Is there some sort of trick, or formula behind getting a flat-ish torque curve when running boost? My max torque runs out almost immediately after peak
First off: the turbo stuff in the game should be considered preliminary, it will be reworked when we get other FI things into it.
For now, try higher AR ratios to keep the torque going for longer 
1.40 AR and peak torque lasts for 400 RPM
4.0L Turboost V8Rev0.lua (56 KB)
Well, I would recommend using close to similar sized compressors and turbines*. Also, as Killrob said, the A:R ratio** can help. Any boost pressures above 0.5 Bar (~7 psi) tends to not mix with the engine’s torque curve alone, leaving a noticible difference. I’ll take a look at the engine later on today and get back to you.
*Doesn’t always work.
**Also doesn’t always work.
Just a silly engine i had previously made. 40 psi from this with a decent flat torque curve. 1.4 AR 71mm Compressor 45mm Turbine.
The turbo stuff is a little bit fidgety but its workable
i generally found in a very simplified sense that for big revs you increase the A/R. and big power you have a turbine 60% ish the size of the Compressor. Reduce the A/R for more response/less revs and smaller engines and scale the Wheels sizing accordingly.
Very demanding and expensive to make tough!