passive crossover

I have changed out all of the speakers in my car. THe car came with 2" speakers mounted in the top of the door panel. They are currently connected directly to the radio, everything else is driven on external amps. I can turn the head units amp on and off via the menu.

Now with the 2" speakers on, with every music except for hip/hop and rap they sound great and add to the amplified door speakers. The only problem with them is there are no crossovers for them and if I move to hip/hop and rap I have to shut them down which kicks the imaging more to the rear of the car. I am getting ready to re-vamp the stereo system but I would like to put a capacitor inline temporarily so that I can leave them on.

I read and re-read the section on passive crossovers here and just need a bit of clarification.

First the speakers in the door running off of the amp are Polk 5x7 2 ways getting about 75-80rms. What crossover frequency should I use for the 2" speakers on the top of the door. They have been running full range off the head unit but on certian music they sort of pop a slight bit when the bass kicks in. I was thinking somewhere between 500 - 1000hz.

According to the chart 500 would require a 80mfd, and 1000 would require a 40mfd. Or if another suggestion comes up I can find that on the chart. The main confusion is what type of capacitor to use. I found these at Radio Shack http://www.radioshack.com/product/index.jsp?productid=2102484&cp=&pg=1&sr=1&y=7&retainProdsInSession=1&origkw=capacitor&x=13&kw=capacitor&numProdsPerPage=100&parentPage=search

but they are 100mfd. The confusing part is they have 35V 20%, 50V 10%, ect.

Any help would be great.

The V part is voltage, referring to the thermal limitations of the component.  Use the highest you can afford, such as the 250 V limitation the PE caps are spec'd at.  The percentage is tolerance in quality of design, materials and workmanship.  With 20%, expect that the particular cap that you buy may vary from specs by 20%.  10% is expected and tolerable, 5% is better, etc.

The OEM 2" speakers should not be part of the picture at this point because you have installed coaxials to replace them. 

Thanks for the help. I am thinking I will cross them over at about 2500hz and see where it gets me.

As for the factory speakers not being in the picture I agree. The only problem is even with the 2-way's in the door its not enough up front. The door speakers in a Mustang are in a terrible position pointing directly at your knee. Like I said I am currently re-designing the system building kick panels to house some Focal components, building a baffle to house a 6" midbass in the door, taking the Infinity 3 ways off the back deck and replacing them with 6" midbass on a narrow band crossover for just a bit of rear fill.

Hopefully if time and energy allow I will be done within the next month or so so for now I just need a band aid.