Sea level air pressure is average 14.7 lbs/PSI. Let's call it 15. 15 divided by 2 equals 7.5 which can also be expressed as 13.3%. So that would translate into about 13.3% lower pressure when compressed in the engine. I dunno how that translates into temperature drop but less air compressed into the same space will have a lower temperature. I do know though that there will be about 13.3% less air available for combustion. So I guess max HP would be lower. But since the diesel engine types I'm familiar with, which are 40 years old or older, always compress as much air as possible since the air isn't throttled, it seems like there would be plenty of air at any setting below max fuel injection. So maybe less air compressed leads to lower compressed air temp which leads to harder starting.
Eric
