the specs on my sub say that a sealed enclosure should be at least 1.00 ft cubed (28 litres)
but I have a mini truck, and after working out the measurements, i found that the best i can do is about 18 litres (that factors in wall thickness (3/4'' MDF))
can having too little internal enclosure volume damage the sub? or will it just not be as loud as it can be?
If it only off by a little bit, like .1 cu ft, it won't make a huge difference, but it won't drop as low and won't get as loud because the smaller the box is, the more internal pressure their is (meaning the sub's cone isn't as free to move). 18L is .64 cu. ft, so it's a pretty big jump. Put a lot of polyfill in there if that's the biggest you can get your box.