question about yamaha ypvs servo thingy

I hope this isn't too stupid of a question but what does the YPVS servo thingy do?

 

I may be wrong, but instead of cam driven exhaust valves they are electric motors that spin the rotary valves of late model engines in time with crank RPM. I've heard that they are same stepper motors used in computer hard drives.

 

On the RZ 350 they're cylindrical gates that hang down in front of the exhaust ports. At low rpm they rotate down and effectively lower the top edge of the exhaust port to retard exhaust timing. At high rpm they rotate up out of the way to allow earlier exhaust timing. The whole idea is supposed to improve low speed performance from cylinders ported for high speed performance. At least that's the idea anyway. Some have said that it's more gimmick than performance.

[This message has been edited by TZpunk (edited 07-17-2001).]

 

on most bikes they call them powervalves

 

YPVS = Yamaha Power Valve System.

Basically, a variable exhaust port timing system. Works off of the ignition system. Improves low end performance for street application.

On race RZ's, some folks just disable the system and position the valves at wide open or maximum exhaust port duration. Others use a manual system, with a hand lever (think thumb brake lever), and use it only on starts. I still have mine on my race bike ... I prefer to have it just in case I find myself in low RPM's. However, if you use aftermarket ignition systems, the YPVS system will not function.

 

On the TZ they are mechanical slides positioned outside of the exhaust ports as part of the cylinder that slide from full open to partially closed (position based on ignition timing.)

They effectively change back pressure and theoretically modify the length of the tuned exhaust system to keep port scavagening (cleaning out spent exhaust gasses)optimal

 

On the '89RGV250 they are a pair of overlapping plates that slide (and have a tendency to get gummed up). The RGV is cool in that they have a test mode, by turning on the ignition key very slowly, it initiates a cycling of the power valves and you can watch the rotary actuator to determine if your slides are moving through their full range.