I studied boost vs RPM for the 2008 Cadillac STS-V LC3 engine in this post but I am back on this topic again.
This graphic shows the Boost from the Supercharger in pounds per square inch (PSI) of pressure in blue, and the incoming air volume through the mass air flow sensor feeding the supercharger in LB/Min in red.
As we have seen before, the supercharger boost is surprisingly spikey. The supercharger in the STS-V spins at a 2.1:1 ratio to engine RPM, so at 6000 RPM the supercharger is spinning at over 12,000 RPM. One would think the boost from the supercharger would be pretty constant.
Of course, constant is a relative term. The blower is spinning up with the engine and the entire graph above is a less than 4 second interval. The spikes and troughs tend to be 4 samples over 1/10 of a second.
Here is a zoom in on the run from 6400 RPM to 6700 RPM and boost values:
|Time||Engine RPM (SAE) rpm||Boost PSI|
All of which happens in 0.3 seconds (3/10 of a second).
I don’t see a correlation between the surges in the Supercharger boost pressure and surges in demand of air volume via the MAF; the air through the MAF appears to have a pretty constantly increasing slope. Perhaps as the RPM goes up and the engine has greater demand for air, and the boost is building, we should see a higher MAF slope than we do? The MAF slope appears to be increasing at 2900-5900 RPM and lower at RPM above 5900? The rate of increase in boost psi also flattens at that point, so the blower may have reached its capacity.
I would like to see a boost graphic for an STS-V with an upper pulley for comparison.
Are there things that could be done to help smoothen the boost output of the blower?
Here is a graph comparing IAT2 Temperatures (intake air after the supercharger) with the boost curve. I see a close relationship here.
As the boost goes up the air is getting compressed into the manifold, and the air temperature goes up, making the air less dense, and eventually causing the engine to start to pull timing via IAT2 advance (retard) on hot days. On this run the engine stays out of that range, but there is the other range that if the IAT were cold enough the computer would add timing.