FIAT A.S.8 ENGINE.

[sibboh]

Sorry, but there is not possible parallel. The possibility to reach 2400 rpm, and even 2590rpm, is clearly stated in the manual. We know how to rev up the engine, and there is not a single statement, in the manual, about the fact thath the throttle doesn't work once reached, say, 2200 rpm. So we have to assume that it works at least until 2590rpm.
On the contrary, there is one described system to overboost the engine, and is clearly stated that this system doesn't work over 2000m, and is clearly stated why it doesn't work. So, if there isn't the description of another system to overboost the engine, we have to assume that this isn't possible over 2000m, at least for the version of the engine described in the manual. There is not a "lack of text". There is a clear statement, followed by an explanation of how the device work and why it doesen't work over 2000m.
However, we can add something, due to pure physics. The possibility for the overboost to be obtained only up to 2000m is intended at takeoff rpm, so at 2140rpm. As well as the "quota di ristabilimento" (the maximum altitude at which the supercharger is able to provide 822 mm hg) is 4000m at 2250 rpm (potenza normale).
Given that the supercharger is directly geared to the cranckshaft, at higer rpm the supercharger rev more, and gave more pressure. So, the automatic barometric control has to mantain the intake valve more closed to mantain the 822 mm hg, so the overboost lever will work at an higher altitude than 2000m.
For the same reason, at higher regime than 2250 rpm, the supercharger can provide 822 mm hg at a higher altitude than 4000m.

I didn't want to do a parallel, but only emphasize the fact that, as for the overboosting (apart takeoff), we have no explanation about the conditions (limits, altitudes, power output etc...). We have only a power chart. I doubt that they included an incorrect or useless power chart. So i wanted to reflect about power output that are marked on that chart.
Your latter explanation seems plausible. I'm not technician. And I continue to learn. I try to start from the questions that come to my observing the data.
Infact all "overboosted" power output in the RC40 engine chart are between 3500 and 4000m. Two are obtained at 2400 rpm (circa 860 mmHg and circa 890 mmHg) and only one at 2250 rpm (circa 854 mmHg). How do you explain the higher pressure at 2250 rpm at 3500m?

 

[sibboh]

It may have been, I don't know or have not read anything about Italian fuel. I do know that US and British fuel was different until sometime in 1941-42. Especially the 100 octane.

The B4 may not have changed. According to many people the C3 fuel did. Early and late examples show different qualities.

Some German engine development was planned around C3 fuel and had to be back pedaled or worked around with the shortage of C3 fuel. ( the use of MW 50 and Nitrous).

With engines taking 4-6 years to bring form drawing board to production the crystal ball used for future fuel availability had better be pretty clear.

I know that Piaggio P.XIX used 92 octane fuel.

 

[sibboh]

Searching on the web i found this:

 

[Dogwalker]

I didn't want to do a parallel, but only emphasize the fact that, as for the overboosting (apart takeoff), we have no explanation about the conditions (limits, altitudes, power output etc...). We have only a power chart.

We have not only a powerchart about the RPMs. We have the clear statement that 2400, and even 2590 rpm are obtainable, and there are nothing, in the rest of the manuale that could make us think that tha statement is incorrect, so we have to take it for real.

How do you explain the higher pressure at 2250 rpm at 3500m?

I can only make speculations about that. Given that, as said, a supercharger that could provide 822 mm hg at 2250rpm at 4000m, could phisically provide more than 822 mm hg up to 3999m, that the same is valid for 2400 rpm, with a parallel and higher power curve (as in the chart), and that the engine is capable to handle up to 890 mm hg, it seems that the chart shows what are the phisical possibilities of the engine. Indeed, this is the manual of the engine, not that of the aircraft. Even if, in its basic configuration, the engine does not seems to have a system able to fully open the intake valve of the compressor over 3500m, as would be necessary to reach the higher points of the curves (also because, looking at the chart, to fully open it at lower altitudes would probably have destructive outcomes), an installation on a particular aircraft could have it.