21 tooth Pinion Opinion

I'm not sure how well the new pinion does on a ST, but it does very well on an OS. That extra tooth is designed to further reduce the engine's torgue load. You engine will have to rev a bit higher to achieve the same target headspeed RPM's than with the 20T. The more teeth the higher your engine will have to rev. Less teeth will reduce the engines RPM's, but increases it torque loading; making the engine work harder to achieve target RMP's. That should apply to any engine with that pinion installed.

Your engine will peak faster and rev higher. That should reduce the potential for the head to bog during aggressive pitch transitions, making your heli more forgiving in the case of poor collective management. If your engine is bogging, it most likely a tuning issue.

Give that 21T pinion a go, I think you'll see a noticeable improvement. It should work well on the ST. Let us know how it goes.