(And now from the land of run-on sentences):
According to my memory, which was fed on various books and reprint articles,
and which can indeed have been fed improperly and also behave poorly, there
are only two differences between the high and low compression Spitfire 1500
engines. At least for U.S. Federal Spitfires (and I don't know about Midgets
but I'd assume they were the same though not necessarily because the weights
and gearing would have been different), all 1500's were 7.5:1 except for the
1976 models which were 9.0:1. All California models were 7.5:1, including
1976. The two different parts needed to make the 9.0:1 version were the
pistons and the head-mounting studs. I inferred this from something I read,
but then later, out of curiosity, looked up the parts in the catalogue from
TRF. As I recall, *all* parts, head included, were the same, i.e. listed
for all years, except that the pistons and head-mounting studs were given
different part numbers depending on year/compression. Now, I was not in the
process of rebuilding an engine, so I didn't exactly look up all the nuts,
bolts, screws, washers, fittings, elbows, bearings, and rods that might have
been different, but I *did* look at all the first-glance obvious "usual
suspects" like head, valves, etc. (I have been interested since I got Percy
in the various technical mods that the entire car had, just in case I might
have to do a big rebuild someday, and I wanted to make sure I didn't pick up
the wrong camshaft or anything.) So I am reasonably certain, but won't put
money on it yet, that those are the only differences. In any case, the way
to solve this is to go to the TRF catalogue with a greater sense of purpose
and look up whether the head etc. carries different part numbers depending on
year and/or desired compression ratio.
And anyway, why should you necessarily *have* to change the head (assuming of
course that the designers kept the head the same)? Dishing, or not dishing,
the pistons will conveniently change the TDC cylinder volume, and then it's a
simple matter to calculate what the compression ratio change will be. In fact,
the "original" version of the 1500 (which may even have used 1300 pistons but
had a different crank) problaby had the 9.0:1 ratio. Certainly the home-market
engines did. So the dish to lower the compression would have been the add-on,
and it would have been a simple matter to not-dish them for 1976, and thus
commonize the parts with those used in Europe. I have never heard of any of
those other compression ratios that someone quoted here (there was a list of
4, wasn't there?). This may not apply to the Midget 1500, since the procedure
for verifying emisisons was (or at least has been for some time, if it wasn't
then) dependent on weight and gearing, which the Midget had very different
and so had its own different emissions problems.
And, besides, mjb, I *did* send a response, but it was to the enquirer only,
and anyway, it bounced!
Am I done now?
Jim Muller
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