Effect of boost control cutout prior to +12 psi boost modifications

[Viper2000]

Originally Posted by Redroach

I still don't get what the problem is, sorry. Your argument about the Boost cut-out reads somehow self-fulfilling to me... you don't drill the little hole and you get 17psi, therefore, detonation will occur quickly with 87 oct. fuel. However, I've yet to see an engine detonate in any condition and moreover, why assume that the boost cut-out is modelled without the pressure bleeding hole? Strange.

Did you read read the documents I linked to in the first post?

The boost control cutout mod is part of the process of clearing the engine for +12 psi combat rating.

If you're running on 87 octane and therefore are not cleared to use +12, why would you go to the trouble of modifying the ABC?

As for not seeing detonation, we don't get a message about it. That doesn't mean that it isn't being modelled implicitly or explicitly as the root cause of engine failures.

In any case, that's another debate. The point which I am trying to make is that the effect of operating the cutout is not correctly simulated at present, and that the evidence suggests that the effect of the cutout should change depending upon whether or not the engine has been modified for the +12 combat rating. Since we currently appear to have 87 octane fuel and +6¼ combat power in the Spitfire I, we might as well have the model finished so that it behaves in accordance with the data.

Then (after what I'm guessing would be a minimal amount of work) we should move towards getting the correct engine ratings for the Battle, ie those associated with 100 octane fuel rather than 87 octane fuel.

Originally Posted by Redroach

Furthermore, the way I read it, 6 1/4 psi is connected with 100 oct. fuel (Spit I pilot notes mention "100oct. only for operational units, 87 oct. fuel for 'other units'")

The combat concessions for 100 octane fuel are in section (vii) on the next page. Otherwise the ratings given are for 87 octane.

Originally Posted by Redroach

Soooo... I've yet to test the boost cut-out but in case everyone fails to reach 12 psi with boost cut-out engaged, wouldn't it be the right side of occam's razor if you just assume the boost cut-out is bugged, instead of assuming that every plane runs on 87 oct.?
I've just tested the spit I in level flight and tried to go all-out without engaging emergency power. With the radiator as closed as I felt comfortable with (3 'ticks' before closed, iirc), I went along just shy of 250mph at sea-level. Which I feel is pretty spot-on with the diagram someone posted earlier (it says 246mph on sea level, though that's on lean mixture, apparently, and I flew on fat mixture). Since I further assume that this diagram was made considering 100oct. fuel (it just makes no sense otherwise), my bet would be on 100oct. fuel and perhaps a non-working (buggy?) emergency power.

RAE speed data, complete with your 246 mph TAS in weak mixture:


Note that +12 is substantially faster.

Indeed, +12 for Merlin III at sea level will be faster than +12 for Merlin XII at sea level since the latter has a higher supercharger gear ratio.

I'm somewhat disappointed with this graph really, because it's dated 1941 and has no data points for the pencil lines at +12 and +16. I know that +16 was eventually only used in the Sea Hurricane (for which engine life was not an issue) . However, it's better than a poke in the eye with a sharp stick.

The documentary evidence as to the use of +12 during the Battle is pretty solid, so the fact that the speed graph is less than perfect doesn't materially affect the argument.