Budarooski wrote:Wiki says MAPP requires 4.98% Displasive Volume Percent, but then on the next page it says 4.6%, so I'm not sure what to believe. Then again, everything I read says Propane requires 4.03%, yet the second wiki page says 4.2%.
"Displasive" volume percentage is the percentage of the chamber volume you need to inject in propane if the propane injection displaces air from the chamber (out a cleanout, out a vent hole, past the projectile, etc.). The idea is that, since the propane displaces air, there is less to burn the fuel in, and the percentage must be adjusted.
The other percentage is the percentage of the chamber volume to inject if the fuel injection displaces no air (a completely sealed chamber). In reality, the chamber doesn't need to be completely sealed either, because the fan will have homogenized the fuel mixture by the time the slight pressure caused leaks.
Overall, there shouldn't be much performance difference between which percentage you use. I've always used 4.2% for propane and 4.89% for MAPP. If you are fueling a hybrid launcher, then the method and figures used start to matter, but for a basic combustion it shouldn't matter a whole lot.