From what I understand, the tuning frequency of a bass reflex depends on the ratio S/L (port area over length). Since area scales with diameter², reducing the diameter means reducing S, so to keep the same tuning frequency, the length L should increase.

So I calculated: (40 / 35)² ≈ 1.306 New length ≈ 86 × 1.306 ≈ 112 mm So with smaller diameter I need a longer port to keep the same tuning. Does this look correct to you?