I was just reading about differences between Mk.3 and MK.4 engines...
Mk.3 and early Mk.4 have smaller valves.
Mk.3 crank is lighter and stronger than Mk.4, and of a stronger material (EN40
vs EN16, I believe). The Mk.4 crank also has larger bearing surfaces, leading
to higher bearing drag. Mk.3 rods are also lighter.
Apparently the hot Spitfire engine combination is Mk.3 block and crank with a
later (large valve) Mk.4 head and a Mk.3 25/65 cam. If you really want to go
for it, fit standard Vitesse/GT6 pistons - they're .040" over in a Spitfire,
and raise the CR to about 10:1. Combine this all with std Mk.4 carbs, 1500
exhaust and a straight through silencer...you should be able to get about 88bhp
out of this, according to one tuner, with power from 2800-6500.
And the Mk.3 gearbox is to be prefered because it's quite a bit lighter than
the later boxes!
The Mk.4 and 1500 engines can be made to produce gobs of torque low down, but
never really develop much power - it takes forever to get the heavier pieces
spinning at high speed.
|