i've got a friend who has got a 90, he likes how my 5" exhaust sounds. stepped it up from the 3" to the 5" right behind the t'case (lack of room closer to motor) i didn't notice any power difference really which didn't expect any, just lots of turbo sound which is what we all want. but my friend did the same thing and his rpm dropped to about 650 or so from the 750-800. 5-spd non-intercooled just like mine. why did his drop, but mine didn't?

 

I can't imagine why the idle would drop, I don't think the two things are related.

 

i could be wrong. but my only guess was not enough back pressure, or not allowing the motor to warm up fully due to open of exhaust. but i have heard these diesels only need like 2 percent back pressure. but that does not explain why mine did not change. he took his exhaust back off and his rpm went back up.

 

If the exhaust change caused the idle to change, there is a problem somewhere. The information you gave is the exact opposite of a restricted exhaust system. Something else has to be going on.
PS; you need backpressure like you need EGR.