N/A FC Final Drive Gear Idea

Whizbang · 12-12-2008, 03:27 PM

So after much searching and looking into parts compatibilities, i have found no known supplier of gearing for the NA FC aside from the 4.3 (which was OEM anyway).

BUT i came across another idea. If this works....oh man.

The FC differential is, as you know, an IRS setup. BUT it still uses a front loader style differential. For what ever reason an FB front loader can attach to the FC housing. There are two extra bolts on the FB housings, but generally, it lines up well.

The FB as you know, is the longer pinion design and has a varying ring and pinion options.

Now this DOES present a problem. There is no front diff mount location on the FB component. Screwed? Not yet. The mount is pretty simple. Either some welding could work OR

Enter the Kia sportage front differential. As i have written about before, this is compatible with the 84-85 FB. But has a large extension with a hole from suspension piece on the kia. (see below)

Its also on the same side as the FC mount, so my plan is too then weld an adapter piece that mounts to the Kia diff and the FC differential mount (at least the location the mount itself attaches to the sub frame).

The axle stubs of the FC should with whatever LSD i put into it. Drive shaft might have to be lengthened. I am going to try and cobble this together soon enough. I just need the parts to get things working.

jgrewe · 01-07-2009, 08:55 PM

Why not just lengthen an FB pinion with an FC pinion and be done with it? I like your outside the box thinking but its a wheel that has already been invented.

Whizbang · 01-07-2009, 09:01 PM

most due to the fact that things that bolt or stationary things being welded together is much easier and cheaper for me than something that has tolerances, very important angles, heavy loads and lots of motion being welded together. if i had some problem tools on how to do it or money to pay someone else to, then that option would be best. Also, i hate one of items. If there is a problem with the pinion, then i have to find someone to make another one, etc.

jgrewe · 01-07-2009, 09:42 PM

I'm all about using what is laying around the shop and firing up the hot glue gun as well. I almost never buy stuff that can be made with a torch, welder, band saw, lathe etc. If you take an FC pinion and the FB pinion of the gear you want to a machine shop you will probably spend less than $100.

I am interested in how things go though...

Don't try to weld to the cast pig though unless you really know what you're doing, I would make something that bolts through that hole and is made out of some square stock or angle iron.

Whizbang · 01-07-2009, 09:49 PM

Quote:

Originally Posted by jgrewe

I am interested in how things go though...

Don't try to weld to the cast pig though unless you really know what you're doing, I would make something that bolts through that hole and is made out of some square stock or angle iron.

that is more or less the plan. This project might be on hold due to joining the air force til i know where im getting posted.

NoDOHC · 01-07-2009, 11:29 PM

Congratulations on joining the Air force.

I have always wanted about a 3.55 final drive ratio to help fuel economy.
I also wanted to see if a RX8 diff would fit (4.77:1 and IRS).

I do know that the mazda 929 differentials bolt into the turbo II differentials (learned from Ray on v8rx7forum.com) they only have a 3.73, 3.90 and 4.10 ratio available though.

Rotary#10 · 01-16-2009, 07:18 PM

wow a 3.73 diff in a turbo 2 diff ? wouldn't 60 mph be about 2,250 rpms or even lower ? i know 3.90 in my n/a is at 60 mph is 2,350 rpms! haha i love it.

Whizbang · 01-17-2009, 04:04 PM

but as far as the gearing goes. I really don't like low geared cars unless you happen to be making such a stupid amount of power that you need to have a lower gear to use it.

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.

sen2two · 08-14-2010, 08:14 PM

Why can't you just take the 4.78 ring and pinion out of the Kia rear, and install it into the FC NA diff in place of the 4.10 ring and pinion that in there?

sen2two · 08-15-2010, 02:04 PM

^After looking at the Mazdatrix and Racing Beat site, I'm guessing that this is not possible. The 72-85 pinions are very short, and the 86-92 uses a very long pinion gear. So swapping the entire ring and pinion swap is not possible.

But can you swap in the ring gear from the Kia into the FC diff? This would give you a 4.3 ratio without having to find the very rare GTUs rear.

01-07-2009, 08:55 PM	#2
jgrewe Rotary Fan in Training Join Date: Feb 2008 Location: On the Beach iTrader: (0) Posts: 59 Rep Power: 17	Why not just lengthen an FB pinion with an FC pinion and be done with it? I like your outside the box thinking but its a wheel that has already been invented. __________________ New rat from the sinking ship.

01-07-2009, 09:01 PM	#3
Whizbang Respecognize! Join Date: Jul 2008 Location: Δx = ħ/2Δp iTrader: (5) Posts: 3,190 Rep Power: 20	most due to the fact that things that bolt or stationary things being welded together is much easier and cheaper for me than something that has tolerances, very important angles, heavy loads and lots of motion being welded together. if i had some problem tools on how to do it or money to pay someone else to, then that option would be best. Also, i hate one of items. If there is a problem with the pinion, then i have to find someone to make another one, etc. __________________ For current updates and event coverage check out Follow on Twitter! @WhizbangRally Whizbang Rally's Webpage \| Facebook

01-07-2009, 09:42 PM	#4
jgrewe Rotary Fan in Training Join Date: Feb 2008 Location: On the Beach iTrader: (0) Posts: 59 Rep Power: 17	I'm all about using what is laying around the shop and firing up the hot glue gun as well. I almost never buy stuff that can be made with a torch, welder, band saw, lathe etc. If you take an FC pinion and the FB pinion of the gear you want to a machine shop you will probably spend less than $100. I am interested in how things go though... Don't try to weld to the cast pig though unless you really know what you're doing, I would make something that bolts through that hole and is made out of some square stock or angle iron. __________________ New rat from the sinking ship.

