Sky High Car Audio FXB-8 D4
8″ subwoofer · dual 4 Ω · 500 W RMS
The Sky High Car Audio FXB-8 D4 is a 8″ subwoofer. With Qts 0.77 and EBP 50 it suits a sealed box: tight bass in a small enclosure; a ported box would need a lot of volume. Long excursion (Xmax 0.55″) — built for loud music and high power.
| Source | skyhighcaraudio.com · manufacturer data |
|---|---|
| Checked against source | October 2026 |
| Thiele/Small parameters | complete |
| Mounting dimensions | 3 of 4 — missing: magnet diameter; the designer will use an estimate |
Recommended box
For a sedan, one sub, 500 W. It’s the same math as the designer’s step-by-step mode, where you can fit the box to your space, vehicle and amp and get plans, a cut sheet and DXF.
Sealed
smaller, tighter| Bass character (Qtc) | Tight and punchy · 0.94 |
|---|---|
| Fc | 54 Hz |
| F3 (−3 dB, no cabin gain) | 45 Hz |
| Example box (outside) | 10 5/8″ × 10 5/8″ × 10 1/16″ |
Ported
deeper, louder| Tuning (Fb) | 41 Hz |
|---|---|
| Port | slot 1 15/16 × 14 5/16″, 86 3/8″ long |
| Port velocity at 500 W | 32 ft/s |
| F3 (−3 dB, no cabin gain) | 34 Hz |
| Example box (outside) | 31 1/2″ × 15 3/4″ × 10 1/16″ |
The volume and tuning are the recommendation, but the port doesn’t fit this example box. Size the box and port in the designer.
Open ported box in designerDeeper: 1.17 ft³ (33 L) tuned to 36 Hz, 27 3/8″ × 15 3/4″ × 8 1/16″ outside — a bigger box with flatter deep bass. Open in the calculator
Projects with this subwoofer
Boxes that users designed and published: plans, cut sheet and response graph.
Thiele/Small parameters
| Size | 8″ |
|---|---|
| Voice coils | dual 4 Ω |
| RMS power | 500 W |
| Fs | 44.3 Hz |
| Qts | 0.77 |
| Qes | 0.89 |
| Qms | 5.67 |
| Vas | 0.19 ft³ |
| Re (one coil) | 3.2 Ω |
|---|---|
| Sd | 30.2 in² |
| Xmax | 0.55″ |
| Vd | 16.6 in³ calc. |
| Mms | 131 g calc. |
| BL | 11.4 T·m calc. |
| EBP | 50 — suits sealed calc. |
| Sensitivity, 1 W / 1 m | 79.1 dB calc. |
Mounting dimensions
| Cutout diameter | 7 1/2″ |
|---|---|
| Mounting depth | 5 1/2″ |
| Overall diameter | 8 3/4″ |
Where it fits in a vehicle
Spots from our vehicle guides where the cutout, mounting depth and recommended box fit. The spot sizes are approximate — measure your own vehicle.
- F-150 SuperCrew — under the rear seat sealed and ported box
- Ram Crew Cab — under the rear seat sealed and ported box
- Tacoma Double Cab — under the rear seat sealed and ported box
- Tacoma Access Cab — behind the front seats sealed and ported box
- Bronco 4-door — behind the rear seat sealed and ported box
- Wrangler 4-door, hard top — behind the rear seat sealed and ported box
- Wrangler 4-door, soft top — behind the rear seat sealed and ported box
- Silverado / Sierra Crew Cab — under the rear seat sealed and ported box
- Spare tire well sealed and ported box
- Polaris RZR / Ranger — under the seat sealed and ported box
- Can-Am — under the seat sealed and ported box
- FJ Cruiser — behind the rear seat sealed and ported box
- El Camino 1978–87 — behind the seat sealed and ported box
- Cherokee XJ — cargo area, spare moved out sealed and ported box
- Excursion — behind the third row sealed and ported box
- Mustang 2015–2023 coupe — trunk sealed and ported box
- Challenger / Charger — trunk sealed and ported box
- Tesla Model 3 — well under the trunk floor sealed and ported box
- Club Car Precedent — under the rear seat sealed and ported box
Cone area (Sd) calculated from Fs, Vas and Mms: 30.2 in².
