so yesterday i noticed that my lights were brighter than usual and my blower motor was blasting air better than ever. looked at my voltage gauge and it read 16v, it isnt higher than this because when i turn on the windshield wipers it drops to about 15.5v. my power window works great! headlights snap to attention. windshield wipers are fast and powerful. the car runs better than it ever has in the year and a half i've owned it. my 3800 rpm bog is gone. unfortunately my sub cuts out at about 15.5. obviously my alternator wasnt giving enough power before. im worried about fragile things getting fried, aka ecu, etc. i have heard that the fd alternator is much better, is it safe to assume that it will put out a little more power than the fc one? i have a bosch reman right now.

 

You're risking a lot running with elevated voltage...the ECU is actually the least (expensive) of your worries.
The main switchgear (lights/wipers) is also at risk and more expensive to replace.

The FD alternator is theoretically rated at 100A so yes, it is somewhat "better".
The FD alt has a slightly larger output shaft and comes with a pulley that won't accept V-belts, so you'll need to bore out the stock FC pulley (which is two piece) to fit.
If you have a S5, the wiring is identical, so the FD part is plug-n-play.

That said, many other Mazdas use similarly framed alternators...mount points and wiring are identical, output is the variable.
My FD alt finally died last week and I'm now running a 929 part which is nominally rated at 90A but tested at 110A (for whatever that's worth).
At idle, fully warmed up, with every electrical accy. ON (lights, heater fan, rear defroster, brake lights, efan) it shows a steady 14.2(ish)V.

The FD alternator is expensive ($160 for a NAPA reman) and almost impossible to find in a junkyard, so the 929 (and 626, MX-6) parts are an attractive option, especially given the ease of install.

Jackchild has a nice writeup on adapting a Ford Taurus 130A alternator (here), which is probably the ultimate in "poorboy" (i.e., junkyard) swaps but it requires bracketry to work (apparently quite simple though).
Certainly worth considering if your system draw requires a more robust solution.

 

Thats like what, 30 jdm horsepower running at 15.5v? Fix it before you blow out all your bulbs and electrical components.

Amps: FD>FC

 

Haha, yea its actually faster, just kinda scares me I dont know what is getting more.voltage to.do that. So the fd alt should theoretically fix the 3800 rpm bog?

 

isnt the 3800 problem a ground issue?

but as for replacements the plug for an S5 is quite common, hell so is the S4 plug. just go to the junk yard and youll be able to find a replacement no problem. and must junk yards you can pick up an alt for 10-20 bucks

 

ok so i for a temp fix i want to put an in line voltage regulator on the positive line coming out of the alt. i've been looking around and i dont really know what i need, any suggestions?