Quote:
Originally Posted by Crumpp
It seems very likely that RAF pilots were authorized to "pull the tit" on their aircraft using 87 Octane fuel with a lower manifold pressure boost gain. It appears to be independant of 100 Octane fuel use.
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Dear Crumpp,
I think this is incorrect about the Spitfire boost control cutout. It actually had quite different roles pre and post modification.
The original boost cutout was a true "cutout" in case of failure of the boost controller (which controls at +61/4psi). It gives full throttle valve control back to the pilot, enabling him to get +20psi boost (full unthrottled supercharger output) at ground level if he desired. Of course with either 87 or 100 octane this would not be a good idea.
There is no documented use of the original boost cutout as a combat boost system I am aware of, but it would be possible. The pilot would need to be very careful not to exceed a boost level that caused predetonation or engine damage, and the boost would continuously change with rpm, height and throttle position.
That document giving 10.55 psi boost is very interesting. As Banks suggested, it appears to be the boost attained with full throttle and (unmodified) boost cutout pulled at height. The height is about right. Interesting in that such a high boost was usable at all in a Merlin III on 87 octane.
However it is very unlikely that pilots were ever authorised to use the unmodified boost cutout as emergency power because the risk of instant degradation of engine performance was high (especially at low altitudes)
When the boost cutout was modified it was actually no longer a "cutout". It became a boost setpoint changer from +6 1/4 to +12 psi. It then became authorised as a practical combat boost (in conjunction with 100 octane fuel) that Dowding could fret over.
camber