Exhaust rust

[basement]

So, it's still winter here up north. And I thought I was doing the right thing by going out maybe twice a month to start up my car that is waiting patiently under its car cover. I have been allowing her to run for about 15-20minutes at a fast idle before shutting it down. The outside temp has been around 29f when I did this and someone told me that I am eating away the exhaust pipe cause I don't let it run long enough (though the temp gauge reads 'at temp' for about half the run time). and that moisture builds and ends up sitting in the exhaust pipes allowing them to rust. Do you feel 20minutes is long enough to let the engine idle? Or am I better off just leaving things sit until spring?

[Sublime]

Don't even bother starting it. Its not necessary no matter what you hear.  Some people are religious about having to start it up once in awhile but its really not needed - you are doing more harm than good.

[Mopar73340]

Engine temp is irrevelent to the exhaust temp. Look at the tailpipe if there is steam or water still comming out then the exhaust system has not got hot enough to clear out the moisture.   

[Oldschool]

I crank the Beast and drive it around for 5-10 minutes every couple of weeks. ( I took it out today).  The main reason I do it is to let the valve springs that have been sitting in the compressed state decompress.  With the big lift roller cam, the springs are really compressed.  When I shut it down, perhaps different springs are compressed and it allows different springs to sit compressed for a couple of weeks.  10 minutes usually gets it up to temperature before I get back home and shut it down....