Prophanger1 is right TT stands for thunder tiger. Just a guess but i would say the factory mufflers are about the same on performance. I was just letting you know this in case you wanted to try an aftermarket like a MACS or something. Your absolutely right about the prop factor and different brands or styles. I seem to pick up a few RPM with an APC compared to some other props ive tried. Prop factor numbers will keep you in the ball park when trying new props.