01-07-2009, 11:29 PM	#6
NoDOHC The quest for more torque Join Date: Sep 2008 Location: Sheboygan, Wisconsin iTrader: (1) Posts: 855 Rep Power: 17	Congratulations on joining the Air force. I have always wanted about a 3.55 final drive ratio to help fuel economy. I also wanted to see if a RX8 diff would fit (4.77:1 and IRS). I do know that the mazda 929 differentials bolt into the turbo II differentials (learned from Ray on v8rx7forum.com) they only have a 3.73, 3.90 and 4.10 ratio available though. __________________ 1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers) 1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic)

01-16-2009, 07:18 PM	#7
Rotary#10 Rotorhead Join Date: Sep 2008 Location: "the town " Oakland iTrader: (1) Posts: 219 Rep Power: 17	wow a 3.73 diff in a turbo 2 diff ? wouldn't 60 mph be about 2,250 rpms or even lower ? i know 3.90 in my n/a is at 60 mph is 2,350 rpms! haha i love it. __________________ Currently running: 90 GTUs converted to S5 TurboII. Track car. 90 N/A GTUs.. Running project car 89 N/A GTUs in Black #2 10/6/18 Blown coolant seal undergoing rebuild and restomod.

12-12-2008, 03:27 PM	#1
Whizbang Respecognize! Join Date: Jul 2008 Location: Δx = ħ/2Δp iTrader: (5) Posts: 3,190 Rep Power: 20	N/A FC Final Drive Gear Idea So after much searching and looking into parts compatibilities, i have found no known supplier of gearing for the NA FC aside from the 4.3 (which was OEM anyway). BUT i came across another idea. If this works....oh man. The FC differential is, as you know, an IRS setup. BUT it still uses a front loader style differential. For what ever reason an FB front loader can attach to the FC housing. There are two extra bolts on the FB housings, but generally, it lines up well. The FB as you know, is the longer pinion design and has a varying ring and pinion options. Now this DOES present a problem. There is no front diff mount location on the FB component. Screwed? Not yet. The mount is pretty simple. Either some welding could work OR Enter the Kia sportage front differential. As i have written about before, this is compatible with the 84-85 FB. But has a large extension with a hole from suspension piece on the kia. (see below) Its also on the same side as the FC mount, so my plan is too then weld an adapter piece that mounts to the Kia diff and the FC differential mount (at least the location the mount itself attaches to the sub frame). The axle stubs of the FC should with whatever LSD i put into it. Drive shaft might have to be lengthened. I am going to try and cobble this together soon enough. I just need the parts to get things working. __________________ For current updates and event coverage check out Follow on Twitter! @WhizbangRally Whizbang Rally's Webpage \| Facebook

01-17-2009, 04:04 PM	#8
Whizbang Respecognize! Join Date: Jul 2008 Location: Δx = ħ/2Δp iTrader: (5) Posts: 3,190 Rep Power: 20	but as far as the gearing goes. I really don't like low geared cars unless you happen to be making such a stupid amount of power that you need to have a lower gear to use it. __________________ For current updates and event coverage check out Follow on Twitter! @WhizbangRally Whizbang Rally's Webpage \| Facebook

04-26-2010, 07:49 PM	#9
Akagis_White_Comet Comealong busts your nuts Join Date: Jan 2009 Location: Columbus, OH iTrader: (0) 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, 10:01 PM	#10
Black91n/a Rotary Fan in Training Join Date: Mar 2009 iTrader: (0) 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.

04-26-2010, 11:39 PM	#12
Akagis_White_Comet Comealong busts your nuts Join Date: Jan 2009 Location: Columbus, OH iTrader: (0) 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? __________________

04-30-2010, 11:37 AM	#13
Black91n/a Rotary Fan in Training Join Date: Mar 2009 iTrader: (0) 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.

08-14-2010, 08:14 PM	#14
sen2two KTEC Join Date: Jan 2009 iTrader: (2) Posts: 642 Rep Power: 0	Why can't you just take the 4.78 ring and pinion out of the Kia rear, and install it into the FC NA diff in place of the 4.10 ring and pinion that in there? __________________ what I have: 1985 Gs:12a All motor Drag car - 1973 Rx2: Play car What I had: 93 Touring: TRADED - 91 Coupe: TRADED - 90 GTU: RHD - 88 10AE: SOLD - 87 Base: SOLD - 86 Base: SOLD - 1985 GSLSE - 85 Gsl: SOLD - 80 Gs: TRADED - 1972 Rx2

08-15-2010, 02:04 PM	#15
sen2two KTEC Join Date: Jan 2009 iTrader: (2) Posts: 642 Rep Power: 0	^After looking at the Mazdatrix and Racing Beat site, I'm guessing that this is not possible. The 72-85 pinions are very short, and the 86-92 uses a very long pinion gear. So swapping the entire ring and pinion swap is not possible. But can you swap in the ring gear from the Kia into the FC diff? This would give you a 4.3 ratio without having to find the very rare GTUs rear. __________________ what I have: 1985 Gs:12a All motor Drag car - 1973 Rx2: Play car What I had: 93 Touring: TRADED - 91 Coupe: TRADED - 90 GTU: RHD - 88 10AE: SOLD - 87 Base: SOLD - 86 Base: SOLD - 1985 GSLSE - 85 Gsl: SOLD - 80 Gs: TRADED - 1972 Rx2 Last edited by sen2two; 08-15-2010 at 02:13 PM..