Look at title. As an example, a 15" Fi SSD both D1 and D2 have the same specs except for a 40% higher BL on the D2, can someone please explain what is happening that makes this configuration have that much more motor force?

Ohms law, coil is wired in series..more turns in the gap with a D2 coil than a D1 coil.

 

A dual 4ohm coil will end up with about another 50% BL across the D2

Thank you,tried telling my buddy that but he wouldn't believe me

I have noticed that too- so does that translate into a harder hitting sub?  enough to be audible?  I would think that it that were true- no one would want D1's in favor of D4's.

No. It's not audible or more hard hitting.

great post

Thanks Duran.

I think one thing that people forget in car audio is that the lower the impedance, the worse everything performs. 

Keep in mind that while BL increases, Re also doubles which means less current through the motor.

 

Using the standard BL²/Re to express the motor force factor, the D1 SSD15 would have a force factor of ~123 while the D2 SSD15 would have a force factor of ~125.

 

As you can see, they are virtually identical.