[41Sqn_Banks]

Basic guideline:
- Keep engine speed below 2,600 RPM.
- Keep oil temperature below 90°C
- Keep water temperature below 95°C
This are not the absolute limits, higher values might be possible, maybe only for limited time.

Historical reference: AP 1564A Hurricane I Aeroplane Merlin II Engine - Notes concerning the Merlin II engine

(i) Limiting operational conditions.-

Take-off            Maximum r.p.m.         2,850*
(up to 1,000 ft.    Mimimum r.p.m. at  
or for 3 mins.)     maximum boost          2,080
                    (+6¼ lb./sq.in.)

Climb               Maximum r.p.m. at
                    maximum boost          2,600*
                    (+6¼ lb./sq.in.)

Maximum cruising    Maximum r.p.m. at
(mixture control    maximum boost          2,600
"Rich")             (+4½ lb./sq.in.)

Economical          Maximum r.p.m. at
cruising            maximum boost          2,600
(mixture control    (+2¼ lb./sq.in.)
"Weak")

All-out level       Maximum r.p.m. at
(5 mins. limit)     maximum boost          3,000
                    (+6¼ lb./sq.in.)

Maximum dive        Momentary maximum
                    r.p.m. at maximum
                    boost                  3,600
                    (+6¼ lb./sq.in.)                    

*These r.p.m. will not be obtained with a fixed-pitch airscrew.

[...]

(iii) Oil inlet temperatures.-

Maximum for opening up              15°C
Maximum for continuous cruising     90°C
Maximum for climbing                90°C
Emergency maximum                   95°C
(5 mins. limit)

(iv) Coolant temperature.-

The engine which employs ethylene glycol as the
cooling medium, should not be opened up to full
power until the radiator temperature exceeds 70°C.
The maximum permissible temperature in flight is
120°C and the recommended cruising temperature
should not exceed 95°C.