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FD gearbox strength
Hi there.
I don't have an RX7 but i am considering using the FD gearbox to fit in a retro car (engine turbo Honda K24). I was wondering about how strong the standard gearboxes are. The plans are for no more than 400bhp and i won't be running tyres any wider than 205. I'm not into hard launches (drag) although i do like a bit of gymkhana and drifting. The Toyota R154 and the RB25 boxes (expensive) are also an option, but i saw that the RX7 FD boxes were a lot cheaper. Do you think this gearbox would hold up to the power? Any better suggestions? Thanks |
the FD people seem to break these transmissions at around 450lbs/ft in 3rd gear, give or take. this is in a ~2700lbs car.
you might have a look at the gearing, its ok for a car with a low flat powerband, but if you build something peaky, it sucks donkey balls |
Thanks for the reply :) The car will be a lot less than 2700lbs, around 2000lbs. So that should help stress the gearbox less and with maximum width 205 tyres on the rear, it sounds like it could be strong enough and not have to worry about it breaking.
Comparing the ratios of the Honda Integra DC5 with the K20 and a 4.3 final drive. FD ------------- K20 1st 3.484 ---- 3.266 2nd 2.015 ---- 1.88 3rd 1.391 ---- 1.121 4th 1.000 ---- 0.921 5th 0.806 ---- 0.738 They don't look too far away... Maybe playing with the final drive would be an option. |
In a lighter car with smaller tyres it should be ok.
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Yeah, thanks for that... I can now start buying the necessary parts :)
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Should be ok but the adapter plate to a FD trans will likely eat up some of the cost savings.
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Originally Posted by m11rf
(Post 12313389)
Hi there.
I don't have an RX7 but i am considering using the FD gearbox to fit in a retro car (engine turbo Honda K24). I was wondering about how strong the standard gearboxes are. The plans are for no more than 400bhp and i won't be running tyres any wider than 205. I'm not into hard launches (drag) although i do like a bit of gymkhana and drifting. The Toyota R154 and the RB25 boxes (expensive) are also an option, but i saw that the RX7 FD boxes were a lot cheaper. Do you think this gearbox would hold up to the power? Any better suggestions? Thanks |
They're not dealing with an Rx-7...they are ONLY using an FD transmission for a "retro" car build with a Honda engine.
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I think they break at a lot lower than 450 ft-lbs, or rather doubt I was making that much with OE twins back in the day ... :suspect:
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I wouldn't want to put more than 375 ft lbs through an FD box in anger on a regular (track) basis. They'll last just fine up to 425ish on the street though.
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Originally Posted by dguy
(Post 12517952)
I wouldn't want to put more than 375 ft lbs through an FD box in anger on a regular (track) basis. They'll last just fine up to 425ish on the street though.
Or are you mainly thinking about a fully OEM stock trans? |
Originally Posted by madhat1111
(Post 12517930)
They're not dealing with an Rx-7...they are ONLY using an FD transmission for a "retro" car build with a Honda engine.
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Originally Posted by fendamonky
(Post 12517968)
Would that caution/reservation extend to rebuilt/upgraded transmission with sturdier main shafts and treated gears?
Or are you mainly thinking about a fully OEM stock trans? I don't think you'll ever get away from 3rd gear case flex so hardened gears - in my opinion - only delay the inevitable of stripping all those teeth off. I've never seen shaft failures on an FD trans, plenty of gears turned into wheels on 3rd. Also if 5th gear synchros were made of gold, I'd be rich from all the panning I could do from drained fluid. |
any validity to the stronger bottom cover helping to reduce flexure?
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I doubt it personally, it's not like the cover is actually fixturing the shaft and counter shaft. The flex is also up/down, so the thinnest dimension of the cover.
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the liberty mainshaft upgrade has worked perfectly for me for many years after blowing numerous transmissions. No results or signs of shaft flexing after many years of abuse at the track and its holding up fine.
Its not the strength of the 3rd gear that its the problem, its the misalignment with the countershaft |
this 5-spd trans billet cover/stiffening brace from Promaz in AU looks pretty stout if you ask me.
I recognize that the OE shafts will flex and I see that as a different issue, but with an aftermarket gearset it seems like this would add resistance against case twist/flexure: https://shop.promaz.com.au/product-p/box-brace-rx-7.htm looks like maybe 1” (25mm) thick by my eyes . . |
I dont see how this will help.
as a side note. I do have aftermarket gearset on my race transmission. The case deflection was never an issue but i am not making 1000 hp . |
Originally Posted by TeamRX8
(Post 12518128)
this 5-spd trans billet cover/stiffening brace from Promaz in AU looks pretty stout if you ask me.
I recognize that the OE shafts will flex and I see that as a different issue, but with an aftermarket gearset it seems like this would add resistance against case twist/flexure: https://shop.promaz.com.au/product-p/box-brace-rx-7.htm looks like maybe 1” (25mm) thick by my eyes . . |
Originally Posted by dguy
(Post 12517991)
I don't think you'll ever get away from 3rd gear case flex so hardened gears - in my opinion - only delay the inevitable of stripping all those teeth off. I've never seen shaft failures on an FD trans, plenty of gears turned into wheels on 3rd. Also if 5th gear synchros were made of gold, I'd be rich from all the panning I could do from drained fluid.
HOPEFULLY a bit of caution and calmer shifting on my end will help alleviate the 5th gear syncro issue! Though I think that's definitely going to haunt all of us as long as we keep the syncros.
Originally Posted by R-R-Rx7
(Post 12518103)
the liberty mainshaft upgrade has worked perfectly for me for many years after blowing numerous transmissions. No results or signs of shaft flexing after many years of abuse at the track and its holding up fine.
Its not the strength of the 3rd gear that its the problem, its the misalignment with the countershaft As an added precaution, we're also adding in an oil bar to get oil directly to the gears that need it most. I really hope it all results in a far more resilient trans. I definitely plan on putting more than 400tq through the trans. |
Originally Posted by fendamonky
(Post 12518234)
From what I've heard, the problem is that the stock shaft flexes there, that's why we always see that 3rd gear failure. Liberty's upgraded main shaft is meant to remedy that.
HOPEFULLY a bit of caution and calmer shifting on my end will help alleviate the 5th gear syncro issue! Though I think that's definitely going to haunt all of us as long as we keep the syncros. That's great news! Mine is currently being rebuilt at liberty. I'll be incorporating their main shaft, along with the other hardening and surface finish treatments. Ideally that'll make a noticeable difference! As an added precaution, we're also adding in an oil bar to get oil directly to the gears that need it most. I really hope it all results in a far more resilient trans. I definitely plan on putting more than 400tq through the trans. |
I suppose a PPF delete and supporting it independently rather then relying on the case to carry the loading would help relieve things as well, but at that point using a stronger box likely makes sense.
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Originally Posted by Monika
(Post 12518196)
Might as well get the attain/ksp billet pan. I have it and works well. Has stand offs to bolt a magnet too. It adds an extra liter of fluid as well. But if you're sinking huge dollars into a oem box...might as well get a TTI 6 speed. Cheap with the exchange now.
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