Chris,
I believe that the terminology of "Stage I, II, III, IV" is not unique to
the Spitfire, but is common across much engine tuning. Each stage I think
does refer to a specific level of re-work, but I'm not sure exactly what the
definitions are - at the milder end they involve polishing and porting the
cylinder heads, moving through high-lift camshafts and enlarged valves.
Also it is possible (likely?) that everyone has a slightly different
definition, or even makes up their own.
Rimmers have a catalogue of Rover V8 engines to various specs (better than
any underwear catalogue!), they sell Stages I, III, and IV, described as
follows:
Stage I: Polished and lightly ported cylinder heads featuring bulleted
guides and standard size valves. Suitable for standard and mildly tuned 3.5
to 4.6 litre engines.
Stage III: Fully polished and ported, gas-flowed cylinder heads, featuring
bulleted and shortened guides and fitted with Big Valves (41.45mm inlet,
36.5mm exhaust). Together with the right camshaft and fuel system, these
heads offer a substantial power and torque increase, and are suitable for
Performance engines up to 5.2 litres.
Stage IV: Ultra Big Valve heads. Fully polished and ported as Stage III,
but featuring 43.11mm inlet and 38.1mm exhaust valves. These cylinder heads
are suitable for serious Performance applications, in engines of 4.6 litres
and above, and require suitably modified fuel systems.
Not really the full answer, but some idea at least. If someone can provide
a definition of Spit engine upgrades that might be more useful...
Richard & Daffy
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