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Old 10-13-2018, 04:15 AM   #1  
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Default Transmission brace INCREASES g'box case flex?

It seems there are a few folks out there with 400+hp miatas who are trying to preserve the stock gearbox by bracing it in addition to the PPF, or doing a PPF delete, and mounting the rear of the stock box in a way similar to the Kmiata mounts. I'm not talking about bracing the PPF itself, a la the Goodwing powerbrace thingy, but trying to reduce case flex in the stock box by bracing the rear of it. If anyone has a picture of theirs, it would be great to see.

So, I fired up the welder and started mucking around, and came up with this:
Brace with the factory PPF bracket sitting on top








I had almost finished the brace, and was about to drill holes in the floor pan, when it hit me - if the bellhousing end of the gearbox is twisting as the engine revs, then wouldn't bracing the rear of the gearbox to the chassis actually INCREASE the torque and therefore the amount of flex put through the gearbox casing???

I mean, there isn't much engine movement with solid engine mounts anyway, but it's the theory of the thing I am trying to get my head around. If you are wanting to reduce the gearbox flexing down its length, why would you bolt one end to the chassis? Wouldn't it make more sense to actually make TWO gearbox braces to chassis - one at the front, using the bottom 2 bellhousing bolts, and the other at the end of the gearbox? Something like this:

Does anyone with direct personal experience want to weigh in at this point, and perhaps upload a pic of their stock, braced gearbox?

For those who are thinking "NVH will be off the charts", doesn't matter because track car. Do Kmiata put the poly bushes in there to reduce NVH, or to allow a little movement because of the issue raised above?

And, yes, I know that if you have a 400+hp miata then the Kmiata trans swap should be done, and I plan to - but I need the stock box to last till then. The Kmiata stuff is awesome, but it is over $8K landed in Australia. And before other internet import experts howl that down, the homework is thus (in $AUD):
- Kmiata Gearbox kit = $4248
- Kmiata rear diff mount = $492
- clutch = $403
- shipping = $808
- GST & Customs** = $1,844
- 2nd hand BMW gearbox = $1000 - 1800
Total = $8795 - $9595 AUD
**Where VoTI (value of taxable import = CV (customs value) + 5% Duty (Car Parts) + Shipping = 31% of CV.
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Old 10-13-2018, 08:00 AM   #2  
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You're running down the wrong path here. No amount of bracing of the tail shaft will do anything for you. The flex everyone is talking about is the main shaft and Jack shaft are trying to push themselves apart due to the nature of gear teeth angles. So you would need a brace that kept the trans case from expanding radially. Just compare the 5 speed to the 6 speed cases and you'll see all the extra bracing on 6 speed case for this very reason.

Last edited by Bronson M; 10-13-2018 at 09:49 AM.
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Old 10-13-2018, 09:34 AM   #3  
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Ahh, interesting - because of the curved teeth, the gears are pushing themselves apart? So you are talking about “banding” the gearbox, like the steel hoops around the old-fashioned wooden barrels. That would be damn hard to fabricate.
You reckon the gearbox case flexes outward, rather than flexing longitudinally...
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Old 10-13-2018, 09:43 AM   #4  
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All spur gears push themselves apart, regardless of tooth shape or angle. The flexing is internal to the gear box; the bearing pockets each shaft ride in are trying to move away from each other. The case itself needs the bracing, not sure how you'd DIY that effectively. Running wooden barrel bands around it wouldn't do much if anything IMO.
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Old 10-13-2018, 12:46 PM   #5  
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you wanna keep the shafts together.
kinda like this:
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Old 10-13-2018, 06:01 PM   #6  
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awesome, thanks guys for helping me grasp the concept. I guess the only feasible way to make sure the box lasts is to just turnnthe boost down until I can go Kmiata!
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