Audio Systems: Car audio impedance problem, ohm resistor, power resistor

Question
Hi,
 I'm planning to add a component speaker set to my car, and I have the followings:  dynaudio x250 crossovers @ 4ohm, 8ohm 8" dynaudio drivers, and 1" dynaudio tweeter MD100 rated @ 4ohm.  All the components are not from the same set so the impedance do not match.  Since my car's amplifier's rear channels are splitted, so it can only handle 8ohm impedance.  Is there any ways I can add resistors so that the output impedance will be kept @ 8ohm?  Although the crossovers are stated for 4ohm setup, but if I use a 8ohm driver and a 8ohm tweeter(after adding resistor), will the crossover point maintains at the same frequency?  Will the amplifier sees the setup as 8ohm as well? Thank you very much for you help!

Answer
You have a system that will forever be out of balance.......  but will it work?  Yes.  Will it sound good?  Yes, probably.  Will the crossover frequency change when you put 8 ohm speakers on it? Yes.  Will there be technical things to worry about?  Yes.  See below.

ONE:  Easy solution first.  You can use the tweeter by putting a 4 ohm wire wound 10W power resistor in series with it.  This will make it 8 ohms.  Further, the sensitivity of this tweeter is 91 db and the woofer is 88db.  Therefore the tweeter is 3 db more efficient (or loud).  You are in luck.  Putting the 4 ohm resistor in series will reduce the power input by about 3db and you will have a perfect match. You can find a resistor at Radio Shack for not much money.  It may come in a value of 3.9, 4.7 or 3.3 ohms.  Any one of them would do.

TWO:  The frequency of crossover will change.  How much?  I would need to see the schematic diagram of the crossover network in order to give you an opinion, but my guess is it will not change all that much, at least not in terms of the overall sound quality.  With 8 ohm loading of the drivers it will likely go up by about half an octave; from 2800Hz to about 3400Hz or a little more.

THREE:  What will the impedance to the amplifier look like?  That is an important question.  Will it be 4 ohms, 8 ohms or somewhere in between?  Actually, the impedance, will, if the circuit is conventional and has good coils in it,  become close to 8 ohms.

If you contact with Dynaudio they may be able to give you more specific details.  I could not find their schematics online anywhere, so I cannot tell you for sure what will happen.

Best wishes,
C