I've got several HB PDP engines and like them very much. They run great but don't make that much power by todays standard. A non-PDP .61 makes about the same power as a Modern OS AX .46 and the PDP Chrome Blitz engines (Black Heads) don't make that much more.

They're perfect for models like the Astro Hog, Hog Bipe, The Telemaster, etc. They're very reliable and easy to tune.

If you want More Power in a .61 size engine there are better engines out there that don't need a tuned pipe or the hassle that goes with it.

I've got two OS FX .61 engines that spin an 11 x 7 better than any of my HB's

I'd keep the stock muffler on them and enjoy them as is.

Just my opinion.