Impedance Manipulation

Well, it would be a lot easier is the sub was DVC - Dual Voice Coil.  It allows more flexibility in connetion.

is yours DVC?  Does it have 2 sets of speaker wire connectors (4 total)?

I don't believe I've ever heard of an 8ohm DVC sub. If its 8ohm SVC then I don't think there is much you can do with different wiring configurations. As coppellstereo said, a DVC speaker is much more flexible...

You should leave that home speaker in your home and get something for your car.    

Just curious, what kinda speaker is this we are talking about?

Some drivers are built for versatility.  And any drivers will work, but may prove to live a short life because of the weather extremes.  There are so many good drivers on the market to pick from that it wouldn't be the wisest move to plan a new system using drivers made for home use...especially if you're putting a lot of effort and expense into it.  Fact is, the latest woofers that I've been selecting for my home system would do just fine as far as the weather is concerned, due to the materials used in the surround, e.g.  The impedance isn't really a factor, because you know that the  impedance load is the result of the number of drivers and the way they are connected.

There's also a cone weighting factor that Mr. Kephart speaks of from time to time, and that I'm not entirely knowledgeable about.  But it has to do with mass added to the cone in the manufacturing stage (on one or the other types) and is provided because of the "room" acoustics...which is far different in cars than it is in the house.  That is to say, a lot of drivers built for home acoustics won't perform as you might expect them to in the confines of an auto....or in the smaller enclosure spaces that might be necessary.

Many years ago I put a 15" home subwoofer in my car and it was ruined after about a year or two.  The foam surround rotted and fell apart.

I have a question.  Why do you WANT to try and lower the impedance?  To "get more power?"  Or because you believe your amplifier cannot supply an 8-ohm load?

I use and recommend 8-ohm speakers for car audio all the time.  (And to whoever posted the question there are DVC 8-ohm and higher woofers easily available.)  Indeed, I ALWAYS use my subwoofers and mid-woofers at 8-ohms or higher because that results in far less amplifier distortion than lower impedance loads.  A (solid state) amplifier will operate perfectly into any impedance load that is higher than the listed minimum.  If an amp lists ratings at 2-ohm and 4-ohm this means it will handle as low as a 2-ohm load and that (usually) the listed ratings were measured at 4-ohms.  4-ohms is only used because this is the most common load presented by car audio speakers.  It also means the amp will operate perfectly into 3, 6, 5, 8, 16 or 100 ohms.  The amp will only "care" if too low an impedance is presented.  Higher loads makes the amp produce less current, operate cooler and with less distortion.

If it's power you are concerned about (why oh why do car audio people obsess so much about power?  It's less about power that it is about system efficiency!!) then you simply need to use an amplifier designed to produce the power you want at the impedance load you decide to use.

Yes, I understand that an amp can operate at any ohm greater than its minimum. And I understand that running the amp at 8ohm can yield greater benefits with regards to current draw, operating temp, and distortion. My question was more of a hypothetical one, not that I necessarily planned on doing it.

Yes, a concern of mine would be getting enough power, realizing that if an 8ohm speaker required 50w I would probably need an amp with 100w @ 4ohm. Of course, this would require a greater amount of $$$. If I were set on a particular 8ohm speaker this would not hinder me, but I would just have to be a little more patient in my saving.

As always, I appreciate the education. Also, thanks stevdart for what you wrote on driver durability. It was something I had a little awareness in, but not to that detail.

One additional point.  I realize what I'm about to write may not be what you intended, but you used the phrase "if an 8ohm speaker required 50w" which causes me to say this.

The only way a speaker "requires" any particular amount of power is if the user is trying to achieve a particular SPL level.  Speakers do not "require" any particular amount of power.  Loudspeaker power ratings are the maximum average power the voice coil can dissipate as heat.  It is not any sort of requirement, and indeed most professional installations I have done strive to avoid using amplifiers that are close to the loudspeaker's thermal ratings.

Granted, some speakers tend to sound better with higher power input, especially if they are relatively inefficient and do not begin producing acceptable dynamics until they are in a particular "power" band.  But others produce more than acceptable SPL and lively dynamics with one or two watts input, or even less.

The only time a particular amount of power is "required" is to achieve an SPL target.  Then a calculation can be done using the loudspeakers sensitivity and the overall system efficiency to predict how much power input may be necessary to create a target SPL number.  Other than that, speakers will operate on any power you feed them up to the point where they fry from excessive input.  Period.