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Discussion Starter #1 (Edited)
The MKIV subwoofer replacement in the stock box discussion has seem to have died many moons ago, but these 6.5" micro subs have come a long way!

I'm rocking the old-school 1990's SAS Bazooka tube pushed with 200W of Alpine deliciousness, it does the job very well, but I want my trunk space back for the targa. I think a 6.5 sub will fit with a custom 6"x8" wooden plate on top of the stock sub box and no trimming of the stock box. The stock box appears to be vented, which is perfect for these little subwoofers, but I don't know the exact specs (internal volume, port specs, tuning frequency) of the stock box. Anyone know?

I'm thinking the SKAR EVL 65, Toro Fierce 6, or the Kicker CompRT 6.75" may fit nicely. The Kicker has a Freq response of 30-500, but Fs of only 55hz. Pushed with a real amplifier, these little subs may sound awesome... or they may sound like poo!

With a small spacer ring, even an nice 8" shallow mount sub may also work on top of the stock box!
 

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That would be pretty slick! I was thinking of a similar upgrade for my MR2 Turbo's old factory sub. Keep us posted on what you do!
 
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Master Shit Fixer
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I have a rare 6x9 Kicker sub in my NA's stock subwoofer spot, with exactly what suprajake describes...a plastic 6x9 ring and handmade adapter to fasten it to the stock 5x7 hole. The ring/adapter joints are sealed. A JL Audio amp powers it, and it sounds & feels great!
 

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Not being able to find an aftermarket 5x7 sub option down here in NZ, I bought a 5x7 Earthquake speaker to replace the oem sub that had siezed the voice coil. I also cut the tweeter wires off it to avoid any signal possibly leaking past the LPF on the amp or headunit. The amp is a half decent Kicker unit, and Im running it bridged to the sub, with LPF on and frequency cut off at 85hz

No matter what I do, I cannot get it to sound half way decent, and Ive been into car audio for 25+ years, so I have to presume its not operator error. I have fully sealed the back of the speaker frame to the OEM sub box, so there are no air leaks around it, and I have played with the settings endlessly, but it just sounds like a wet fart when its turned up, and at no volume does it ever sound tight.

Its ruined my audio enjoyment in the car to the point that I almost never listen to anything when I drive it, as if there is one thing I hate, its good music played badly :(

Subscribed for results
 

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Discussion Starter #7
@Ree, using a full range speaker (minus the tweeter) still won't give you subwoofer results. Try something else, and see if it's any better. Any lower power 5x7 subwoofer should sound better.

@bitshftr, nice! Are you still able to use the stock grill?

After extensive searching, there are so many 6.5" subwoofer options: Savard, Sundown Audio, Toro, SKAR, Audiopipe, Rockville, DD Audio, Massive, Memphis, JL, Kicker... my head is spinning!!! I just don't know of anyone using these monsters. Most have a mounting depth of 4-5", so a mounting ring would be required. The good news, many of these are less than $100.

I may remove the OEM box, fill with water and measure the exact volume. The vented port area is about 11.5 sq inches total. This may help match with a subwoofer for best results. The OEM box material seems rigid enough, so I'm not very concerned about droning, harmonics or port noises just yet. If it does sound like poo... it's back to the Bazooka tube!!!
 

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@bitshftr, nice! Are you still able to use the stock grill?
Yes, although since it's a 6x9, I had to cut out part of the edge of the grill that faces the back seat. You can't see the cut when standing behind the car looking into the hatch.
 

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From the land down under
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@Ree, using a full range speaker (minus the tweeter) still won't give you subwoofer results. Try something else, and see if it's any better. Any lower power 5x7 subwoofer should sound better.
Jake, I fully agree with you, and always knew it would never be as optimal as a proper sub, but it was a case of when needs must, as there was no other option on shore here in NZ. Also, when you look at the OEM "sub" that came in the car from the factory, it was a flimsy, paper cone, with a small magnet, so I figured that getting a 20 year newer full range speaker and clipping its tweeter in conjunction with using the LPF on the amp, would be an improvement, but I was VERY wrong🤣

I will look into the 5x7 that Bitshftr linked me to, and see how I get on :)
 

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I don't have any experience with this but JL makes a 5x7 / 6x8 woofer: JL Audio C5-570cw-RP
That is a component woofer, also known as a mid woofer. They are not sub woofers. They're just like the 6.5" woofer part of 6.5" component speaker set, and meant to go with a crossover and tweeter (all in a pair). They're just in 5"x7"/6"x8" sizing to fit certain vehicle's front doors, like Ford trucks.
Would it work, sure. But it's not going to give really low, deep bass. That's just not their intended purpose.

C5-570 - Car Audio - Component Speakers - JL Audio

You're better off going with an actual sub woofer, 6.5" or larger. The JL 6W3v3 is a great little sub, just don't expect too much out of it. I sold and installed many of their 8W3v3 sub woofers, especially in their prefab ported enclosures (CP108LG-W3v3). Awesome little subs for the size. Very musical and surprising output from such a relatively small package. The 6W3v3 is about the same, just a little less output wise.

It also just really depends on what kind of music you like and how you like to listen to it as well. If you like a lot of strong heavy base they're not likely going to cut it.
 

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You might reach out to Thomas at O'Brien Audio, he was showing me some rather small subs a while back, and said he was rather impressed with the output for the size, that they would likely be a good fit for our cars. For what it's worth, he has a Mk4 as well, so it's definitely a crossover of interests for him.
 

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Discussion Starter #12 (Edited)
Well, it can't be done. After several days of taking measurements and building templates, no matter how, a 6.5 subwoofer is too big to fit in the original sub enclosure. I spec-ed out my top 2 picks for this install: the Sundown SA-6.5 and the Savard RAP 6.5. They are supposedly amazing little subs.

I was excited to try because the stock subwoofer box is some weird type of transmission-line or bass reflex. It appears to have 2 ports, so if they are dissimilar lengths, it may even be a band-pass enclosure! I'll keep trying to see where a 6.5 sub will fit, but it's not looking promising because of my rules to modifying my supra: 1) do not cut carpet, 2) do not cut plastic trim pieces and 3) no new holes.

BTW, yes... I know all about Wicked C.A.S. boxes. (Thanks, Nazar.) Not looking at that route right now and I do believe the OEM subwoofer assembly does provide some rigidity for the chassis. (It's been mentioned before, but I could be wrong.)

I'll take some more measurements and mock-ups, where the Styrofoam spacers are around the spare tire, but again... "no cutting the carpet."
 
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