One of my 850 turbo test guys has a bm tune.
Flyweight is a mirror of engine torque. More torque, more flyweight to maintain "rated rpms". If the engine is rated at 7950 rpms and now you add 10 pounds of torque at 7950 rpms, then you have to add more flyweight to keep from overrevving past rated rpms.
Stock (with iBackshift clutch tune)
984 ramps
Flyweight @ 102 grams
Primary spring @ 100-340
After going to bm tune stage 3 non ethanol w/diamond-s can. (with iBackshift clutch tune)
984 ramps
Flyweight @ 108 grams
Primary spring @ 100-260
Had to add 6 grams of flyweight and take the primary spring end force and lower it 80 pounds to keep the engine from overrevving.
its kind of a rule of thumb that for every 1 gram of flyweight you have to add, there is an addition of about the equivalent of 5hp.
This is easily proved, since for every 1000 ft elevation increase, there is 5hp loss. Every 1000 ft increase, an estimated 1 gram must be removed to maintain rated rpms.
Therefore; IF 1 gram = 200 rpms and 1 gram for 1000 feet and 5 hp per 1000 feet, THEN for every 5hp added, you need to add 1 gram.
5hp loss for every 1000 feet, I got that out of a Rotax 503 fan aircraft shop manual, so they are not my words, they are Rotax's words.
Going from 102 to 108g then that's like there was an addition of 5hp x 6 grams = 30hp increase. Then there is a rule of thumb of for every 30 pounds primary spring end force change, there is a hp change to match it. The calculation is a little bit more complicated because you have to factor the track speed to know what forces are being used by the spring, at-that-track-speed.
I would put my neck on the chopping block and safely say there's a 30hp+ gain to pull off that amount of flyweight gram increase and not put forth how much more hp there is based on primary spring change.
