I changed to a 19.5 x 11.5 PN apc yesterday. Alsoinstalled an exhaust diffuser on the exhaust stinger tube. Both worked great; the jump on pipe is easily manageable

The sound in the air got even lower, more like an electric driving a gear box.....Really cool

<span style="color: #0000ff"><span style="font-size: larger">EDIT: Dag's idea of valving the exhaust stinger at lower speed may be easy to actually do if the exhaust stinger is made of soft silicone rubber tube. In a way, that's what I did earlier on Dean's suggestion to reduce the exhaust area by constricting the stinger, except this wasn't a controlled situation. Rather, I reduced the exhaust area about 60% but it was constant from full rpm to idle.</span></span>