Merlin engine limits & failure behaviour

[Viper2000]

Merlin marks II, III, IV and V were covered by the R.M.1.S. rating, which in its final state was:

Combat power = 1310 bhp/3000 rpm/9000' at +12 psi

or

1440 bhp/3000 rpm/5500' at +16 psi (Sea Hurricane only)

Takeoff power = 880 bhp/3000 rpm/SL at +6¼ psi.

There was also an R.M.2.S. rating which allowed increased takeoff power for 100 octane fuel before the +12 psi combat rating for R.M.1.S was introduced:

Combat power = 1000 bhp/3000 rpm/15500' +6¼ psi. (provisional rating, not confirmed by type test)

Takeoff power = 1000 bhp/3000 rpm/SL at +8¼ psi.

[This rating is a bit strange; my suspicion is that the +8¼ psi may actually be the 87 octane combat rating with boost cutout engaged, because otherwise if everything was rated at +6¼ psi boost then there wouldn't have been any purpose in fitting the cutout before 100 octane fuel was used...]

Cruise and climb were the same for both ratings:

Cruise power = 2600 rpm +4½ psi
Climb power = 2600 rpm +6½ psi

Power isn't specified for either of these ratings; you'd get what you got!

The Merlin XII had a higher supercharger gear ratio (9.089 vs 8.58, and it therefore achieved a higher FTH (but less power due to the increased supercharger drive requirement). It was covered by the R.M.3.S. rating:

Combat power = 1280 bhp/3000 rpm/10500' at +12 psi

Takeoff power = 1175 bhp/3000 rpm/SL at +12½ psi.

Cruise power = 2650 rpm +7 psi
Climb power = 2850 rpm +9 psi

Data from The Merlin in Perspective - the Combat Years, by Alec Harvey-Bailey, 4th edition, Rolls-Royce Heritage Trust.