RG500 Disc Valve Timing

Kevin Cameron did an article on the Aprilia some time back, and he mentioned that if the valves open too early, the piston doesn`t create the proper vacuum in the crankcase when the valve opens. If you flip the RG valve over, you can alter the stock timing so the valve opens later and closes later, giving the piston that extra 6 or so degrees to pull a vacuum before the intake cycle begins. The same effect can be had by slipping the valve a tooth, but flipping them over gives a far less drastic effect.

- Rob

I would think that in the context of GP tuned factory race motors, theoretical limits of rotary valve intake timing (both opening and closing) are appropriate to explore. I am sure that the Gamma RV disc in stock form does not approach either extreme. The RG's RV is carried on 13 splines. 360dg / 13 = 27.7dg per spline of potential change from repositioning the disc. This would allow the valve timing to be either advanced or retarded by 27.7dg. By reversing the disc, you change the relationship between the valve opening and the splines, giving 2 options. One position will delay opening and closing by 11dg. The other will open and close earlier by 16.5dg. Keep in mind that by changing the disc's position on the spline to open later (to increase initial vacuum draw) you are leaving it open too long, allowing the now pressurized crankcase to force intake charge out the still open RV. Not a good idea. In my opinion, the stock rotary valve timing is pert' nere optimum for both stock and sensibly modified street motors.

- Rick