Using While Lid Closed on MBP 15

mine has been closed all day, and is running 48deg cpu.

the top is slightly warm, but the whole case is aluminum. It wont hurt.

 

I have mine open for the most of the time for some reason. But when I close it I dont notice any differences. So go for it.. If it gets to warm you can alwasy open the lid again

 

Mine is closed all the time. If it gets hot, you will hear the fan. It has so many sensors that it should be safe.

Get Temperature Monitor to check for yourself.

As to what's hot and what's not, there are specs by every chip manufacturer. Acceptable temperatures vary from 60-110 degrees Celsius, depending on type of chip (CPU, graphics, ICH), brand, and model.

 

It decreases the life of your screen to run with the lid closed, especially intensive things. This is due to the heat.

 

i usually run mine in clamshell mode. but i also have it on a notebook cooler that has two fans i had to reverse one to suck the heat away from the notebook as the aluminum does a good job at passively releasing the heat away.
but i also open the notebook about an inch or two while running in clamshell so it can dissipate the heat alittle better and not put too much heat on the glass.

my temps vary from 99deg ~ 124deg Fahrenheit (CPU); GPU 70 ~ 92 dF; open or clamshell so it stays about average for me.