N/A FC Final Drive Gear Idea

Akagis_White_Comet · 04-26-2010, 07:49 PM

Very impressive idea, I had a feeling that Mazda or someone would put to use the mythical super-low 4th rear end ratio
4.3:1=43/10 (43 teeth on ring gear/10 on pinion) (GTUs only)
4.1:1=41/10 (FC manual)
3.909:1=43/11 (FC automatic)
3.727:1=41/11 (combined AT and MT gearing)

Since everyone in the 20B sector prefers a lower ratio to match the 20B's torque, perhaps this would better suit it instead of a 13b?

Black91n/a · 04-26-2010, 10:01 PM

You could alternatively use a 1.8 Miata diff, which uses the same aluminum rear section as the FC but with different bushings and has factory ratios of 3.9, 4.1 and 4.3 if I'm not mistaken, plus a 3.636, 4.77, 4.88 and 5.12 are available as well. Pretty much covers all the ratios one could want.

This will require a new front mount and a longer driveshaft as well, but the Miata has the provision for a mount, since that's where the PPF attaches.

RETed · 04-26-2010, 10:08 PM

Quote:

Originally Posted by Akagis_White_Comet

Since everyone in the 20B sector prefers a lower ratio to match the 20B's torque, perhaps this would better suit it instead of a 13b?

Are you trying to imply to use non-turbo rear diff parts for a 20B application???

-Ted

Akagis_White_Comet · 04-26-2010, 11:39 PM

Not necessarily Ted, just illustrating what the possibilities are by mixing the original ratios together. By putting a manual diff's pinion gear on an automatic diff, Mazda ended up with the GTUs's ratio of 4.3:1. To get 3.727:1, you simple do it the other way (auto pinion in manual diff).

in short:
US-spec Auto diff + US-spec Manual diff = one 4.3:1 GTUs diff and one 3.727 diff (7" ring gear family)

J-spec Auto FC diff + US-spec TII diff = one 4.3:1 Infini IV diff and one 3.727:1 diff (8" ring gear family)

FD Auto diff + FD Manual diff = one 4.3:1 FD diff and one 3.727:1 FD diff (8" ring gear family)

Plus, if Aaron Cake can put down 407hp and 360ft.lbs on a N/A drivetrain with open diff for 2 years straight with no issues, suffice to say that it is hearty enough for a 20B running stock twins at least untill the owner can afford a proper TII drivetrain swap. But with a single turbo conversion, it'd probably be pushing a N/A drivetrain too hard as you suggested.

Now all we need to get are part numbers for the J-spec FC Auto diff's pinion and ring gears. Can someone overseas hook us up?

Black91n/a · 04-30-2010, 11:37 AM

Won't that not work because of the differences in ring gear thickness to compensate for the different pinion gear sizes?

For the 7" ring and pinion, in the short noze size, you can get all those ratios that I mentionned as factory matched sets made for each other. No, it's probably not the best for a 20B (maybe NA), but for someone wanting differnet gear ratios without the limitations of the long pinion housing, or the expense and possible reduction of strength from a welded pinion, it's a realistic option I think. I think that if one wanted several diffs in different ratios for differnet tracks for instance, it'd be the cheaper, easier way to go.

04-26-2010, 07:49 PM	#1
Akagis_White_Comet Comealong busts your nuts Join Date: Jan 2009 Location: Columbus, OH Posts: 29 Rep Power: 0	Very impressive idea, I had a feeling that Mazda or someone would put to use the mythical super-low 4th rear end ratio 4.3:1=43/10 (43 teeth on ring gear/10 on pinion) (GTUs only) 4.1:1=41/10 (FC manual) 3.909:1=43/11 (FC automatic) 3.727:1=41/11 (combined AT and MT gearing) Since everyone in the 20B sector prefers a lower ratio to match the 20B's torque, perhaps this would better suit it instead of a 13b? __________________

04-26-2010, 11:39 PM	#4
Akagis_White_Comet Comealong busts your nuts Join Date: Jan 2009 Location: Columbus, OH Posts: 29 Rep Power: 0	Not necessarily Ted, just illustrating what the possibilities are by mixing the original ratios together. By putting a manual diff's pinion gear on an automatic diff, Mazda ended up with the GTUs's ratio of 4.3:1. To get 3.727:1, you simple do it the other way (auto pinion in manual diff). in short: US-spec Auto diff + US-spec Manual diff = one 4.3:1 GTUs diff and one 3.727 diff (7" ring gear family) J-spec Auto FC diff + US-spec TII diff = one 4.3:1 Infini IV diff and one 3.727:1 diff (8" ring gear family) FD Auto diff + FD Manual diff = one 4.3:1 FD diff and one 3.727:1 FD diff (8" ring gear family) Plus, if Aaron Cake can put down 407hp and 360ft.lbs on a N/A drivetrain with open diff for 2 years straight with no issues, suffice to say that it is hearty enough for a 20B running stock twins at least untill the owner can afford a proper TII drivetrain swap. But with a single turbo conversion, it'd probably be pushing a N/A drivetrain too hard as you suggested. Now all we need to get are part numbers for the J-spec FC Auto diff's pinion and ring gears. Can someone overseas hook us up? __________________

04-26-2010, 10:01 PM	#2
Black91n/a Rotary Fan in Training Join Date: Mar 2009 Posts: 39 Rep Power: 0	You could alternatively use a 1.8 Miata diff, which uses the same aluminum rear section as the FC but with different bushings and has factory ratios of 3.9, 4.1 and 4.3 if I'm not mistaken, plus a 3.636, 4.77, 4.88 and 5.12 are available as well. Pretty much covers all the ratios one could want. This will require a new front mount and a longer driveshaft as well, but the Miata has the provision for a mount, since that's where the PPF attaches.

04-30-2010, 11:37 AM	#5
Black91n/a Rotary Fan in Training Join Date: Mar 2009 Posts: 39 Rep Power: 0	Won't that not work because of the differences in ring gear thickness to compensate for the different pinion gear sizes? For the 7" ring and pinion, in the short noze size, you can get all those ratios that I mentionned as factory matched sets made for each other. No, it's probably not the best for a 20B (maybe NA), but for someone wanting differnet gear ratios without the limitations of the long pinion housing, or the expense and possible reduction of strength from a welded pinion, it's a realistic option I think. I think that if one wanted several diffs in different ratios for differnet tracks for instance, it'd be the cheaper, easier way to go.