According to the Wallace Calculator;
With no other changes, and stock cam, at 800ft elevation;
here is what the different iron headed-318 compression ratios bring to the party:
@8.0 =136psi / 113 P/V(abysmal), burns any gas all the time
@8.8 =154psi / 128 P/V still on 87gas, most of the time
@9.2 =163psi / 136 P/V now requires 91gas at WOT, and 136 is getting peppy
@10.1=184psi /154 P/V only works with alloy heads and burns 87 again
read about P/V here;
V/P Index Calculation
Now theoretically speaking, assigning a value of 100% to P/V of 113, here is how much stronger each engine will be, in the critical bottom end, for street-use;
113/113 =100%
128/113= plus 13.3%
136/113= plus 20.35%
154/113 =plus 36.63%
If your 318 makes 50hp at 2000 rpm, you might expect the power to increase by these percentages so; 13.3%=6.5hp, 20.35%=10.2hp, 36.63%=18.35hp.. This is how you get back the bottom-end that is usually lost when you up-cam, and neglect compression ratio, which then requires a higher stall to overcome.
These higher P/Vs allow you to run lower stalls, lower rear gears and the engine will gain fuel-economyFuel economy can be expected to increase but I don't have estimates for that.
As to gearing, you can decrease your gear easily by half the P/V percentage increase. Assigning 3.55s to the 113P/V this means that;
pv of 128 can run 13.30/2% less gear so 3.32s; and
pv of 136 can run 20.35/2% less gear so 3.19s, and
pv of 154 can run 36.63/2% less gear so 2.90s
with no loss of low-rpm performance........
each stronger P/V allows you to decrease your cruise rpm by at least half the percentage difference, with no loss in low-rpm performance .
If I assign 3.55s to P/V of 113, then the others, with 27"tires and 3% cruise slip; come out like this
with 113, 3.55s, 65=2960
with 128, 3.32s, 65=2770
with 136, 3.19s, 65=2660
-and 154, 2.90s, 65=2420
and that reduced cruise-rpm is another boost to the mpgs, and;
NONE of this requires an overdrive.
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Check out what those alloy heads can do for you; going from 3.55s to 2.90s, the cruise rpm drops 540 rpm or the same as a .82 overdrive. Maybe not .69 like the A500, but cruising at 2420 instead of 2960, I have a formula for that which says an rpm reduction of 1.00 less 81.2%= 18% which should yield an increase in fuel-economy of half that, so 9%; which takes a 20mpg combo to 21.8mpgs . If you buy 600gallons of fuel per year at 20mpg, but at 21.8 only 488 , how much money will you save? Now, factor in burning 87 with the alloys versus 91
( see note 1) with iron, and I get an annual savings of about $660 at last weeks pre gas-gouging prices.
How many years will it take to pay for those alloy heads just in the fuel-economy difference, at those prices?
Since my car was, in those days, a DD, I estimate that it took between two and three years. Over the life of the body, about 16/18 years, I saved a lotta lotta money.
EDIT
note-1
What I mean to say is that 91 is best pump-gas that you can buy and limits the pressure you can run with iron heads to around 160/165psi.. Whereas 87 will support alloy heads to at least 185psi, and I have successfully run a tic over 190 still on 87. Thanks to YY1 for calling attention to this seeming error