1st off I love 18" subs:
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Manufacturers typically have a marketing dept that will usually go to great lengths to sell their products.
70-90in2 of port in around 5.5ft3net- 7ft3net usually makes most 18's happy. Dependent on power.
For example, this 1750watt 18 was overpowered a fair bit... and was in 5.6ft3 net with 77in2 of aeroport for a reason. Recommendations claimed 6-7ft3 for this sub on their website. Designed around the power and installed for the vehicle it did numbers like 153db+ garage and 152.4db usaci for 2 years with only a slight coil tap when hitting the lanes.
I think I was close/at the subs ultimate potential... 3K clamped worked with a 1750rms Fi ferrite sub.
It wasn't a burp box either, it was tuned to my musical tastes it went as low as I needed it to and got loud for what it was. It sure flexed and slapped windshield wipers when I needed it to...
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To go with under 5ft3 you'd probably need a stronger motor, that means a beast sub with more output potential and THAT means... $. The capable 18's I've seen cost and need power and port to use in the 5ft3 range.
Space makes bass... especially in lower power setups.
As stated... space makes bass
Food for thought