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 *****

 

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.

 

Yeah, thanks for that... I can now start buying the necessary parts

 

Should be ok but the adapter plate to a FD trans will likely eat up some of the cost savings.

 

hi. Any update on your swap. Curious why are you running such thin tires on the fd.

 

They're not dealing with an Rx-7...they are ONLY using an FD transmission for a "retro" car build with a Honda engine.

 

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 ...

 

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.

 

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?

 

what car is it gonna be in. Still. With 400hp at the wheels… 205 seems really small.

 

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?

 

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.

 

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 .

 

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.

 

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.

 

It wasnt financially reasonable to have liberty rebuild my transmissions because both of my gearboxes are in cyprus. So i didnt end up doing the treatment/plating for the remaining of the components for my street transmission otherwise i would have loved to do it. The 5th gear syncho is currently the weakest part of the tranny

 

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.

 

I wasn’t promoting the other in particular, but belive the price is AUD. Which would only be $344 USD, but I have a CD009 6 spd conversion waiting to be installed so moot point. In retrospect I might have gone with Liberty had I known they now offer a short ratio replacement gear option.