RE: MVVS Exhaust option
Depending on the prop -pipe setups used - look for 500-1000 rpm losses .
One combo on the 26 MVVS= 2000 rpm difference! That's about half the power -
look at the "power calc's on props for a rough view of power to run given props at specific rpm's.
That 400 rpm can easily be 20% of power .
My own foolin around bears this out -- for example -- take a 26x10 mejzlic -pretty good sample prop for testing as they are very consistant in size/shape etc..
On Older in cowl muffler on ZDZ80 -- 6000 rpm
Now a 80cc tuned can - 6700
finally a tuned pipe -aprox 7000.
These are worst to best cases I have actually tried.
Look up hp differences on 26x10 menzs or Mejzlic -whatever - and note power for each 1000 rpm
On smaller engines the rpm spread is different but the curve is about the same.--In this instance -from about 7-11 hp spread
not perfect but a fair idea as to losses n gains.