We talk boost all the time but we often neglect airflow. The bottom line is that an engine need CFM airflow to make power. According to some very detailed calculations (and I am no engineer or mathematician so these are NOT my calculations!) a basic 3.8 needs about 350cfm naturally aspirated. At 10psi and 5000rpm, it needs about 550cfm. Based on the charts I have for the M90, this is exactly what the M90 puts out meaning that these calculations seem to hold up. It also supports my notion that Ford very carefully designed the SC as a package that works together. No more, no less. Now keep in mind that no amount of porting can increase the output of a positive displacement blower. All porting can do (in terms of airflow) is extend the rpm range capability of the blower. So if we assume that porting has been 100% effective in increasing the rpm range of the blower, we can calculate that after taking into account factors for temperature and M90 blower efficiency, 900cfm airflow will be needed to support the same motor at 6000rpm and 18psi. Here is where things get ugly. To generate 18psi and 900cfm on a motor with 100% VE requires 24000rpm with the M90 which, if in theory is even possible through porting, the net output after taking into account heat generation and loss of efficiency is a net of 675cfm. Herein lies the secret as to why people can't make high rpm power with their M90's and why Coy Miller needed a cog drive to spin it. The more boost you make, the more heat eats into your output, combined with an VE rating of about 72-75% at 18psi and you have passed the point of diminishing returns. We do not have hard data on the MPII regarding output at high rpm and high boost so unfortunately we have to make some guesses, but I've used best case scenario for comparison purposes.
