What is the highest rpm a large diesel engine, such as in a semi truck, can reach if there was no governor. How much can the rpm be safely increased by?
I know the governor is there to make the engine last longer, but I’m just wondering what the maximum SAFE rpm on most large diesel engines is, that won’t cause it to fail catastrophically. 
the governor is there for a reason, if you want the engine to last, leave it alone.
There is no limet untill it blows apart
depends on the reason for the governor. .
Some are used by trucking companies to keep the speeds down on their trucks but 95mph in a semi can be done. . .
Most limiters are there to protect the engine as most diesels will redline at 4000 RPM. . any faster and they can come apart