Act 1 for first stage engines, top speed around 2200mph. Take it to 60,000ft and then turn off first stage engines (Act 1 again) and activate second stage scramjet with act 2. Fly up to 100,000 and it'll just keep accelerating. I once got it to 50,000mph. Also, I recommend flying on a clear night, cause the scramjet engine looks awesome.

The Sr-92 is a black project created to replace the Sr-71 after its retirement. It was built with lessons learned from the experimental XB-70. It uses a two stage engine system to achieve insane speeds at low earth orbit. First stage is a turbo-ramjet similar to the engines of the sr71,which take it to Mach 3. Once at altitude, second stage ramjet is activated to take it to top speed. Enjoy!