Pre-compressor mist and "apparent" boost reduction

[masterp2]

a number of reports from different sources say that boost readings decrease when WI is activated.

I have my own idea on why this may be, but would like to start a discussion.
It might be helpful to differentiate turbos. Some are wastegated (mechanically work limited) and others are variable geometry with MAP feedback controlling plenum boost (not work limited). Most newer vehicles fall into this category.

What is your explanation?

[Richard L]

I guess there is a possibility some water has gone into the wastergate diaphragm chamber and cause the rod to extend further then normal?
I am not sure.

Drop in IAT temperature may cause drop in boost pressure?

[masterp2]

In a VVT, boost is commanded via a MAP signal produced, normally at the intake plenum. I would expect that boost at the intake to remain identical before/after WI.

Wastegated turbos are another matter. Not sure how to rationalize that.

[ride5000]

Originally Posted by Richard L

Drop in IAT temperature may cause drop in boost pressure?

that's my vote.

[masterp2]

are compressors constant volume devices, or constant mass devices?

[ride5000]

Originally Posted by masterp2

are compressors constant volume devices, or constant mass devices?

depends on how hard you're running it.

[hotrod]

It depends on the design of the compressor. If the system uses a positive displacement system like a roots blower you will see a drop in boost for a given blower rpm because your cooling from the WI lowers the temperature enough to drop the pressure ----- but the actual mass flow will be the same or greater depending on where the WI occurs. Boost does not equal air flow.

Same goes with some turbos. If the turbo is at max volume flow WI will drop your max boost but you still will be moving the max flow the turbo can create. If you are using pre-comressor injection on a turbo that is maxed out you will increase the actual mass flow by a few percent.

If the system uses a turbo that has plenty of capacity the WI should not effect the peak boost much unless you are running way too much water and cooling the exhaust so much that the turbo looses drive to the turbine.

Short answer yes there are circumstances where WI would reduce boost, that does not necessarily mean total air flow dropped though.

Larry

[masterp2]

then with VGT, and boost sensor (MAP) feedback, the boost will remain identical at the intake plenum. The difference will be that lower turbo rpm will be required to attain it. In my case that means higher compression efficiency, and higher charge density. (if all my t's are crossed )