OK, I'll give as much info here as I can, may be a bit long winded. Bike: 2005 YZ450F w/~32 hours, PC TI-4 slip on, YZ 250 filter cage, jetted perfectly (or as close as possible), stock gearing (14/51) w/o-ring chain, RG3 3-clamp, Pro Taper, Dunlop 773 front and 756 rear and a Dr.D 8oz flywheel. Rider: 35yo male, 6'3", 240lbs, 8 yrs woods exp, very little MX exp, no riding for last 3 years (military), last bike 94 YZ250. Location: Durhamtown Plantation, GA..MX tracks with hardpacked red clay, groomed red clay (one day only), somewhat dusty. Miles of trails with dry and wet clay, sand, loam and mud in various locations. Also a clay drag strip. Conditions were Sunny, 65-80 deg. 40-65% hum. The reason for the mod: Looking to tame the bike some as it is wearing me out after only a few laps as I am not used to the power. I can't control the throttle well on jumps or in the rough stuff. The power hit, or fear of the hit, causes me to loop out, nose in and fatigue/pump up prematurely. After a bit of research I decided to go with a Dr. D 8oz flywheel due to reputation, cost and the help I received when calling. The Dr. D is an OEM flywheel with the weight welded to the perimeter of the flywheel. This is better than most bolt ons as they often have some of the weight in the center hub area which is not as effective and the reliability of the welded weight is greater, no parts to come loose and fly around inside the case. When I opened the box I was happy with the look, quality and design of the flywheel. Installation was a snap with the flywheel removal tool I also purchased on the same order. Everything fit as expected and no problems occurred. Costs were $89.99 + $200 refundable core charge for the flywheel, $11.99 for the puller, and $22.00 next day air to GA. Quite reasonable for the support you get from Dubach Racing. Upon starting the bike, I could tell no difference. She still fired on one kick and I did not have to kick any harder than before. She still revved quickly and idled the same. At this point you would not even know it was installed. Before going to the tracks, I had to test traction on a pure sand road. Where I would spin like crazy before, I found some traction, lifting the front wheel in a matter of a foot or instantly instead of feet or yards it took to gain traction previously. On the tracks, the bike had a whole new attitude. I found new confidence in turns that I could not handle before as the rear used to slide out. I was jumping more jumps and clearing some I could not before cause I had confidence in the throttle control. I had almost complete control of the attitude of the bike where as before I felt like landing front wheel first or last was up to someone other than me. I seemed to be riding harder because I was not AS afraid of losing control of the bike. And the best thing is my endurance increased a bit as I was riding 1-2 additional laps before fatigue/arm pump made me stop. There was one turn, a flat 100deg hardpacked dust covered turn that I had to go slow on because I would break the rear loose and fishtail on. Well the rear was hooking up so well that I eventually washed the front out in the turn. Also I was able to use any gear on the track and could use 3rd in all but the slowest stuff without stalling. In the woods it was the same story. I had traction everywhere. Wet clay from morning dew that I used to coast on, I could use moderate power on. Sand and mud was almost too easy. Rough sections became almost smooth as I could roll the power on and control a wheelie through them without worry. Tight sections were no trouble as stalling was lessend. In fact I only stalled twice as I tried 3rd gear starts thinking I was in second, because I had been in 4th gear in places where I normally rode in 3rd...DOH!! Now for the big question......How much did it hurt total power. None that I could tell. With perfect traction, I am sure your 1/4 mile time may be off a few tenths, but in all other cases you are faster because of the new found traction. The power is still there it just feels like it got spread out and is more controlled. But crack the throttle wide open and all the power I remember being there is still there. At the drag strip I did not loose once out of 7 runs. Even when I was on the right lane (damp clay as opposed to tacky on the left lane) and got off the line late, I would catch and pass the other bike or quad. Conclusion: I would recommend this mod for anybody having difficulties with the power or traction on their bikes (weight size may be different for your application.) I was faster in the woods and on the tracks because I was more comfortable with the bike. It did not beat on me as bad and was more predictable. That in turn allowed me to have better throttle control, bike control and improved my endurance a fair bit. I enjoyed riding a whole lot more and should improve at a faster rate. Now if I can control my blisters on my hands for a 4 day weekend of riding....that may be another write up in the future.