540: Cam Gears and Prop Gov Ratio Question

[lr172]

Bryan's post got me thinking. I am rebuilding a 540 ND and I am replacing the 2 piece cam with the one piece cam. I also know that the WD's have a different prop gov. ratio than the ND's.

I am now wondering if the difference in ratio comes via the gear on the cam or the driven gear on the idler shaft. I knew that Lyc recommended a straight swap from the 2 piece to 1 piece, so I assumed the ratio change must be coming from the different driven gear between models. I also knew that the parts collection on the idler shaft is different and that seemed to support my speculation.

Can anyone confirm that my logic is correct here? I don'[t have the one piece cam in my possession yet to compare the two gears.

Thanks,

Larry

[Toobuilder]

Regardless of the accessory drive speed, the cam and crank will always maintain a 2:1 relationship. Any variation in accessoriy drive ratio will build off that foundation.

[lr172]

Originally Posted by Toobuilder

Regardless of the accessory drive speed, the cam and crank will always maintain a 2:1 relationship. Any variation in accessoriy drive ratio will build off that foundation.

I understand that. I am referring to the gear on the forward end of the cam that meshes with the driven gear on the prop gov idler shaft at the forward end of the engine. Given that the ratios (crank speed to prop gov speed) are different between the WD and ND, one of the two gears or both, could be different. Or it could be the two gears that convert the rotation 90* for the prop gov driveshaft are different (which was my assumption). All of this is based upon the standard assumption that the cam always tuns at 1/2 of crank speed. However, something is different that is creating the different ratios. I just want to confirm it is NOT the forward gear on the cam that is creating the difference.

Larry

[659JR]

It is not the gear on the end of the cam. The two bevel gears in the case are different between the ND and WD engines.

Jesse