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Old 07-11-02, 02:42 PM   #1
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4.875 / 5.12 Rear-End N/A gears

I know some people are interested in this.

I called SpeedSource about getting the MazdaComp ring and pinon 4.875 fitted to the N/A LSD rear end.

They said $375 to lengthen gears to fit and then you have to install them in the rear end. I guess you have to load it right and a bunch of other stuff over my head to make sure they don't blow up, so I asked how much if we just send the entire pumpkin to you and have you install the gears into the rear end. Thats another $150. Plus parts to rebuild the entire rear diff, he said when it's all said and done 5.12's cost a customer around $600, excluding the cost of the gears.

The cost of the 4.875's from mazdacomp is $281.40

While the cost of the 5.12's are $561.65

Hmm...decision's decisions, personally going with the 4.8's sounds pretty cost effective, basically 800 bucks for the whole shot...It would be better if I knew how to rebuild and install the gears into a rear end, I wonder how hard it'd be. Because for $656 thats a decent deal, when the 4.33's are 500 alone from racing beat.

I wonder how useless 1st gear would be in the stock N/A tranny?



Part No.
1312-27-110A
Description
RING AND PINION, 4.875
Notes
Short pinion design. Fits 79-85 RX-7, 1994-01 Miata, RX-2, and RX-3 SP. Can also be lengthened to fit 86-91 RX-7 non turbo.
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Old 07-11-02, 05:05 PM   #2
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Screw that! Just go to parts trader and find a chunk already set up with the gear you want for$300. Or you can buy the ring and pinion (used if possible). And have a local transmission or full service shop set it for you. Call a tranny shop and ask them how much to set the gears if you bring in the chunk. I bet it is hella cheaper than you where quoted.

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Old 07-11-02, 05:16 PM   #3
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You've seen 2nd gen rear diffs setup with 4.88s on PartsTrader??????????????

PaulC
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Old 07-11-02, 07:54 PM   #4
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Quote:
Originally posted by Silkworm
You've seen 2nd gen rear diffs setup with 4.88s on PartsTrader??????????????

PaulC
Actually, I meant post an add In the wanted section. Thats usually the way I find the parts I need on the Parts trader. Works very well for me.

I've never seen any 2nd gen diffs on parts trader but I've never really looked. I do see first gen diffs on there regularly though and have even bought one from there.
I know it's not the same thing.
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Old 07-12-02, 12:32 AM   #5
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Depending on the intended use of your car, a 5.12 may be a bit much. The 5.12 is intended to minimize the need to go to second gear and in the long run, may save you some shifts, thus helping longevity.

I have a few gear charts for the 4.88 and 5.12, I will attempt to post. Keep in mind, in my ITS car I have a .756 GTUs fifth gear.

GearChart4.88:
Gear 1 2 3 4 5 Final
Ratios: 3.475 2.002 1.366 1.000 0.756 4.880

RPM Limit: 8500
Power @ Peak RPM: 190
Tire Size: 225/45 x 15
Tire Diameter: 22.97 in
Tire Circumference: 72.16 in


Gear Mph per 1000 RPM Mph @8500 RPM
--------------------------------------------------------
1 4.03 34
2 6.99 59
3 10.25 87
4 14.00 119
5 18.52 157

Mph RPM (in Gears)
-------------------------------------------------
1 2 3 4 5
-------------------------------------------------
5 1241 715 488 357 270
10 2482 1430 975 714 540
15 3722 2145 1463 1071 810
20 4963 2859 1951 1428 1080
25 6204 3574 2439 1785 1350
30 7445 4289 2926 2142 1620
35 5004 3414 2499 1890
40 5719 3902 2856 2160
45 6434 4390 3214 2429
50 7148 4877 3571 2699
55 7863 5365 3928 2969
60 5853 4285 3239
65 6341 4642 3509
70 6828 4999 3779
75 7316 5356 4049
80 7804 5713 4319
85 8292 6070 4589
90 6427 4859
95 6784 5129
100 7141 5399
105 7498 5669
110 7855 5939
115 8212 6209
120 6479
125 6748
130 7018
135 7288
140 7558
145 7828
150 8098
155 8368

