I've seen A LOT of posts about making HP with a 318-So why is it so ???

To PRH's point, look how much that change in seat to seat affected the predicted torque.
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The model used for the above is Fill and Empty simulation.
Sea level. Air Temp 68* F, RH 0%
340 std bore, 10:1 CR, 77 cc chambers, Open Wedge Head, Flow numbers X heads posted by Shady Dell
Carb flow 660 cfm, Dual Plane Std flow intake, H.P. Manifolds with Mufflers.

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Compared with HR, Mopar Muscle's baseline is very similar. The power gains from the add-ons (carb swap and intake swap) show more gains..read into that what you will. Neither article mentioned what rate of acceleration was used.

The problem with building 318 simulation is similar. We need a good baseline sim to really get a sense of what various changes might do.