Hey guys,

I have very little knowledge about car audio and need some advice/info.

I am buying a clarion srw1251 12" sub in a sealed box. the sub specs are

400 Watts RMS
Single 4 Ohm Heat Resistant Voice Coil
Polypropylene Electronic Spun Aluminum Dust Cap
Fiber Glass / Paper Cone Design
Nitrile Butadiene Rubber High Excursion Surround
Dual Strontium Ferrite Magnets
Hyper Extended Vented Pole Piece
Spider Exhaust Technology (SET) Cooling
Dual Gold Plated Terminal Blocks
Linear Cotton Spider with Integrated Tinsel leads
Custom Stamped Steel Powder Coated Chassis

does this mean i can only run the sub at 4ohms? or can i still run it at 2onhs?

if it helps this is the amp i also am buying

Clarion APX1301

400 W × 1 into 2 Ohms, 20 Hz ~ 20 kHz @ <0.1 % THD
300 W × 1 into 4 Ohms, 20 Hz ~ 20 kHz @ <0.1 % THD
Selectable Bass Boost; 0/6/12 dB @ 50 Hz
Power MOS-FET switching transistor
Adjustable Low Pass Crossover @ 30 ~ 300 Hz
Gold Plated Connectors; Cinch/Speaker/Power
Speaker Level Input

You can run 2ohm but it will not last as long.

300 rms will still sound good.

INO, I would buy an amp that is happy with 450-500RMS.

As LTDHO said, it will work fine. Should be a reasonable amount of punch. I'd reccomend sound-deading the boot, I'm running a similar set up in my EF Ghia and the Boot rattle is incredible.

With the Ohms thing, thats the resistance that the Sub places on the amp. I'm not sure how you would wire it to present a 2 Ohm load, A dual coil Sub would be better if you want to get the most out of the amp.

The Ohms measurement is measuring the load (resistance) the sub places on the amp. It's not measuring what the amp gives to the sub.
In simple terms, if you've got a 4 Ohm sub, you can't make it run at 2 Ohms.

The more Ohms load your speaker has, the less power (Watts) will come out of the amp because less current can flow. (Current is measured in Amps).
The opposite is a lower resistance will make more current flow. There are limits to this though because it gets closer to a dead short (0 Ohms) and creates more heat. Amps are rated at different minimum loads - in your case it appears to be 2 Ohms. Any less and it will blow a fuse or melt.

So, for the amp you're looking at:
4 Ohms = 300 W
2 Ohms = 400 W
and approximately, 8 Ohms = <200 W

So, to summarise, you can't turn your 4 Ohm sub into a 2 Ohm sub. If you had 2 subs you could wire them as either 8 Ohms (in series) or 2 Ohms (in parallel).

And to preempt your next question there's no problem running your 400W sub on a 300W amp.

BTW - It's not the resistance.
Ohm of a speaker is the impedance.

They measure it by applying DC, so it's resistance, regardless of what they call it.
It's all null and void anyway, because as already answered above, your 400w sub will be fine with a 300w amp.

buy 2 subz then bridge to create 2 ohms nd runnin 2 subz