Author Topic: Power Valve Operation???  (Read 267 times)

Offline MotorGeek - Jerry Hall

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Re: Power Valve Operation???
« on: September 04, 2020, 06:45:05 pm »
Been searching and can't seem to find any info on how the power valve works on these engines...specifically how the gear driven governor makes the valve close at upper RPMs.....I've heard different oil viscosities and temperatures can change the RPM the valve closes as well?

Also is there a general RPM setting the PV should close at??? I've heard 5800RPM on the 500's but nothing on the 250's.Obviously the pipe/engine combination will have some affect but just looking for a general starting point......Just trying to educate myself on the operating theory and gain some info.Thanks

Thanks Jerry..... ..Another question,does oil viscosity affect when the power valve closes??? For instance if you are using automatic transmission oil and set your PV to a certain RPM and then change to say a heavier 15w40 oil will the RPM the valve closes change drastically?
 6300 to 7000 rpm on the Lt250rs is range for LT250s.  The Power valve closing RPm point depends upon the porting and pipe combination.

I have not performed the test you described with ATF and 10w40.  I have seen the closing RPM point  change a few hundred RPM when doing 3 to 4 back to back dyno runs where there is only 45 seconds to 1 minutes between the 1st and 4th dyno run.

A radial thrust bearing is sandwiched between a gear face and a actuator plate that has the power valve actuator rod attached to it. The friction from the rotating gear and the thrust bearing, imparts rotational force onto the actuator plate causing the push rod to move and rotate the power valve spool in the bottom of the exhaust port.  Anything that changes the frictional rotational force imparted on the actuator plate will cause the power valve to close at a different RPM.  I would think that the viscosity change in the ATF would be less than the viscosity change of the motor oil with the same temperature change.

When the spool closes the path between the exhaust port and the chamber in the engine cases, the helmholtz frequency of the exhaust pipe is changed.  When the helmholtz frequency is changed the engine sees an entirely different exhaust system.  Now we have one exhaust system that is basically tuned to enhance the power at a lower RPM and another pipe that is tuned to enhance the power at the higher RPM.  Each of these two exhaust systems generate entirely different shaped power curves.  The two power curves will intersect each other at one unique RPM point.  The RPM intersection point is where the power valve needs to close very rapidly to essentially change pipes at that RPM point. 


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