Maybe so but, per the factory literature
a 318 in 69 was rated 230hp@4400IIRC, (which is only 193 ftlbs torque) and 340 ftlbs at 2400 rpm :
whereas the 340 didn't get to 340 ftlbs until 3200, and peak power of 275@5500 translates to 356 ftlbs.
Engines do not make horsepower.
Engines make torque over time, and horsepower is the computed result of that.
340 ftlbs is the same no matter at what the rpm is measured at.
So then;
340 at 800 is 52hp
340 at 1600 is 104hp
340 at 2400 is 155hp
340 at 3200 is 207hp
340 at 4000 is 259hp
340 at 4800 is 311hp
340 at 5600 is 363hp
Loss of torque with rpm dropping, is usually a loss of efficiency. As the rpm drops, there is more and more time for the piston on the intake/compression changover, to push just-inducted fuel/air charge back up into the intake, thru the late-closing intake valve. This is why big cams idle at low vacuum.
Another thing that can happen, more so with headers, is, as the rpm comes down, there is more and more time, for, on the overlap cycle, for the headers to pull fuel-air from the plenum, straight across the piston and into the primary pipes. That's Idle/low speed fuel charge that never got burned in the chamber, therefore did not contribute to torque-production.
These things, with aggressive cams, can continue from idle to at what-ever rpm it takes ,to build peak vacuum, usually in the window of 1600>2200, or more with bigger cams.
A third factor is the duration of the power stroke.
The longer your exhaust duration is, the shorter your powerstroke has to be. The stock 318 has, IIRC, 122 degrees of power stroke, so by the time the exhaust opens, there is as good as zero energy left in the expanding gasses. But a long-period cam might have 100 or less degrees of power-stroke, so needless to say, there will be plenty of energy left in those gasses.
So then in these two examples there will be a difference of say 20 degrees of, what I call, Power-extraction. Obviously, at low to mid rpms, the more energy that you can extract, the more torque she will make.
Here's how the factory low-compression 318s can still make torque;
1) The early-closing intake traps the charge, and
2) the near total lack of overlap, traps the A/F charge in the intake, and
3) the very long extraction period puts as much of the energy as is possible, into the crank. but
4) the short intake period limits power-production.
