The better the sub can breath in an enclosure that's been properly calculated for port size, and the more distance the bass has to propagate the wave uninterrupted (vehicle), the better it will sound.
The ported enclosure is the first box, your vehicle is the second box. Try cramming too much punch in to too little space and you defeat what you're trying to accomplish. Plus your rig just rattles like some cheap hoopty vehicle.
Depending on what you're after tuning in both an EQ and amp may be required. You can say something like "oh but bass isn't directional" but if the wave can't propagate at least a good % of the actual wave length, it's going to sound like distorted PUNCH, not smooth authoritative bass you want with William Orbit, Paul Oakenfold etc. U2's Rattle and Hum ends up just being rattle and rattle.
Wavelength
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