A Little Cobb Tech article on boost controls

Here is a little tech artilce that Cobb sent out to it's protuner users. Kinda just a fyi on how the boost system/ecu works.

I wanted to let you know specific details about the ECU and what it does internally and
what it is capable of communicating through its data stream outputs. The ECU uses Absolute
Pressure internally whenever "boost" is referenced. The ECU actually reads this value in
millimeters of mercury (mm/Hg) but we will leave it at psi for this conversation. The only time the
"Relative Pressure" value is used or calculated is to output the datalog. Otherwise, the ECU has
no care or use for a relative pressure value. The reason why ~18.42psi is the highest you see out
of the data logged Relative Pressure reading is due to a limitation in how the ECU outputs the
data.

The ECU uses closed-loop boost control which references absolute pressure (we
translate the maps into relative pressure to make it easier to tune). The ECU can internally read,
target, and adjust boost up to 36.98psi of absolute pressure, which is ~22.28psi of relative
pressure (at sea level). That will be the maximum limit of how much boost the stock boost control
system can target.

Now let’s get into some physics; this is how some of our earth works. At sea level we are
under ~14.50377psi (1 Bar) of atmospheric pressure. Our bodies have evolved (or been
intelligently designed if that is your train of thought) and are accustomed to and can withstand this
pressure (that is why we do not implode while we walk around . Generally speaking, as you
increase in elevation the atmospheric pressure decreases. For a turbo to hit 10psi of relative
pressure (boost) at higher elevations your turbo will need to increase its shaft speed or work
harder to do so. This is why hitting target boost on stock vehicles becomes more difficult as the
vehicle rises in elevation. At some point a turbo will not be able to increase pressure because it is
out of its efficiency range. Once this happens your turbo turns into a flame throwing time bomb.

This is not taking into account weather changes which have a direct effect on atmosphere
pressure. For instance, when a hurricane comes in at sea level atmospheric pressure can
decrease from ~29 in/Hg down to ~25.4 in/Hg which is what we see here in Salt Lake City on a
daily basis.

So here is the basic equation I would like you to be aware of:

Absolute Pressure – Atmospheric Pressure = Relative Pressure (Boost)
less equals

You will need to let the ECU know the Boost Targets you want to hit then give it a wastegate duty
cycle target to work with (some vehicles have one wastegate duty cycle table, some have two).
The ECU will then modify the wastegate duty cycle using the compensatory wastegate duty cycle
tables (Wastegate Duty Cycle Air Temp. Compensation A, etc.) then use the Turbo Dynamics
tables to adjust the wastegate duty cycle in order to achieve the dictated target boost. The only
limitation is the data logging feature of the stock ECU limits the recordable "output" of relative
pressure to ~18.42 psi, this is a non-functional limitation. In other words the ECU will be able to
read and calculate relative boost pressure up to about ~22.28 psi, but it cannot output that it is
doing so. Make sense?


Russ