Hello,
Last weekend I finished messing with my engine (1972
late MK IV, 8:1 compression).
I installed the "fast road" cam that SpitBits sells
(with new lifters), and I had about .090" milled
off my head, which gives me a compression of about
9.25:1, as far as I can tell.
These changes make the engine more or less like the
non-emission 1300 engines.
The car already had twin HS2s and a Pacesetter header
installed (with stock exhaust).
It definitely has more "zing" now. The 1972 was originally
the wimpiest model year. In stock form, the car did 0-60
in about 15.5. Now, after the cam is broken in, I
could probably do it in about 11.5 seconds.
It runs well, and still does not knock on regular 87-octane gas.
My one question is: What is the optimum timing? I do
not mean the idle value but the actual advance in the power
band (say 4000 RPM) at full throttle. (I have no vacuum connection
to my distributor, since it is the "retard" style and there is
no way to connect that to the HS2s).
After looking at all the tables of data for the various MK III and
MK IV distributors in my Bentley book, it looks like the
non-emission cars actually had about 27-28 degrees advance
at 4000 RPM. But it is hard to say for sure since the different
distributors all had different vacuum units.
One bit of trivia: All of the distributors I had data for
(five of them) have about the same shape advance curve above 1500 RPM.
The change in advance from 1500 to 5000 RPM ranges from 10 to 12 degrees.
The only major difference is the amount of advance from 0 to 1500 RPM,
which ranges from 10 to 20 degrees. The models with the large amount
of low-RPM advance are supposed to have retarded idle timing to
compensate.
Doug Braun
'72 Spit
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