Gear Change RPM drop (change @190) RPM drop (change @8500)
-------------------------------------------------------------
1 -> 2 -81 (to 109) -3603 (to 4897)
2 -> 3 -60 (to 130) -2700 (to 5800)
3 -> 4 -51 (to 139) -2277 (to 6223)
4 -> 5 -46 (to 144) -2074 (to 6426)


GearChart5.13
Gear 1 2 3 4 5 Final
Ratios: 3.475 2.002 1.366 1.000 0.756 5.130

RPM Limit: 8500
Power @ Peak RPM: 190
Tire Size: 225/45 x 15
Tire Diameter: 22.97 in
Tire Circumference: 72.16 in

Gear Mph per 1000 RPM Mph @8500 RPM
--------------------------------------------------------
1 3.83 33
2 6.65 57
3 9.75 83
4 13.32 113
5 17.62 150

Mph RPM (in Gears)
-------------------------------------------------
1 2 3 4 5
-------------------------------------------------
5 1304 751 513 375 284
10 2609 1503 1025 751 568
15 3913 2254 1538 1126 851
20 5217 3006 2051 1501 1135
25 6522 3757 2564 1877 1419
30 7826 4509 3076 2252 1703
35 5260 3589 2627 1986
40 6012 4102 3003 2270
45 6763 4615 3378 2554
50 7515 5127 3754 2838
55 8266 5640 4129 3121
60 6153 4504 3405
65 6666 4880 3689
70 7178 5255 3973
75 7691 5630 4257
80 8204 6006 4540
85 6381 4824
90 6756 5108
95 7132 5392
100 7507 5675
105 7882 5959
110 8258 6243
115 6527
120 6810
125 7094
130 7378
135 7662
140 7945
145 8229

Gear Change RPM drop (change @190) RPM drop (change @8500)
-------------------------------------------------------------
1 -> 2 -81 (to 109) -3603 (to 4897)
2 -> 3 -60 (to 130) -2700 (to 5800)
3 -> 4 -51 (to 139) -2277 (to 6223)
4 -> 5 -46 (to 144) -2074 (to 6426)


Hopefully this will give you an idea of what rpms you will see with the respective gears.
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Old 07-12-02, 12:36 AM   #6
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Sorry but the spacing set up in the message screen will not transfer with the post. If anyone would like the charts emailed to them, just let me know. I can paste them into the body of the email, or attach them as a Word document. Your choice.

Sorry it didn't work. You may still be able to interpret the data, it's just a bit harder. You might also attempt to reconstruct the data by copying/pasting to your own Word document and going from there. Good luck.
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Old 07-12-02, 01:28 AM   #7
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bcrx7 o_block
Are higher ratio gears legal in SP??
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Old 07-12-02, 02:06 AM   #8
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No, according to my 99 Solo2 rule book.. Maybe the rules changed between 99 and now, but I doubt it
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Old 07-12-02, 05:55 AM   #9
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I would apprecaite the chart.
Can you run it with MIATA transmission gear ratio's?
Thanks John
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Old 07-12-02, 11:38 AM   #10
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I don't think you can run the 5.12, but maybe the 4.88. The 5.12 will put you at about 145 at 8300 in 5th with the GTUs gearset. This setup works very well at tracks here in the Southeast. I know the 4.88 is more popular elsewhere, but here the 5.12 dominates. Isn't the Miata 5th gear like a .81 or .83? That may equate to too much rpm/not enough top speed.

Remember that the tightest ratios and lowest rear gear are not always the best combo. Having a setup that requires you to shift more is sometimes more costly on the stopwatch than having a bit less gear but saving a shift.

John, I'll have to forward you the chart from home this evening. If you have all of the ratios for the Miata gears, I can plug them in and provide that info as well.

