On Tue, 5 Mar 1996 lesnyd@bb1t.monsanto.com wrote:
> I have a 1968 Spit Mk3, one of the early ones, but the engine and gearbox in
> it came from a 1971 Mk4 Spit. It seems to run real hard at highway speeds,
> and the speedo reads pretty optimistically (reads 60 at 50).
>
> Could it be that the 68 car has a 4.11:1 diff, and would a Mk4 gearbox be
> geared differently, like for a 3.89:1 diff? I know that this gearbox is
> all synchro, while the mk3 is supposed to have a crash-first box, at least
> I think so. If this is the case, I'm going to try to locate either a Mk3
> 'box or a Mk4 diff.
You're pretty much right on all counts. The 68 would have come with a
crash-first box and a 4.11:1 diff, and the 71 would most likely have had
a 3.89:1. The greatest difference in the two gearboxes (other than a
syncrho first) is that the later box has not quite so tall (stump-
pulling) a first gear -- above and beyond the difference in rear ends.
There are, I think, also slight differences in intermediate gear ratios.
I'm not clear on what the problem is you're describing, but a Mk.3 box
won't help at all here. The difference between the 3.89 and 4.11 diffs
is not outstanding, but it is noticeable.
Of course, if you toss a Mk.IV diff in, you'll have to do some other
upgrading, as input and output yokes are larger than for the Mk.3, and
you'd also have to drill two more holes for your present spring mount or
upgrade to the swing-spring and fatter front bar.
Check your numbers: the Mk.3 gearbox would have an "FD" prefix, the
diff. should be "FC". The Mk.IV would have an "FH" gearbox and an "FH"
or "FK" diff. Just to further confuse, the 1972 Mk.IV in the US used a
4.11:1 ratio version of the newer diff.
Hope that all helps.
--Andy
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Andrew Mace e-mail: amace@unix2.nysed.gov *
* *
* Mrs Irrelevant: Oh, is it a jet? *
* Man: Well, no... It's not so much of a jet, it's more your, er, *
* Triumph Herald engine with wings. *
* -- The Cut-price Airline Sketch, Monty Python's Flying Circus *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
|