Final Gear Ratio / Alternate Sources

[gruff]

I am looking at working my final drive ratio. I have an 86 sport model with the GTU fifth gear and the stock 4.10 final drive.

I have heard that I can use a GTU diff, but not a TurboII diff. I was under the impression that these differentials were the same part. The update/backdate rules would seem to allow the use of complete TurboII housing/diff/gear assemblies in ITS as long as no housing grinding is required to fit the unit. How do the update/backdate rules apply to my differential choice?

I have read about the hot setup of 5.12 final drive ratio and the GTU fifth. On my race circuit of choice, I top out at 120, in fourth, at about 6700 RPM. I am obviously giving away some acceleration at the start of the straight. By my calcs, I lose about .8 seconds of the 15 second sector compared to a 5.12 due to gear ratio suckage alone. (I'm not commenting on driver suckage :) )

Is the 5.12 durable? If I ran a 20% taller gear (e.g. 4.88) and just drove my car as a four speed, I would be giving away a small bit of acceleration at corner exit. It might not be worth it if the 5.12 requires giving away some durability. A mechanic friend of mine (nascar guy) seemed to think that a 5.12 was awfully short. I countered with rotaries are low on torque and rev freely, plus we're not slamming the gears like a drag racer. Still, I don't have the budget for fixing blown up diffs on a regular basis. If someone here can confirm the durability of the 5.12 that would help me to decide.

Also, has anyone done any new research into compatible gears from other vehicles? Hollanders provides some hope, but I don't have enough information. I see that some really short gears are available for the Kia Sportage front diff that are being used in S2000s and 3rd gens. These would still need the long pinion mod to work, but it is a source of a number of ratios from 4x4 modders. I am not sure how a person procures parts from Korea, but it might be worth looking into.

I read that the gears are 7" or 8" depending on model. Is that true? Being Japanese, I wonder if the gears are actually 180mm and 200mm which are very close to 7" and 8" respectively.

Who made Mazda's final gears? I suspect it's Toyo-Kogo, but I haven't found anything authoritative.

It's a huge post, I know. I have read beacoup sources on the net and poured through Hollanders looking for gears trying to find a less expensive solution the long pinion mods. I am trying really hard to find those gears that I presume to exist. I have a hard time believing that a gear manufacturer made a onesy-twosy custom application for a single make/model of car.

Thanks,
Jason

[Chris Wire]

I am looking at working my final drive ratio. I have an 86 sport model with the GTU fifth gear and the stock 4.10 final drive.

I have heard that I can use a GTU diff, but not a TurboII diff. I was under the impression that these differentials were the same part. The update/backdate rules would seem to allow the use of complete TurboII housing/diff/gear assemblies in ITS as long as no housing grinding is required to fit the unit. How do the update/backdate rules apply to my differential choice?[/b]

Here's a shot at some of the information you're seeking.

Since the Turbo model (indeed no Turbo model) is listed on our spec line, you are not allowed any update/backdate of parts unique to the Turbo. Therefore, the Turbo diff unit is a no-no. :018:

The "any diff" allowance in the rules pertains to the differential inside the housing only.


I have read about the hot setup of 5.12 final drive ratio and the GTU fifth. On my race circuit of choice, I top out at 120, in fourth, at about 6700 RPM. I am obviously giving away some acceleration at the start of the straight. By my calcs, I lose about .8 seconds of the 15 second sector compared to a 5.12 due to gear ratio suckage alone. (I'm not commenting on driver suckage :) )

Is the 5.12 durable? If I ran a 20% taller gear (e.g. 4.88) and just drove my car as a four speed, I would be giving away a small bit of acceleration at corner exit. It might not be worth it if the 5.12 requires giving away some durability. A mechanic friend of mine (nascar guy) seemed to think that a 5.12 was awfully short. I countered with rotaries are low on torque and rev freely, plus we're not slamming the gears like a drag racer. Still, I don't have the budget for fixing blown up diffs on a regular basis. If someone here can confirm the durability of the 5.12 that would help me to decide.[/b]

Yes, the 5.12 is extremely durable. It's a well proven part in competition (as is the 4.88). Most of the fast RX7s in the southeast run the 5.12. Works well at the tracks we run. If you are cash-strapped, you could run the 4.88, but the money difference is only about $300. Still, some tracks work better with the 4.88 run as a 4-speed as it eliminates the 4-5 shift. Sometimes that will offset the perceived lack of corner-out acceleration, sometimes not. Another added benefit would be less wear on the tranny.

[gruff]

Since the Turbo model (indeed no Turbo model) is listed on our spec line, you are not allowed any update/backdate of parts unique to the Turbo. Therefore, the Turbo diff unit is a no-no. :018:
[/b]

That proves that I really don't understand the update/backdate rule.

The GTU doesn't appear on the spec line or any spec line either. In fact, GTU doesn't appear in the GCR anywhere. I guess the crux of the matter is, "What constitutes a model?" I wouldn't think the GXL/GTU moniker makes a difference. Apparently it doesn't as people can race GTUs. Obviously, a turbo car is specifically disallowed from ITS, so I can understand that update/backdate using parts from the Turbo isn't allowed.

A GTU is the same make (mazda), model (rx7), and engine (13B) per GCR 9.1.3.c. Is that the specific criteria by which a GTU car is allowed and/or update backdate to GTU parts is allowed?

But looking at other cars in the GCR the spec lines do include the specific "sticker" monikers. The Civic SI or Porsche 944 and 944S.

Thanks,
Jason

[Scott Nutter]

So if the ring and pinion from the turbo model will bolt into your non-turbo differential, then you can swap them (ring and pinon gears) into your housing.

[mbuskuhl]

I recently found myself in your situation. I had the 4.10, the 5.12 was a lot of money to justify considering everything else I had just done to the car. I had never tried a 5.12 or 4.88 for that matter. Fortunately I came across hardly used (2 race) and reputable 5.12 on the secondhand market and went for it.

The 5.12 is the best purchase I have made for the car. You won't regret it, if you are half way serious about being competitive then the 5.12 is a must in my opinion. Do it right, do it once, get the 5.12 - or, if you are going to run in the SOWDIV ever stick with a 4.10 please ;)

[dickita15]

That proves that I really don't understand the update/backdate rule.

[/b]
To help you understand the update backdate rule you can use any part ( or actually the entire assembly) from any car that is included in your spec line listing. You car is listed as “Mazda Rx-7 (13b) (86-91)” so any part from any year or model of these cars can be used. However IT does not allow turbo cars so that model is not part of your spec line. Gtu is a model so you can put a Gtu aluminum hood on your 86.

[Chris Wire]

Actually, the model I think you are really referring to is the GTUs. That is the model that came with the 4.30, the aluminum hood, and the .765 fifth gear. It is, as dickita15 pointed out, a model included on the spec line where the Turbo II is not. The GTU, which is what my car started out as, is more of a base model, with the smaller brakes and non-automatic suspension.