Notice the Mac's specs on the .61 RF (near the bottom of the page)

http://www.macspro.com/tuning.asp

It calls for a 6" header (long) and a 11,300 rpm with the 11-8 Zinger.
Notice the header length on my RF. Mine turned 11,400 with 15%
PowerMaster with some added castor. I left it alone.

BTW, that engine is not likely to "come on the pipe" no matter what you
do to it. It is a long stroke engine.

If you have never tried the Zinger props, give 'em a try. If all you use
is APC's, you're in for a suprise.

FBD.