The premioum gas sold now is not that far behind in octane number than what was
available in the 60's. It simply reaches the high octane with non-lead octane
boost additives. At worst, you may have to retard your timing slightly from
stock specs. Even that is unlikely as the stock specs were probably a bit
conservative. Reducing compression ratio is a bad idea, as there is a very
serious loss of performance.
If you are rebuilding your engine, consider the following. Instead of using the
tiny stock cam (about 260 total duration), use a larger cam simlar to the HP
440 (270-280 w/ 45 degrees overlap). You will lose a little torque at the low
end, but you will get higher torque at the upper end (more horsepower). Since
these cars are highway beasts, for me its worth the sacrifice. Along with dual
exhaust, you will be right on 440 HP specs (except for the exhaust manifolds,
but I doubt these contributed much). The higher cam will give more tolerance
to lower octane fuel and/or more spark advance.
I have both my 68's advanced as much as possible before knock at premioum gas
(92-93 in Austin TX). This allows for maximum performance. On the LeBaron
which has a cam of about the stock HP, I have it advanced to about 15 degrees,
as I recall (may be its 12). You will never hear knock with 92 octane.
Sometimes in cooler weather, I blend the fuel with 89 octane, and then you
sometimes can hear knock, but only if the engine is super warmed up during a
medium speed highway drive (75-80) and only at WOT. This car has a 190
thermostat, and in spite of this, it tends to run cool at low speed. The Sedan
has an even biger cam, and the timing is advanced to about 20 degrees and no
problem (I only use 92-93 in this car). This car has a 180 thermostat, but
does not run as cool as the other car at low speeds. Both cars though need
carton boards in front of the radiator to get them to operate at 180-200F in
the winter (operating temperature, engine knock, and engine performance and
efficiency are all inter related).
D^2, 2x68s
Quoting Ken & Tracie <kjosephson@xxxxxxxxxxxxxx>:
> Hello All,
>
> I would appreciate your opinions, thoughts and experiences with rebuilding a
> late 1960s 440 for use with today's gasoline.
>
> I plan to go with the necessary modifications to accommodate unleaded
> gasoline. But what about the compression ratio? Will the currently available
> 91 octane "premium" grade work in a '68 or '69 440 rebuilt to stock
> specifications? Or should I have my machinist take the steps required to drop
> the compression ratio to
> 9:1? I know about the tricks such as retarding the timing or using a water
> injection system. But I also know that some older engines can be modified
> with different distributor advance curves and/or cam shafts with different
> lift and duration specs to allow older engines to retain their original
> performance
> characteristics with a lower compression ratio.
>
> Thanks in advance for any replies.
>
> Ken Josephson
> '68 Crown Four Door Hardtop
>
>
>
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