some interesting charts of power v rpms for GY6 engines
these were posted on a french scooter board in a thread discussing GY6 heads and cams. the posts are in french so you need to translate the pages to make sense
www.scooterchinois.fr/modules/new....SC&type=&mode=0these charts are on page 3
exhaust manifold 23 x 650
I think the colors are
red stock Kymco 125 with stock valves/head
black Kymco 125cc with autotech355 big valve head
green 158cc Kymco with stock valves/head
blue 158cc with autotech355 big valve head
this does not exactly translate to a 50cc small case GY6 engine, but expect the small case GY6 to behave in a similar manner when comparing a 50 to a 72 or 82
what it appears to show is that porting and polishing plus larger valves increases power by about 20% and shifts the power band to a lower lower rpm range (about 500 rpm) and flattens the peak somewhat.
edited: no port and polish...
you also see the power falling significantly from peak to 9000 rpms
9000 rpm is kind of THE magic # for the small case GY6 as with stock gearing, 9000 = 50mph
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this is with different cams and carbs
cams are stock, a9, and juani. I do not know what juani is
green black red 125cc, carb 24mm, exhaust 23.4mm x 650mm
tan purple blue 150cc, 30mm carb, 27 x 650 exhaust
a lot more power from larger valves & exhaust plus more aggressive cam