ZDZ 40 , what props are you using?
Zinger Pro 22x10 is about same load as 22x8 in menxzS
we did a lot of test flying on ZDZ40 with in cowl muffler (original Pitts type sold by RCS) and that was the best overall performance on a 14 lb H9 Cap (72" span )
the rpm was -I think-- in mid 6000 range - do not remember exact - but the prop worked well
The Diablotin muffler runs about the same --
the 22x8 ZingerPro just revved -was noisy and performance was not as good