LEGAL DISCLAIMER: IT racing does not permit the changing of ratios within the gear box. However, the GTUs box is a legal upgrade, and since the 5th gear is the only different ratio from the others, most guys just change 5th gears. I don't know about SOLO and the different class allowances, but in IT it's a no-no.
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Old 07-20-03, 05:02 PM   #11
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did any company any low final drive gear for 2 gen TURBO. I am runing ITE (13B PP) in SCA. the only thing I can find is 4.3 from MAZDA comp. just wondering ??
the stock TII ratio is 4.1
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Old 07-21-03, 10:00 AM   #12
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the 5.12 and 4.88 should have the same installation "problems". I believe both of these ring and pinions are modified to fit the second gen N/A diff. I have seen many 5.12 setups and this seems to be the EP setup of choice. R&P are modified and then the installation is the same as any other R&P. Speedsource unfortunately gets a ton of money for their parts.

As for the turbo II diff the only other gear is a 4.33 available from Mazda. I believe this was an option on the turbo II and GTU cars.
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Old 07-30-03, 10:14 AM   #13
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Quote:
Originally posted by tims
the 5.12 and 4.88 should have the same installation "problems". I believe both of these ring and pinions are modified to fit the second gen N/A diff..... R&P are modified and then the installation is the same as any other R&P. Speedsource unfortunately gets a ton of money for their parts.
Try ISC racing at www.iscracing.net . Mike VanSteenburg has been building rear ends for years and he is usually cheaper than SpeedSource. BTW, they both use the same machinist to modify the pinion gear, it just appears that SS marks up the job a bit higher than Mike does.

It's worth looking into.
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Old 07-30-03, 07:47 PM   #14
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So the million dollar question is who is the machinist. I've talked to a friend that runs a BIG machine company and he didn't want to touch the project. Another fellow RX-7 racer / machiniat does his own but has snapped them too so it seems to be quite the little art to modifying the gear. You SW guys ought to know the source and how to cut out the middle man.
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Old 07-31-03, 11:10 AM   #15
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Quote:
So the million dollar question is who is the machinist. I've talked to a friend that runs a BIG machine company and he didn't want to touch the project. Another fellow RX-7 racer / machiniat does his own but has snapped them too so it seems to be quite the little art to modifying the gear. You SW guys ought to know the source and how to cut out the middle man.
this is quite common with these modified R&P. the 5.12 needs a cooler and pump to keep it alive and would be recomended with the 4.88 also. this setup is a very short lived item. in racing time they are good for maybe one full season of racing. the machine work can be done by any competent machinist, but the setup and installation are the same as a stock R&P.
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Old 07-31-03, 11:24 PM   #16
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Quote:
Originally posted by tims
this is quite common with these modified R&P. the 5.12 needs a cooler and pump to keep it alive and would be recomended with the 4.88 also. this setup is a very short lived item. in racing time they are good for maybe one full season of racing. the machine work can be done by any competent machinist, but the setup and installation are the same as a stock R&P.
Actually, I know many racers that have run the same gear set for many years, over 5 for one individual I can think of.

And we must do without the cooler/pump setup, that was declared illegal in Fastrack over a year ago. I don't agree with it, but that doesn't change the legality.

As to the machinist, it probably could be done by any talented individual, we just haven't found someone willing to take on the job. The setup alone to make the fixture would likely price you above simply having SS or ISC do it for you.
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Old 08-01-03, 10:29 AM   #17
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Actually, I know many racers that have run the same gear set for many years, over 5 for one individual I can think of.
maybe on an IT car but the 3 EP cars I see regularly have all blown the first gear(all are 5.12) before the cooler was added. since then they have stayed together but they wear quickly. for the EP racers around here the diff has been a problem and heat is the reason. several teams now have temp gauges and data aquisition on the diff and have seen some very high temps(can't remember exact number but well over 300*F). cooler has solved the heat issues but the R&P still wear quickly. again on a lower reving and lower HP IT car it might not be as big a deal, but on the faster more powerful cars they do wear out quicker. thankfully I have a fabulous machinist as a close friend so I don't worry about any parts I need made from metal.
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Old 08-01-03, 10:29 AM
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