Power Valve Engine For Duner?

[c_hustle]

Just wondering how a PV engine does on mid and top range? Riding style is fast duning, sliding and climbing with occasional hill drags. Thanks in advance

[fearlessfred]

they are more suited for mx and woods were you need grunt ,not so much for the dunes

[rsss396]

The problem with PV found on any 250r cylinder is it a pressure operated valve, the factory's knew long ago you needed to mechanically or electronically open them to make the best power. Pressure operated valves will want to close in the over rev because pressure drops, if you adjust the valve so it does not close then it will want to open to soon on the bottom end.
CP is no longer going to machine a PV in the sphinx cylinders because of these issues and infact they sell block offs to eliminate them.

[Jerry Hall]

The problem with PV found on any 250r cylinder is it a pressure operated valve, the factory's knew long ago you needed to mechanically or electronically open them to make the best power. Pressure operated valves will want to close in the over rev because pressure drops, if you adjust the valve so it does not close then it will want to open to soon on the bottom end.
CP is no longer going to machine a PV in the sphinx cylinders because of these issues and infact they sell block offs to eliminate them.

I started testing pneumatically actuated power valves in the early 1980s in the Rotax road racing engines and have done  a lot more testing since then and what RSS stated is exactly what my testing results has shown.

There are a lot of other problems associated with the pressure activated power valves and my experience has shown these types of power valves add considerable cost to an engine project with minimal power gains in the part of the power curve they are promised to improve.  When the valves are operating correctly they will usually improve the power 10% to 30% in the lower RPM range where the engines is ready to come on the pipe but not as much as downshifting one gear.    

The dyno chart from a power valve engine looks impressive but is it really significant in the big scope of things?  Consider this sineiro where we have two engines that make the same peak power but one has a power valve and the other does not.  If you are rolling along at an RPM where the power valve will improve the power and someone rolls up alongside you that does not have a power valve.  The other rider gives you the nod and wants to see how your bike runs.  Are you going to try to compete with him by just opening your throttle and have 10% to 30% more power than him at the same RPM?  If he is not brain dead he is going to downshift one gear and double his power and leave you in his dust and you should do likewise if you want to compete.

[FerrinMotorsports]

I agree with you in a drag Race...But not on a TT or flat-track race...Less shifting means faster lap times.....

[rsss396]

I have not had allot of experience with pressure operated PV's but on a snowmobile engine the only thing i did like about the pressure PV's compared to a block off, was it seemed to have cleaner idle and off idle performance even with the PV spring set very lite. Snowmobiles don't need over rev because of the clutching, so I was able to run them with a decent amount of spring pressure to keep them closed a little longer

[Jerry Hall]

I agree with you in a drag Race...But not on a TT or flat-track race...Less shifting means faster lap times.....

Yes a wide power band that allows you to run one gear around the whole track is always faster unless you are running a 1/2mile or mile track.  What engine/cylinders are you running?  What is the lowest RPM you typically see in the corners?  What RPM do you typically see at the end of the straight-away?

[KASEY]

I have a esr 350 power valve motor in my dune bike,,,, when it comes time for a bore job I am going to have it blocked off I see no gains in the dunes at all,, and its just more moving parts to break ,,

[rsss396]

Kasey you can make a shim and block the valve wide open

[udontknowme]

if i can get to it ill run mine with the valve functional then disconect the linkage and prop the valve open to see the difference. i havent messed with it much so i dont know the port timing but with the valve open the exh roof is 37mm down from the cylinder deck and valve closed the roof is 47mm down from top deck. 72 stroke with piston flush with deck at tdc

[Burns363R]

My experience with power valves is exactly what RSSS396 said.  I absolutely hated my PV engine on the MX track.  The lack of over rev power was killer for me.  Yes it had more bottom end grunt but going up a face of a jump and needing to shift right before take off or letting the bike nose over if you dont shift is a tuff thing.   It will send you through the air in a fashion that is not desirable.

I like running BB stroker motor instead of the PV.  You can build more bottom end with the stroker, and still have a smooth power band with ample over rev.  I think a PV would be great on a XC bike where you can constantly short shift and stay in the middle of the power band for long stretches.

[KASEY]

Kasey you can make a shim and block the valve wide open

ok,, but what about the port shape with it open? would it be better with a block off with a shape to help airflow ? I am not sure about what it looks like with it held open from the inside....  just courious....

[Dirt man]

ok,, but what about the port shape with it open? would it be better with a block off with a shape to help airflow ? I am not sure about what it looks like with it held open from the inside....  just courious....

I fixed mine to the full open position with some washers then did some simple port matching to smooth everything out.

[rsss396]

they are very close and until you take the cylinder off for a rebuild it will be fine and you will more than likely not notice a differance

[Tbone07]

I loved the PV on my PC2000 cylinder for woods riding. Top-end definitely felt like it was thirsty for more. Not sure if it was the smaller carb or the PV holding the over-rev back