enginehistory.org

[mabbott]

Just wondering the situation behind the different hp ratings that for example the Germans gave to the BMW IIIa (185hp) vs the post war British report for the same engine giving it a rating of 230hp?

Was this due to differences is the test method/process to arrive at the values or ?

[rinkol]

The BMW IIIa was a special case due to the optimization of the engine for high altitude performance. The main idea was to use an exceptionally high (for the time) compression ratio and throttle the engine below the design full power height so that problems with engine knock would be avoided. Consequently, the power would be roughly constant up to the full throttle height, rather than falling off with altitude starting from sea level. Since the engine was not intended to operate at full throttle at low altitudes, weight was minimized by designing the mechanical components for the intended maximum power. The British did not realize this at the time that they tested the engine and ran it at full throttle. They succeeded in getting a substantial increase in power, but, my recollection is that, unsurprisingly, the engine failed after about ten hours.

The question of the criteria for determining power ratings is a good one. Later, when supercharging became common, it was possible to take advantage of improved fuels to significantly increase the power for a short duration - this was useful for take-off. Depending on the specified duration (e.g., 1, 5, or 30 minutes), one could give quite different figures for the same engine. This substantially complicates comparisons of the engine ratings for engines produced by the different countries in WW2.

Robert