Having rebuilt a spit using dual SU's I feel the need to throw my $.02 worth
into the current thread. Way back in the dim past, the Triumph SC block (the
Spit engine) used a 5 port head just like the BMC A block (Spridget et al).
Since Triumph wanted to race at LeMans and since the class they were to enter
was a Homolgated (sp?) class, they changed the production head design to an 8
port non-crossflow design and promptly were allowed to use a similar head in
the racing engines. The fact that the racing head was a completely
different casting and thus was probably illegal was of little concern to
anyone involved (that's the way the game was played in those days). In any
case, the old 5 port heads were VERY different from the 8 port heads and
since the stud locations were different the 2 types are NOT interchangable.
There are several different flavors of the 8 port head using different valve
sizes and different combustion chamber sizes. All of these heads will fit on
either a 1300 or 1500 cc block (obvious caution about compression ratios when
you change comb chamber size). All of the ancilary bits like intake manifolds
will swap happily right along with the head. The Mk3's imported to the USA
used dual SU HS-4's (1.5"). Beginning in 1970 (?) This rig was reserved for
all spitfires except those imported to the USA and Sweden. The smog laws
required Strom 175's (1.75") on a truely crappy log manifold. In any case,
the MK3 manifold and carbs will fit a 1500 and unless you plan a monster cam
and comsiderable head work, the HS4's should flow quite nicely for you. Once
note of caution, in '71 the spit went through some reasonably significant
changes and became the MK4. There are a number of pieces that come in 2
styles, "all years up to 70" and "all years from 71 on". Hard tops are a
good example. In any case, one of the changes was to the throttle linkage.
As a result, the MK3 linkage won't bolt right up to a 71 or newer. When I
did my conversion I ordered the parts I needed from the Triumph dealer (these
were stock parts on all Spits outside of USA/Sweden). Of course the fact
that there was a Triumph dealer to go to dates this advise some what. The
required bits are pretty straight forward and can be cobbled up but be
prepared. I didn't realize that this was even an issue until I was pretty
far along on the project and the resulting angst was an unhappy surprise.
Another point. The stock 1300cc engine in Eur trim (no smog) was rated at
75HP. This was with dual SU's and 9:1 compression and a un-smog-control cam.
Getting a late SC block (like in a midget) up to this state of tune will
require a replacing the carbs, and cam and addressing the Comp ratio is some
way (head/pistons whatever). By going to 1500cc and maybe 10:1 comp and a
good exhaust system I can see this engine putting out 90-95 HP. That's a
guess. In short, it would require about all of the mod's described
by an earlier poster. This is presented as a piece of advice to the original
poster. The 100HP figure you are targeting is do-able but at a cost. The
mods will cost considerable money. In addition, as the HP increases, the
radical-ness of the engine increases and there is a drivability penalty. Go
easy and remember that all engine mods should be part of a balance approach.
12:1 pistons and stock carbs are just as stupid as 1e06 CFM carbs and a stock
bottom end which is just as stupid as driving an all out race engine on the
street. I've read some bad things about PAECO on this list (I've been lucky so
far) but their phase 1 - phase 4 engines do represent this sort of balanced
approach. Get their catalog and read what mods they put into each type
of engine. You may deviate from their list if you wish but at the very
least, their checklist makes a good starting point when planning an engine
project
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