Interesting![8D]
Are both turbines using Xicoy controllers(I know that the SS is Xicoy)? The reason I ask is that I had the same problem with a Kingtec 80(using Xicoy controller) in a Boomer. The only way I could solve the issue was to reroute the rpm sensor
Went through the motions of removing tanks, enlarging intakes, moving turbine forward and backwards,etc.
Because there are only two possible routes to the back for wiring, we keep turbine wires together and servo wires together. After moving the RPM sensor to the "servo" side away from the turbine wiring and it worked.
I know it does not make much sense but a long shot, worth a try?