Strange/Conflicting boost readings

[scmguru]

I'm getting 2 different boost level readings from the same mechanical VDO gauge and wondered if you folks had any insight.

If I tap the boost gauge into the brake booster hose, (inside passenger compartment footwell) it reads about .4Bar peak boost, maybe .5Bar and it takes much longer to show any boost on the gauge.

If I tap the boost gauge into the wastegate line via a T, I get about .6 or .7 peak boost and it seems to build much quicker.

The car has a manual boost controller (pressure regulator) T'd into the wastegate input and plumbed into the top of the wastegate vent.

When I've got the gauge tapped into the wastegate line, the only way I've been able to get .8Bar of boost is if I've got the pressure regulator wide open (ie max pressure to top of vent housing)

If I remove the manual boost controller from the loop (stock configuration), I'm getting maybe .5 bar.

The following have been checked and are AOK:

1. Intercooler O Rings
2. Wastegate Opening pressure (.9 Bar on compressed air)
3. Vacuum lines
4. Diverter valve checked, new O-rings.




Any help would be very appreciated.

Thanks!

[billjam]

Be careful winding up your boost controller if your gauge isn't measuring the actual manifold boost. If your gauge is telling you 1 bar in the brake booster pipe, you could be much higher than that in the manifold. Could be expensive especially if you're not getting the extra fuel you need to go with the extra air.

I had a similar problem with low gauge readings when I first fitted a mechanical boost gauge. The mechamical gauge had much lower readings than my standard electonic gauge when I tapped into the brake booster line.
When I moved my connection point to a tee in the wastegate boost line, I had similar readings on both gauges (which gave me some degree of comfort).

I have since moved the connection to a tee in the line between the throttle body and BOV. Now I am measuring vacuum and boost and I have been able to set my boost confidently with my manual boost controller.

[911nut]

Bill, is there an advantage to reading vacuum?

[berg0658]

have you though of tee into the back of the inter cooler.

[jwasbury]

Originally Posted by billjam

Be careful winding up your boost controller if your gauge isn't measuring the actual manifold boost. If your gauge is telling you 1 bar in the brake booster pipe, you could be much higher than that in the manifold.

I'm a bit confused by this statement...The brake booster hose is plumbed to the manifold. At least it is in my car. I am able to read both vacuum and pressure on my mechanical guage using the brake booster line for a "signal."

I find that both the factory gauge and my mech gauge show approximately the same max boost level. The difference I have observed is that the factory gauge starts to show pressure much sooner than the mechanical one. I have read that it is normal.

[scmguru]

Current set up has the the wastegate line running from the intercooler to the turbocharger wastegate input (Line A)

At the wastegate input, there is a banjo bolt which has 2 lines coming off it (Line A from the intercooler port) and Line B, which goes to a variable pressure regulator/MBC under the dash.

The mechanical boost gauge is tapped into a 1/8" port on the variable pressure regulator.

I've attached a picture below (pls excuse the lack of artistic ability)

With the way this is currently connected, I never get more than the wastegate limit of .8-.9 bar, the only difference it seems is how quickly the boost builds.

Is this correct?

[billjam]

Originally Posted by 911nut

Bill, is there an advantage to reading vacuum?

Paul,
Not really. It's just that I have a mechanical gauge (Greddy) that reads vacuum as well as boost so I thought I'd hook it up that way. However, it is interesting to see what is happening with vacuum at cruise.
Another side benefit is that when logging data with my LMA-3 aux box (which is tee'd into the line to the boost gauge), the rapid loss of vacuum clearly indicates when I give it WOT and gives a reference point for measuring response, or lack of!

Originally Posted by berg0658

have you thought of tee into the back of the inter cooler.

Yes, the hose to wastegate comes off the back of the IC. I tee'd into this hose for a while (for boost-only readings). The standard gauge is fitted in the same area (on my B6 IC) and both gauges gave same readings.

Originally Posted by jwasbury

I'm a bit confused by this statement...The brake booster hose is plumbed to the manifold. At least it is in my car. I am able to read both vacuum and pressure on my mechanical guage using the brake booster line for a "signal."
I find that both the factory gauge and my mech gauge show approximately the same max boost level. The difference I have observed is that the factory gauge starts to show pressure much sooner than the mechanical one. I have read that it is normal.

Jacob,
I had quite a different experience with this connection in my car.
When I connected to the brake booster line, my mecahnical gauge showed much less boost than my standard gauge (about 0.4 bar instead of 0.7 bar). When I connected the mech gauge to the outlet side of IC at the wastegate connection or at the throttle body below the butterfly, boost readings on both gauges were similar. I have also had my Greddy mech gauge checked and it is dead accurate at 1 bar.
I can't explain the difference in pressure readings at the brake booster pipe. There doesn't seem to be any type of restriction in the hose or fittings. Maybe the connection's position in the manifold, the shape of the fitting, or its protrusion into the airflow stream is inducing some pressure reduction due to venturi effect as air passes over it on its way into the inlet port

As far as lag in the readings, this would only occur if there is a restriction (damping orifice) in your mech gauge or its supply. It may be designed that way. My gauge responds instantly to movements of my right foot. This is especially noticeable when it is connected to show vacuum.

[billjam]

scmguru,
If the gauge port is measuring the pressure in your IN line, you should be OK.
For this to be the case, the IN port and the gauge port must be open to each other within the regulator.

While we are throwing diagrams around, here's how mine is setup.
This is based on the diagram in the back of the Owners Manual.

[gogomobil]

Originally Posted by billjam

my understanding is that an mbc fits in the line between IC and WG as shown in the graphic above, the top wastegate vent on my car still vents to atmosphere (no need to connect to the mbc).