Dean said something important, the original reed spacers were made for Yamaha YZ motorcycles dating from the early 1980s where the reed stops blocked the rear center boost ports that linked the crankcase to the top end. Putting a reed spacer on those bikes made a huge difference. So aftermarket companies made them for everything and claimed 5hp, no matter if its a KX60 or a CR500. Every once in a while they resurface under extravagant advertising claims and sell for a while then disappear.
This is what a reed spacer does;
1) Increase the volume in the crankcase
2) Lengthen the intake tract
3) Lengthen the distance from the piston face to the reed tips
The crankcase volume and compression ratio is different for all engine displacements. Whats the optimum? There is a range of standards from 1.2 to 2.0 to 1. Large displacement bikes tend to have high ratios because of inherent geometry, just the normal sizing of the cylinder, crankcase, and crankshaft flyweight diameters. Small bore engines are challenged to get the crankcase compression ratio high enough for quick throttle response.
If you look into the early karts of the 1970s, there were all sorts of aftermarket accessories that took up volume in the crankcase including piston stuffers that were plastic inserts fastened to the cases and fitted to the underside of the piston. They had ellaborate porting to channel the gasses being squeezed out from the underside of the piston towards the transfer ports. They called them turbo-squeezers, piston superchargers, and high speed gas funnelizers. All funny stuff! I'm surprised that no modern aftermarket company has copied and recreated that old stuff from karting. In 1975 Mercury produced a snowmobile called the Snow-Twister that featured a shielded crankcase with a knife shaped connecting rod that protruded from a narrow slot in the crankcase. The case was fitted tight to the flyweights to reduce windage and volume in the crankcase. KTM copied that shielded case design for the late model 125 and now shifter kart tuners are fitting aluminum tubing to the cases of CR125 and CR80 engines.
I guess all I can say about reed spacers is that they may or may not work for your bike and riding situation. My friend Jim always puts them on his 144s that I build him every year. He swears by them and I swear at them. But as long as he thinks they work and it makes him ride better, who am I to argue.
Two-stroke tuning is subjective and the world of unsteady gas dynamics is a turbulent one :eek: