WG signal from compressor or manifold?

[EspenW]

I'm interested in experiences from those of you who have routed the WG signal hose from a location between the IC and throttlebody instead of the nipple on the compressor outlet.

Experiences with: spool, WG creep, boost taper at high rpm, boost creep, etc. would be appreciated.

I currently source the signal from the compressor outlet, but I am tempted to source from post IC in order to eliminate some boost taper from pressure loss over the IC at high rpm.

[EspenW]

Bumping this one with edit for some added info

[JP]

I've personally always sourced the signal from the compressor housing and the boost controller signal from the intake manifold.

This way you have the most stable and continuous boost signal feeding your wastegate controlled by the signal of the actual pressure left over post intercooler and boost run.

The further away you take the pressure signal to your wastegate the more difficult it becomes to control as it becomes effected by other variables that do not always remain constant like pressure drop through intercoolers, BOV's etc... Same wayt if you take the signal for your boost controller too close to the turbo you wont be left with an actual of what your engine is not using once it has scavenged all it can from the intake.

just my opinion from over the years, not necessarily right but I'd like to think so

[quattro84]

Would be nice to add an extra MAP sensor directly after compressor to monitor both and compare how the signal behaves. I guess using the suggested position makes the system response time slower, but i expect the signal itself to be more stable.

[EspenW]

With electronic boost control, it doesn't matter where the signal is taken from, since the boost profile can be manipulated (within the limits of the turbo) at all rpm.
However, with a MBC, it will regulate to whatever the boost signal source tells it to. This is all fine when it comes to setting the peak boost that you want the engine to see, but the problem is that with high airflow, there is a pressure drop between the compressor outlet and the throttle body, and the pressure drop increases with rpm. The result is that boost (as measured at the manifold) will drop with increasing rpm.
I will move the signal source for the MBC to downstream of the IC, and put a vacuum nipple in either the hose from the IC, the plastic pressure pipe, or the rubber boot attached to the throttle body.
Should be able to gain at least a few psi at high rpm by doing this. Will also check the actual pressure drop by teeing in a boost gauge that reads pressure from the turbo outlet and compare to the manifold readings.
I currently peak at approx 2.15 bar around 3600rpm, it stays above 2 bar for a while, but tapers to approx 1.6bar at 7000rpm. Keeping it closer to 2 bar at higher rpm as well should free up some horses. Will be interesting to see the Vagcom logs and measure how much horsepower it will put to the ground.

[JP]

Yes sort of but not quite

For an EBC you want the feed to the wastegate to be from a constant pressure source as much as possible but not essential more importantly the route needs to be short so compressor housing is ideal here or depending on boost run of a location of the boost pipes. Howeve due to the targeted control of the EBC you can place the wastegate feed pre or post IC.

The source signal that tells the controller is then ideally taken from the lowest stable pressure source which is post IC i.e. the IM (scenario 2). This is because the controller will always aim to target what you have asked it to achieve.

For example imagine you have an IC with a 5% pressure drop loss for ease of calculation sake (I know a bit extreme but it makes it easier to represent).

1st your source signal is taken from a high pressure point in the system near the compressor then and you set it to 2bar. The EBC now targets 2 bar which it will achieve. However at the IM after the IC you are seeing 5% less than that which is only 1.9Bar with a 5% loss.

2nd you take your source from the IM and your EBC targets 2 Bar. So when your IM sees 2 Bar your turbo is actually producing 2.1 Bar of boost as it has to work harder to compensate for the pressure loss you are now reading at the intake manifold.



Now look at it from the perspective of a MBC like a bleed valve. You will create whatever boost you allow it to bleed of. The only importance now is that you use a stable and consistent location to take the feed to the wastegate of prior to bleeding it of and as short a run as possible. Otherwise you end up with inconstancy in the boost values and response issues.
Once again you place a gauge on the IM to read whats left over and adjust your MBC until the desired reading on the gauge is achieved.

Leaving the lines as short as possible to avoid spiking which once again lends itself to putting the feed to the MBC from the compressor housing or in the boost run very close to it.

Which deps ultimatelly on your boost run layout preferably pre IC doesn't really mater where so long it is a stable and constant location not susceptible to irregularities from being too close to the TB or BOV

[EspenW]

That is correct, more stable signal at the compressor outlet. However, due to the pressure drop at high rpm, the only sensible way to get a perfectly flat boost curve (if that is desired) with a MBC, will be to put the signal line after the IC, unless the IC and piping has 0 pressure drop. Artificially inducing boost creep would be the only way to combat the boost tapering, but that is not a very clever idea..
Being able to program boost is a clear advantage of EBC's or N75, however, with the signal source post IC, the MBC should be able to hold a very constant boost.

I see the boost drop problem even at my low boost setting. It peaks at 1.2bar, but still drops somewhat at high rpm. The turbo is perfectly able to hold 1.2bar to the rev limiter, but the pressure drop over the IC, even at such low boost, is enough to cause boost taper. The MBC thinks it regulates a constant 1.2bar, but it doesn't at high rpm.
Interestingly enough, cranking down the WG spring another 2 revolutions was enough to introduce boost creep, raising boost to 1.5bar at high rpm, even with 1.2bar dialed in as max. Boost creep is not good.