Up to 2400 will work successfully, they have been taken higher but cooling fan speeds become excessive and plastic fans blow apart and wreak havoc.

If you're running a Formula series engine upping the rpms that much will quickly take it out of it's max torque range. They are setup to run 1800-1900 rpm for torque and economy.

I usually run it at 1650 1700 when not in a rush but sometimes run 1900 when need to meet time. tons of people say over 1800 it'll come apart

It won't get hurt that easy unless you try to run it above 2500 rpm for sustained periods. A 350 governed at 1900 rpm is a formula engine tuned for fleet economy. Cummins also built a 2100 rpm 350 engine so if you like a little more rev room on top you could very safely set it to 2100 or even 2150 like the older model NTC 350's were governed and you will never come close to hurting it.

As stated earlier, a Formula 350 is set for 1900, factory setting for a nonFormula motor is 2150. Mine is set at 2350 and will run that all day. I've seen them set to run 2600 or higher, but they won't run that for extended periods of time. Cruising rpm for the old NTCs, including the Big Cams is 1700 to 2150. Those with Jakes work best at 2000 to 2150 on the Jake.