Hazmat

It's not under the jet engine section in build mode. You can fit any engine to a jet but it's not specifically a jet engine, even in air mode.

I'm purposely hiding my design for now until I can get the absolute most out of it. I'm using some design tricks that I haven't seen other players do in KSP yet. (no exploits though)

My air intakes tend to cut out around that altitude, I can only get a constant flow of air below 7 km.

For this test I'm only interested in using "jet engines" from the build menu, but for comparison sake I fitted a Rapier to my build and hit 1903 m/s (Mach 5.6) in level flight around 100 meters. I don't really consider this a fair comparison to jet engines even in open cycle.

Surprisingly you can go faster at low altitude. I tried it at high altitude and had trouble going that far beyond 1500 m/s. Last night I pushed even faster (and lower) and hit 1662 in a much more stable craft this time. I was able to sit at 1660-1662 for 30 seconds or so around 100 meters altitude. I'm specifically only using "jet engines" from the build menu, which means no methalox engines like rapiers for this test. It's easy to surpass Mach 8 with methalox engines.

I'm curious if people have been keeping track of this yet, I've seen the land speed record videos but none for air speed using air-breathing jet engines yet. I managed to hit 1641 m/s (Mach 4.87) with my latest design, screenshot below for proof. https://i.imgur.com/WkmhTRh.jpg (I don't know what AeroGUI is but it pops up when I press F12 to take a screenshot.) It was incredibly hard to keep the plane stable at this speed, but I tried to keep it as flat on the horizon line as possible. I might have hit 1643 or 44 when I was angled down slightly.