Snip from Blackdog_Kt's post above:
"First it was the mechanical tachometers and the negative G cut-out in the Spit and Hurri, developers caved in to pressure from a number of people and changed them
for everyone instead of making them optional."
Apologies for the thread drift but just a point of order and to set the record straight on the above snippet.
The Negative G cutout was exhaustively researched and proven too sensitive. Research involved discussion with 2 current UK early model Spitfire pilots and a visit to the National Archives (one of many). The attached jpgs from a UK Archive document on the matter. As can be seen in the archive document the measured critical value is +0.1G i.e. a reduction of 0.9G from 1G flight, this from instrumented aircraft looking into this issue.
With respect the "Mechanical tachos" and needle bounce. The RPM fluctuations initially shown bore no resemblance to the real case with respect to the RR Merlin... though for a poorly maintained M14P engine might have some truth
. What we see now is pretty close to the mark. How do we know that ? Discussion with a couple of current Spitfire pilots flying RR Merlin Spitfires (one a personal friend who flies both RR and Packard spits). (Packard Merlins have Electrical tachos whilst RR Merlins have mechanical). A commercialy available DVD that has a MKII Spit performing a complete display with a lot of really good cockpit footage with the tacho clearly visible. In addition discussion with two current maintenance chiefs for two well known Spitfire operators. All of these discussions came out with the same thing ... pretty much rock steady RPM indications.
Things wernt/arnt changed on a whim but based on the best level of information that can be found.