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Old 06-23-2015, 02:16 AM   #1
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Default FC alternator swap for 1.6 miata

Has anyone done this before? I think my alt might be going out, my car no longer returns to idle, dies at stoplights, and I'm getting no more than 12.8V at idle with dips down to 11.8 or more. Friggen fuel has gone insane, especially near overrun. I'm hoping it's just the super low voltage, because it seems like either the injectors or coils fell asleep and don't like to be woken up. My MTX-l has been acting up lately as well, it will stick on HTr and the "r" will start to flicker after a while, seemingly about the time the warmup timer would have ran out.

Anyway, from a little research it looks like FC or FD alternators fit with maybe only one bolt difference. The FC is apparently 80 amp, and the FD 100. With my new bigger coil packs, boost control valve, megasquirt and all the other electrical stuff I've had to add I don't think the extra amps would hurt much. They're about the same cost, from the same store, any reason why I shouldn't?

Thanks guys!
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Old 06-23-2015, 03:58 AM   #2
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If you do it you might as well look into putting the fd alt on there. It's serpentine drive and more output...
If I remember right the fc pulley will fit on it.
Idk how the mounts look like between the 2 but I can snap pics in the morning,I have a 1.6 miata and an n/a fc.
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Old 06-23-2015, 09:30 PM   #3
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I think both the FC and FD are in stock at my local parts store, I might go check to see if the pulley is interchangeable with the stock one.

The FC was a drop in swap, right?
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Old 06-24-2015, 08:31 AM   #4
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should plug right in, but you need a new lower bolt/nut. The pulley is a lot smaller so you might need a slightly shorter belt (I think that's how it gets higher amps--from rotation speed).



1.6l miata


RX7 FC - turbo - Notice the larger flange, need a longer bolt (or maybe grinding).



If your car is that bad when the voltage is low, you need to tune your voltage compensations (regardless if alt is working or not)--there's one for fueling, dwell time, AND idle control...
Attached Thumbnails
FC alternator swap for 1.6 miata-80-5844169_wrw_14943_alt1_detl_72d219997825467f2da52a064eacaa02d04c493b.jpg   FC alternator swap for 1.6 miata-80-3111938_wrw_14639_alt2_detl_b0e81b33f861866e49dd066fb14d38343ec15826.jpg  

Last edited by Braineack; 06-24-2015 at 08:44 AM.
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Old 06-24-2015, 08:46 PM   #5
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I think I had my dwell settings too low, it looks like the LS2 truck coils I use need massive dwell at low voltage, might be what was falling asleep. I've been messing with idle PWM voltage comp curve a bit as well lately, but I never really had to deal with volts this low. I'm thinking heat might be a problem as well, lowering the alternator efficiency because I'm seeing 100+ IATs at idle and such, but it's 95 degrees out so what can I dooo... Alternators will get weak before outright dying, right?

MS2 is super primitive on injector dead time volt compensation, I don't get a table just a single value. I'm using NigelT's GT500 injectors with the measured deadtime and comp value so I'd rather not change that.

More dwell seems to help, but I'm still dealing with low voltage though, going for an alternator this weekend. Might just stick with the FC because I might under-rev the FD if I have to swap over my miata pulley. I could track down the FC/FD crank pulley diameter and alternator diameter, rpms at idle speeds and all that other crap but I'm thinking that's unnecessary.
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