I have a 4cu box tuned to 33 hz and a sp4 15 in it. if i were to get the hole cut bigger for a sp4 18..what would happen? and is the box too small for the sp4 18 to sound good. btw im in a  pontiac g6

 it would not sound better IMO, you would choke it, and most likely raise the peak,  if your looking for more ouput

 you should look into a better encloser for your 15  and more more power ( maybe i dont know what either of them are looking like)

i have a saz 3k, am getting another to strap. but the box is pretty good now and very loud. just was curious about what kinda change i would see.

are you chasing numbers at all? or just daily banging?

hard to breath, hair trick,  flexing daily lol..thats what i want.

 simply changing a 15 to an 18  will not gain you that much..  your only going up around 78 inches of cone area by doing so... roughly like adding a 10 inch woofer to your car... its around 78 inchs .

post some specs or blue prints of your enclosure, ..  a good box makes a sub....... with that being said a poor box will make a great sub perform poorly,

 

 not saying your box is bad, 

You'll gain more from doubling power then you would just swapping to an 18. But, you'll spend hell of a lot more on amp and electrical to back it up.

better box is going to get results not changing from 15 to 18.

If you only have 1 you might be able to get away with it. A g6 trunk is pretty small.

But you'll likely not see a difference as Jon said ur not gaining much cone area

If you haven't. I'd first look into perfecting other areas of the setup. Possibly sealing off the trunk. // Or Deadening the trunk in addition to adding MLV (dampener doesnt block low frequencies, it just stops whatever mat'l its applied to from vibrating). // Find your trunks "breath" ports and close them off // See what you can do do build a larger box // upgrade alternator and or battery bank // Drink snapple // Eat a slim Jim... etc etc.

If you haven't. I'd first look into perfecting other areas of the setup. Possibly sealing off the trunk. // Or Deadening the trunk in addition to adding MLV (dampener doesnt block low frequencies, it just stops whatever mat'l its applied to from vibrating). // Find your trunks "breath" ports and close them off // See what you can do do build a larger box // upgrade alternator and or battery bank // Drink snapple // Eat a slim Jim... etc etc.

thanks for all of those tips bro i will start doing what you just advised