diff replacement

[Adrian]

Can anyone help with some information? Finally got the v8 installed and running, all is good, off for a wheel alignment tomorrow morning then getting gauges sorted next week. Should be all good for cert after that. I am running the standard diff which i will keep until its all legal but I know I need something stronger. It is also too low geared but will do in the meantime. Question is, I have heard that a rx7 diff is a straight swap, is this true?. Also, does anyone know if you can simply replace the existing crown wheel and pinion gears for a higher ratio set. I believe my ratio is about 4.3:1, would like something around 3.5:1. Any intelligent help appreciated.

[MrGrey]

what sort of diff is currently installed?

this is from miataturbo.net which may answer your questions...

1.6 owners beware. The 1.6 rear end is only 6" ring-gear. Open or Viscous LSD.

They are known to fail on stock power delivery. They can blow without warning or last a lifetime. There is really no definitive power level where they will fail. But one way to help prevent it is to show mechanically empathy (i.e. no clutch dumps or drifting). A number of higher powered Miatas have lasted over 20k on them, others longer, some a lot less.

Does this mean you should replace it? Up to you, but to play it safe you should plan on it.

But if you do decide to there is still $$$ to be had from ditching the 1.6 rear end to offset costs.

What do you replace it with? The 1.8 rear end of course! Both the Open and Torsen LSD utilize 7" ring-gaers, and have been recorded to handling abuse at power levels exceeding 250-300rwhp. The swap is fairly simple, and it requires only the 1.8 rear assembly, half-shafts & stubs, and driveshaft.

There are various end gears availble from different year miatas. The 1.6 rear is geared at 4.30:1, so if you're looking for a bit more top-end try to find a 4.10, 3.909, or 3.636 unit. You'll find 1st gear much more easier to handle and a significant drop in rpms in 5th gear.

The 4.30:1 came on the '99+ Miata (4.10:1 on automatic starting in 2000)

The 4.10:1 came on the 1994-97

The 3.909 came on the 99+ 6-speed Miata.

The 3.636:1 came on the 2001+ Austrailan 6-speed.

86-91 RX7 Non-Turbo Clutch Type LSDs can also fit into the 1.8 gear ring if you're looking for a good LSD as well. Clutch-types have proven superior in Drag and Road course situations as the lock under driveshaft torque, whereas a Torsen should prove better in an autox situation as it locks only when one wheel loses torque...so in the middle of corners.
You can do this conversion by two ways:

1. 1.8 open converted to RX7 LSD:

Parts:
94-05 1.8 Miata open assembly
86-91 NON-TURBO Rx7 clutch-type lsd (89-91 is a viscous LSD so don't bother)
94-95 Miata stub shafts or 86-88 RX7 stub shafts and 94-95 Miata half-shafts
or 96-05 Miata axles (no stubs needed)
RX7 Circlips (fit onto the end of the stubs/axles, miata clips will not fit)
94-05 Miata driveshaft
Miata Ring and Pinion

Proceedure:
Open the 1.8 miata & pull the open diff. Decide what ring and pinon to use (easier to use the miata gearing, costs more $$$ when having a shop set both the pinon depth and backlash). Bolt the correct ring gear to the RX7 LSD. Drop it in, set the backlash (and pinon depth if swapping pinon gear), and reassemble the housing. Swap/install as normal.

2. 86-88 RX7 assembly conversion:

Parts:
86-88 RX7 NON-TURBO differential assembly
94-95 Miata stub shafts or RX7 stub shafts and 94-95 Miata half-shafts
or 96-05 Miata axles (no stubs needed)
RX7 Circlips (fit onto the end of the stubs/axles, miata clips will not fit)
94-05 Miata driveshaft
Miata Ring and Pinion
Miata Pinion housing (pumpkin) (find at RX7 performance shops)
Miata Diff. Bushings

Proceedure:
Open the RX7 assembly. Decide what ring and pinon to use. Bolt the correct ring gear to the RX7 LSD. Swap the pinon gear and lsd guts to the miata pinon housing. Set the backlash (and pinon depth if swapping pinon gear), and assemble the miata pinon housing to the RX7 housing. Drill out the RX7 diff bushings and push in miata specfic bushings. Install as normal.

*note: The 84-85 LSD will fit, however, the drive side axle will not lock in place and the seal will leak
**note: The 89-91 Non-Turbo LSD will also fit, but shy away from these are they are Viscous LSD units and aren't worth the trouble.
***note: Replace the (2) seals on the Diff before reinstallation (MA02-27-238A $5.95)
****note: RX7 Circlips PN is (M005-27-421 $1.95)

[SLYDIT]

no point using an rx7 diff. the driveshaft and diff head are different. stick with the 1800 diff for now. same size and correct mounting for the PPF. running a diff without the PPF is crazy

[siren676]

rx7 diff head is a 7", as you've most likely got the 4.3 thats only a 6" so wouldnt fit the rx7 diff.

Best bet is to go up to a torsen diff with the 3.6 ratio found in the later model NB's

[SLYDIT]

you need an mx5 diff. the rx7 diff head will not fit. it has a longer nose and no PPF mount. the internals are "the same" however in 7" size

[siren676]

Sorry thats what i meant, i get terms mixed up sometimes

[BRM]

pretty sure that you can use a RX7 diff unit with the 1800 housing parts,
Just need to grind out some material from the alloy housing

[Donovan]

know of a guy in Drury with a 1UZ running a RX7 diff at the back that he put in himself, he is always good for a chat and show anyone his car setup, otherwise there is always the holden diff that I run, gear ratio's at 3.08 (Auto), 3.43 (Man) or 3.78 (GTS). Only viscous LSD, so not much cop if you are going to do track work, but ok for cruising if tightened up.

[Adrian]

Cheers for that. Hadn't considered the holden diff but sounds like a good idea. Looks like the grinder and welder are coming out again!