I've got two Irvine .53's and 2 Irvine .25's... I love 'em... If you take care of 'em they run great... and if you abuse them... they still run great....

On the .53's, i'm running 12.25 x 3.75 APC prop on 'em. I'm getting 13,700 rpm. I tried a Mac's header and mousse can, then later a tunned pipe on one of them... got 14,300 rpm... but it was too much for the plane and the transition was a little more difficult to set... So, I like the Tower mufflers on these motors.