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January 15, 2005
Chuck...do not be mislead by compression
ratios which could be had back when leaded ("manly") octanes were available
.
Today's unleaded "garbage" octanes require a little more thought in this di
rection, assuming a cast iron head and
the ignition advance curve you're probably
working with, along with a "mild" cam
with relatively little duration which will favor maximum "trapping" of the
combustion gases in favor of the low
end of the useable rpm spectrum (torque..
as in street application).
With aluminum heads you can go about a
point higher than with cast iron, all else
being equal.
Also, I seem to remember that turbo Metros or MAYBE even the 10.5 Metros
had an "auto" retard ignition feature. I
could be wrong about this but I remember
reading something about this in the past.
Read the book and
do your homework on cams and how profiles "work" with, and affect ignition
advance curves/requirements.
Remember also that our UK/European
listers have the advantage of 98RON pump
premium unleaded "over there".
We do not. Here in the states, add about
three numbers to what you see
at the pump (PON =3D pump octane) to
come up with the equivalent RON. This
is nothing more than a rough guestimate
but close enough for "government work"
so to speak.
Here in CT, we have 93PON available and
a Sunoco station or two carries 94PON at
50c more. The latter would about equal
the 98RON our UK/European buddies have
readily available the their corner pumps.
A lot of areas here in the states go no
higher than 91PON as far as premium is
concerned.
So, do your homework. Years ago with
Sunoco 260 and Gulf No-Nox (100RON+)
leaded around, you might go a little wild
but on today's weaker octanes, a more
conservative approach to compression
ratios is suggested unless you want to
get into the more expensive variable
ignition/timing devices, or artificial
combustion "cooling" (water, water-meth,
etc.).
Intake temperature also has a great deal
to do with useable compression ratio as
well. Read the book so you can make a
wise choice, bearing all factors in mind.
Back when, 10.0-11.0 was OK considering
the advance curves normally worked with
and octanes available at the corner pump.
For the street today, we're generally looking at a full point lower.
Cap'n Bob
'60 Frog (9.75-1 with alum head, Cooper "S"
cam, stock intake valve size, 1.5RR's)
Cap'n Bob
'60 Frog
"Robert E. Shlafer" < PilotRob@... > 35716
Posted on January 15, 2005 04:25 PM
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