Hi Ron,

The 13.75 factor is the right number for most turbo installations.  George made a special case for the TSIO550 saying that he would use the 14.7 number.  It has something to do with the timing which is 24° BTDC on that engine.  The factor for normally aspirated LOP is 15.0.

Also, after reviewing my notes from the APS, those running ROP (normally aspirated) should use 29" as the divisor for the manifold pressure part of the equation I cited.  It'll come out a little closer to the factory percentages.  % power = ((MP/29) * (RPM/2700)) * 100

For normally aspirated engines the following works also.  For each inch of manifold pressure decrease from max (29") reduce the power by 3.5%; and for each 100 RPMs less than max (2700) reduce the power by 2.5%.

For example:  running at 24" MP and 2400 RPM
(100 - (29"-24")*3.5 - ((2700-2400)/100)*2.5) = 75% power

This was promoted as something "you can do in your head".  Not my head - that's why I printed out the spreadsheet.

Hal