Bill...
Stock 1275? Over here in the States,
"H" model was 8.8-1.
Higher compression is just fine with modern electronics varying the
advance
curve, most especially the digital stuff,
automatically advancing/retarding timing
(to one extent or another) depending on
octanes, induction temperature, engine
load, EGT, etc., etc. thus keeping you out
of trouble with respect to pre-ignition and/or detonation, while
providing the
most bang for the buck, octane and
circumstances taken into consideration.
Otherwise, on engines running "fixed"
rather than "variable" ignition advance
curves, readily available octanes determine useable compression ratios
unless you want to spring $$$ for
racing octanes.
There is a "new" caveat here however...
the modern Pierce aluminum heads
run a lot cooler than the stock
cast iron cyl. head, so a higher static compression ratio can be used on
the
same octane, all else remaining equal.
Bill Perry recommends no higher than
9.0-1 considering today's garbage octane
unleaded premiums. With an aluminum
head, this goes to at least 10.0-1 and
some experienced mechanics say to add
a full point and one-half. Bill says a point
and he is conservative, anyway. He's
not interested in seeing his customers
get into "trouble"...bad for business.
Secondarily, the alum. head has a reconfigured combustion chamber
wherein the spark plug sits in the middle
of the chamber....here again, a more even
burn across the top of the piston which
translates into more torque per lb of fuel
burned. In order to do this, the plug is
12mm instead of the more traditional
14mm.
Thirdly, the alum. head has superior porting for more flow...which, all
things
being equal translates into better cylinder
filling in accordance with the cam profile
used (torque curve).
It's also got larger combustion chambers
than the stock 21cc chambered heads.
23.5 I think, so compression ratio-wise,
this would have to be taken into consideration along with overbore, etc.
A rough (very rough!) "guesstimate" would
be .1 pt. for each cc.
Like the Datsun 5 speed gearbox, this
alum. head is probably one of those
upgrades that gives the most bang for
the buck, especially when you deduct from it's outright price the cost
of rebuilding your stock head (pressure
check, zyglow, "unleaded" inserts/seats
for the exhaust valves, new springs, keepers, inlet seals, etc.).
MiniMania carries one with stock valve
sizes and a Stage 1 version with the larger inlet valves for higher mass
flow
(higher rpm applications). Both of these
are "unleaded" applications. Sports &
Classics sells the stock valved version
ready to go for about $800.
You favor low end according
to your post, so the one with stock rather
than larger inlet valves will favor the low
and mid-range to a greater degree than
that equipped with the larger inlet valves.
Cams: Cam timing along the lines of
a Kent 266 or similiar timing will do
the trick for the street...read up on
the cam section of Dave Vizard's blble..
there are a bunch of good ones out there
by various mfgs. along with tips on timing
them in at various settings. Order whatever cam you get in a kit with
matching lifters and valve springs.
Add a medium bore LCB with a 1.75"
diam. exhaust system through a Cherry
Bomb or Thrush glasspack (freeflow for power, economy and decent sound)
and you've got a quick, peppy daily driver for
the street, without going overboard and
spending billions! :)
The stock distributor can be used or you
can pick-up from NAPA a rebuilt mechanical advance distributor under
part number 48-400 (or 48400). This is
a WOT Cooper S ignition curve like
the 23D #40819 curve used on the early
'67 1275 Sprites. The setting in the book
is either "static" 7 degrees BTDC or
a "dynanmic" 22 degrees @ 1200rpm.
NAPA has them rebuilt for under $100.
Like any other part, check it for total
advance as well, just to be on the safe
side. It's a 7k dizzy but you shouldn't
be getting any more than about 32 at
6k.
Points should be just fine with good
coil, wires, etc. Of course, there is
nothing wrong with a Crane, PerTronix,
etc. with matching coil and wires, either!
Cap'n. Bob
'60 :{)
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