Upgraded Rear Differential Options... - Page 4 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 02-12-2010, 12:31 PM   #61
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Thanks for the information guys... This will come in handy to others who are doing high power and still driving it on the street or drag racing their cars. I think I am going to go with the OS Giken unit and put it to the test. :crosses fingers: When I get ahold of it and run it, I will post if it grenades on me or not... Which would bring me to my next question. Is there anywhere that makes NUMEROUS final drive ratios for the Miata 7" rear end? I can only find the usual suspects (3.9, 4.1, 4.3, etc.) Kinda want something a little lower than 3.9, but could live with it if it were a beefier material.
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Old 02-12-2010, 12:34 PM   #62
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Here's some general info on the subject: Solomiata : Transmission : Ring and pinion tooth counts

3.6 gears are available from MazdaComp if you're a member. You can also get them out of a junked '79-'82 Mazda 626. I bought a set last year for about $120, though I haven't installed them yet.

A very few people report having actually laid eyes upon the 3.308 set, however the only known donor vehicle for them is apparently exceedingly rare in North America.

There's been discussion about having an aftermarket gear done, but to the best of my knowledge, one has never materialized.
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Old 02-12-2010, 02:34 PM   #63
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It would be awesome to have one made from a beefier alloy than the current factory metallurgy... I could see that being a weak-point if enough torque was placed on the ring or pinion by snapping teeth off. The things I would do if I had access to a CNC machine and the software. :(
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Old 02-12-2010, 04:17 PM   #64
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...and a good gear engineer, and a good metallurgist.
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Old 02-12-2010, 04:21 PM   #65
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Know of anyone? Haha! I might put the word out to some companies and see if I can get any hits on having some built for our cars. Would be nice to have beefier and better final drive options out there that are readily available.
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Old 02-12-2010, 04:27 PM   #66
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You are not the first to travel this path...

Go search around at m.n on the diff gearing threads, and call 949Racing, Flyin Miata, etc. I'll bet that you will not be able to beat the cost of the m.n thread research, something like $600+ a set with a minimum buy of 50 sets or so. I forget the details, but it is all in the thread. It fell through due to lack of interest at that price.

I reverse-engineered/designed a gearbox and gears several years ago. There is a lot more to the geometry, metallurgy, and mounting than you might think. I have a dedicated book on gear design that is three inches thick and still did not cover everything I needed to know. And I was just working on straight-cut spur gears, not hypoids. Also you may not be able to just CNC a gear and bolt it in. The metal might require carburizing, nitriding, or through hardening for wear hardness, which might require post surface treatment finishing. We cannot know what Mazda did on the metallurgy without a thorough metallurgical analysis, unless someone did it already or it is documented somewhere in the public domain.

And before you ask, no I'm not interested in doing this. I would rather buy/find the 3.63 gears or do the 8.8 swap, cheaper and easier
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Old 02-12-2010, 04:30 PM   #67
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I will look for it tonight when I'm at home and see what I can dig up...
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Old 02-12-2010, 07:53 PM   #68
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Just curious as I'm not sure about how the differential that was placed into the S2000 is related to the Miata one, but they do offer reinforced differential housing for the S2000, I wonder if this would be a viable option, or if I am just dreaming. They were relatively inexpensive at ~$400 (+ core) too... Haven't seen the housing on the S2000 in person to say if it would bolt into place on the Miata diff housing or not though.

Example: CorSport Online Store: Honda - 00+ Honda s2000 - 550-040
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Old 02-13-2010, 12:53 AM   #69
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Quote:
Originally Posted by thagr81 us View Post
Just curious as I'm not sure about how the differential that was placed into the S2000 is related to the Miata one, but they do offer reinforced differential housing for the S2000, I wonder if this would be a viable option, or if I am just dreaming. They were relatively inexpensive at ~$400 (+ core) too... Haven't seen the housing on the S2000 in person to say if it would bolt into place on the Miata diff housing or not though.

