 0.1425 kg is accelerated to the aforementioned speed of 55 m/sec every second.
The required "thrust" can be calculated as F = m*a = 0.187 * 74 = 7.84 N.

e) A force of 7.84 N distributed over an intake cross-sectional area of 0.002 m² results in approximately 3,900 N/m², which is roughly equivalent to a pressure requirement of 40 mbar for accelerating the intake air or a pressure loss due to the narrow intake opening.

. . . which practically corresponds exactly to the read value
However, the energy initially expended to accelerate the air seems to be permanently lost (due to friction, turbulence, etc.?).

The calculation must be adjusted "somewhat" for TDIs, taking into account the fill level.

Intake manifold pressure loss = (RPM * Absolute boost pressure * Displacement)² / (Intake port cross-sectional area² * 671.4)
(Inputs in 1/sec, bar, m³ and m², do not forget the squares in the calculation, result in mbar).

Hmmm...

Selling ASZ.

ASV to acquire.

'Chipping'

AGR kills.

LMM fake.

Build a ram-air intake.




SCNR.

CU Gremlin.

PS: How do you achieve -40 mbar in the airbox? That would be okay at the charger, but directly in the airbox? I only managed to get 7 mbar there. See above.

Gremlin wrote:

How do you achieve -40 mbar at the air box? It would be okay at the charger, but directly in the air box? I only managed to get 7 mbar there. See above

Perhaps your path to the measurement point is more streamlined, or a backwater effect may occur, which distorts the result.