There are quite a number of changes from the Mk 3 to 4, which is why we don't offer upgrades. The complete rotor assembly (shaft, turbine, compressor etc) is different, and that's an expensive item. The parts which fit closely against the rotor - front intake and NGV - would also need to match the new rotor, and there are other items too.

All our thrust figures are conservative, we give figures that all engines should be able to achieve but most do better.

Sara Parish
Wren Turbines