John, I can't imagine anyone engaging the seperator on any machine at anything but lowest idle. This would be the easiest on all components in my view. As far as high idle, check original specs, and go with that. My 2 cents.
John, I do not see any benifit to slowing down the idle to below 1000 rpm, you have to have enough rpms (torque) to start the seperator turning. On my 81' N6 I pulled my injector pump and sent it to lee as Midwest Diesel in Fort Dodge Ia and he recalibrated my pump to put out 220 hp at N5 rpms. This slowed the motor down about 200 rpms. I also had to put a bigger pulley on the pto (12.5") to compensate for the slower engine speed. The 670HI seem to runs allot smoother with no loss of power. Pete Hinrichsen