Why don't diesels use superchargers? That way they would have boost at all times?

 

The power loss is the reason. In order to run a supercharger at 20psi+ boost levels at 3500rpm and lower you'd have to overdrive it heavily and the parasitic drag would be outstanding compared to running a turbo. Detroit Diesel many years ago ran both a turbo and a supercharger on the same engine

 

The main reason is Superchargers arent as efficient as turbos.. You would need to overdrive the hell out of a supercharger to make it as efficient as a turbo. Like Zippy said, Detroit Diesel ran superchargers on all there "V" series engines, most of them still using turbo's in conjunction..
Piers Diesel experimented with a supercharger on a Cummins. It was so overdriven, that it was making 5 psi of boost at idle, and 40 psi at 3000rpms, which eventually led to its demise due to the head unit not being able to support that amount of boost..

 

a diesel puts out so much more exhaust gas volume than a gas engine it only makes sense to make use of that volume by slapping on a turbo and turbos produce more boost more easily

 

Ahh, I see. Well that makes sense. Thanks for the replys

 

I still work on those old Detroit's at my work. We rebuild 3-53,4-53, 4-71,4-71ti,6-71,6v-53,6v-71, and 6v-92. They all have roots style superchargers but the 6v-53, 6v-92, and the 4-71ti are turbocharged also.