Example: CorSport Online Store: Honda - 00+ Honda s2000 - 550-040
Look closely at the Corsport illustration; what corsport is selling is for $370 (plus shipping of a heavy center section) is nothing more than a set of cap straps for the stock S2000 carrier bearing caps. They are still retaining the caps/straps with bolts which would seem to be suboptimal vs. studs. I'm not wild about their cap strap design either; a cap strap does not have to be contoured to the entire cap surface to be effective. Proven cap straps (Rousch's 1970's design for the 2 bolt main 351W block and numerous current designs for the 2 bolt main 429/460 blocks) are usually thick and rigid horizontal pieces machined out of bar stock like this:

||||_______||||
|_____________|

This design provides strength by providing an inflexible "bridge" to tie the two main studs together. The vertical ends press directly on the "holed ends" of the caps and the very top of the cap surface is usually milled slightly so that the center of "the bridge" bears on it.

For $370, you should be able to buy some high quality studs and get a local machine shop to make a proper set of cap straps for your Miata rear end. You'll save on the shipping too.
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Old 02-14-2010, 11:34 PM   #70
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OK as shown below, the Explorer IRS, fully assembled, dry, weighs 74 lbs according to my reasonably accurate bathroom scale.


Contents listed above.
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Old 02-15-2010, 08:05 AM   #71
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Thanks for sharing the weight on that ZX... Now, does anyone have a weight on a comparable Miata unit?
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Old 02-15-2010, 01:35 PM   #72
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Just weighed the 7" housing in my garage. 68.5lbs for complete manual trans dif housing (open, 4.10) with stub axles inserted and standard rubber bushings in the mounts. Ask Stein what his bottom cradle and top hangar weigh and you should be able to get a good comparable weight between the Miata & 8.8. I suspect a Torsen/Giken/OBX weighs a bit more than an open too; the 8.8 weight already includes the Traction Lok LSD.

Last edited by sn95; 02-15-2010 at 01:38 PM. Reason: clarification
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Old 02-15-2010, 01:43 PM   #73
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I'd say my diff mount is in the 8 lb range, maybe a bit less. I cut off the two stock hangers from the subframe. That probably saved me a whopping 1/2-3/4 lb.
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Old 02-15-2010, 02:15 PM   #74
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So it appears that you are looking at a 5-10 lb weight gain by going with a Ford 8.8 as compared to the Miata diff with LSD (added 5 lbs to the Miata weight to get that number as a guess). So overall not too bad for the options and such that you get with the Ford 8.8 as well...
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Old 02-15-2010, 02:55 PM   #75
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Sounds like a reasonable weight difference. Even if it is 20 lbs that is not bad IMO for what you get.

It's not a Traction Lok by the way (clutch type) it is a Truetrac (more like a Torsen).
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Old 02-15-2010, 03:00 PM   #76
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I agree... The weight difference would not effect it that much as it is down near the center of gravity anyways. Sounds like a decent setup for the big HP drag and V8 guys out there for sure.

I still think I'm gonna put the OS Giken to the test though...
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Old 02-15-2010, 03:01 PM   #77
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Old 02-15-2010, 03:03 PM   #78
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Quote:
Originally Posted by thagr81 us View Post
So it appears that you are looking at a 5-10 lb weight gain by going with a Ford 8.8 as compared to the Miata diff with LSD (added 5 lbs to the Miata weight to get that number as a guess). So overall not too bad for the options and such that you get with the Ford 8.8 as well...
Probably more like 10-15lbs by the time you add in the weight of the heavier Ford/FFR axles. It's really a lot less than I would have guessed. I wonder how much heavier the iron carrier 8.8 is than the AL version? Maybe Stein can supply a weight for his iron 8.8 setup. You also gain the ability of being able to adjust/preload the pinion angle which can't be easily done with the stock PPF setup.
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Old 02-15-2010, 03:06 PM   #79
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True... Forgot about the axle weight being added in. But still not too shabby. I could imagine the cast iron carrier being a bit more, but not TOO MUCH as it wouldn't have to be casted as thick as say an aluminum housing to get the same strength. That is also true about the pinion angle adjustment as well...
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Old 02-15-2010, 03:54 PM   #80
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The cast iron 8.8 are in the 85-89 lb range IIRC. They are heavy. Add the heavier axles (at least 8-10 lbs more) and it adds up.

I anticipate my rear swap will be a full 50 lbs more than the Miata diff/axles that it replaced. That said, it helps regain some of the front/rear balance that I am losing with the addition of the V8.
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