CM871 Engine cycle...

[Hammerhead]

(12-22-2016 )axe Wrote:  My CM870, stock form and tune, will boost around 5lbs less on cold days. Teens and below. Just wondering?

The air operated VGT is not "position reported" like an electronically controlled VGT therefore the ECM cannot "verify" the command of closing. So an 870 uses the mass air/charge flow calculation, and the ECM can calculate that the colder air is more dense, therefore it commands less closing, reducing the boost because it "knows" there is enough air molecules (the density) for the amount of fuel.

Think of it like this:
One cubic foot of air is one cubic foot of air, but we all know that the air is thinner at 10,000' than it is at sea level. At 10,000' there are less molecules of O2, N2, and the other gases that make up the atmosphere because there is less pressure, there is more space between the molecules which is why the volume is the same.
Temperatures have the exact same effect on atmospheric gases, so the colder it is, the more molecules of O2, N2, and such in each cubic foot of air, so the ECM knows it needs less boost to get the correct number of O2 molecules in the cylinders to burn the amount of fuel being injected.
The @M*M^2 tune uses input commanded VGT positioning from the table, so when the air gets more dense, it's putting more molecules of atmospheric gasses in the cylinders, so the percentage of closing needs to be cut back a couple of points to keep from over boosting. The MM tune is set for very fine control, so when these conditions change it becomes noticeable when you watch your gauges closely.