“≈” means the value isn’t in the spec sheet and a typical value for the size is used; “calc.” means it’s calculated from the published parameters. Check the manufacturer’s spec sheet before you build.
Data sources
Voice coil versions
The same sub with different voice coils: the parameters are nearly the same — pick by your amp and wiring.
| Model | Voice coils | Re | Qes | Vas | Sealed | Ported |
|---|---|---|---|---|---|---|
| Sky High Car Audio FXB-8 D2 | dual 2 Ω | 1.9 Ω | 0.86 | 0.17 ft³ | 0.35 ft³ | 0.49 ft³ · 41 Hz |
| Sky High Car Audio FXB-8 D4 | dual 4 Ω | 3.2 Ω | 0.89 | 0.19 ft³ | 0.39 ft³ | 0.49 ft³ · 41 Hz |
Other Sky High Car Audio 8″ models
- Sky High Car Audio BMX-8 D1 8″ · dual 1 Ω
- Sky High Car Audio BMX-8 D2 8″ · dual 2 Ω
- Sky High Car Audio FE-8 D2 8″ · dual 2 Ω
- Sky High Car Audio FE-8 D4 8″ · dual 4 Ω
- Sky High Car Audio FMX-8 D1 8″ · dual 1 Ω
- Sky High Car Audio FMX-8 D2 8″ · dual 2 Ω
Similar 8″ subwoofers from other brands
Close in Fs, Qts and Vas — a box of similar volume will suit them.
- DD Audio 508e D2 0.32 ft³ sealed · 0.74 ft³ ported
- DD Audio 508e D4 0.32 ft³ sealed · 0.74 ft³ ported
- Kicker L7 8" 2-Ohm DVC (41L782) 0.53 ft³ sealed · 0.85 ft³ ported
- DD Audio 1508d D2 0.46 ft³ sealed · 0.53 ft³ ported
- DD Audio 1508d D4 0.46 ft³ sealed · 0.53 ft³ ported
- Kicker CompR 8" 4-Ohm DVC (43CWR84) 0.64 ft³ sealed · 0.46 ft³ ported
FAQ
What box does the Sky High Car Audio FXB-8 D4 need?
Recommended net volume: 0.39 ft³ sealed, 0.49 ft³ ported tuned to 41 Hz — for one sub in a sedan at 500 W. Fit the box to your space and amp in the free calculator, with plans and a cut sheet.
What are the Sky High Car Audio FXB-8 D4 Thiele/Small specs?
Fs 44.3 Hz, Qts 0.77, Vas 0.19 ft³, Xmax 0.55″, Sd 30.2 in².
What is the Sky High Car Audio FXB-8 D4 mounting depth?
Mounting depth is 5 1/2″, cutout diameter 7 1/2″. The inside depth of the box must exceed the mounting depth by at least 1″.
How much power does the Sky High Car Audio FXB-8 D4 handle and how do I wire it?
RMS power rating: 500 W. Dual 4 Ω voice coils: series gives 8 Ω, parallel gives 2 Ω at the amp. Wiring for several subs and amp power — in the wiring calculator.
Is the Sky High Car Audio FXB-8 D4 better sealed or ported?
EBP 50: a sealed box (0.39 ft³) suits it best — tight bass in a compact enclosure. Ported is possible (0.49 ft³, tuned to 41 Hz) but larger.
What port does the Sky High Car Audio FXB-8 D4 need?
For the 0.49 ft³ ported box tuned to 41 Hz: slot 1 15/16 × 14 5/16″, 86 3/8″ long, area 28.2 in²; or a round port Ø6″, ≈ 86 3/8″ long. Port air speed at 500 W: 32 ft/s.
What box do two Sky High Car Audio FXB-8 D4 subs need?
Two subs need twice the volume: 0.78 ft³ sealed, or 0.99 ft³ ported at the same 41 Hz tuning with twice the port area. One shared chamber or one per sub — whatever fits your vehicle.