Merlin engine limits & failure behaviour

[klem]

Does any of this explain why I get the following results when I push/pull the Boost Cut-out in the Spits and Hurris? (NB I have to jiggle the throttle or hit the WEP key to make it actually go into higher boost)

Hurricane DH 5-20 (AKY) @ ~8,000 feet in level flight
Full Throttle boost: 6.25lbs
Pull Boost Cut-out: 6.25lbs

Hurricane Rotol (FTN) @ ~8,000 feet in level flight
Full Throttle boost: 5.2lbs
Pull Boost Cut-out: 5.5lbs

Spitfire I (ZPE) @ ~8,000 feet in level flight
Full Throttle boost: 6.2lbs
Push Boost Cut-out: 6.2lbs

Spitfire Ia (YTR) @ ~8,000 feet in level flight
Full Throttle boost: 6lbs
Push Boost Cut-out: 6lbs

Spitfire IIa (OVC) @ ~8,000 feet in level flight
Full Throttle boost: 5.5lbs
Push Boost Cut-out: 8lbs

In the Hurricane DH 5-20 and the MkI/Ia Spitfires I don't see an increase in boost and the Hurricane wit the Rotol prop only gives an extra 0.3lbs.

Is that correct?