EmboRotary

Hi all,

Will be putting my PP motor together soon and the topic has shifted to what gearbox I'm going to pair it with. For context this is an '85 FB with the original smoothcase gearbox in it. I've been considering an RX8 6 speed as a swap, but I know the shifter is too far forward to fit in an FB, does anyone have any experience relocating the shifter back a bit? Also, does anyone know what sort of drivetrain losses you get at high rpm with that box?

On the other hand, I've also got the option to just swap in an MX5 gearset. This would obviously be a lot cheaper, no fab work required. MX5 boxes are really cheap here in the UK so I could afford to have it break every now and then, not that I'm particularly hard on gearboxes, but I assume a PP would shred them. I could also have the option to upgrade in the future to a Quaife gearset.

What's everyone's thoughts on this?

Eydes2Rotor

iTrader: (3)

My experience with RX-8 6 speeds is that they are relatively weak and not worth the hassle. At the shop I used to work at, our shop car was an 06' RX-8 with a street-port Renesis making about 195 (dyno-proven) WHP. It was an unusually strong motor, miss that car.
After shelling out three 6-speeds in a row on track, we did the popular JDM 5-speed conversion on the car. This issue wasn't a lubrication related, we were using Redline lubricants gear oil, which we had no issues with in other cars.
The RX-8 5 speed, only available in Japan, IIRC, is an FD gearbox with a bell housing that fits up to a Renesis. IMO, you'd be best suited fitting a T2 or FD gearbox, not the 6 speed. Closer gears are cool and all, but not when you lose one every few races.

EDIT: I did some digging. The JDM RX-8 5-speed is actually an "improved version" of the FD gearbox with newer syncho design.

diabolical1

iTrader: (3)

my input on this is purely from reading things, not personal experiences ... so get your grain of salt. also, don't view this as me disputing anything in Eydes2Rotor's post. his is from experience - more than i have, but i just want to include a few things here.

first, let me say, from what it sounds like you want to do, i probably would say just go with the Miata gearset and keep your original. you'll keep your driveshaft and have a working speedo. win, win and win.

now ... for the Rx-8, i think the S1 cars had the problematic trannies, or rather, the weaker ones. from everything i've seen so far, it sounds like the S2 cars had a much better tranny - stronger and doesn't mind high revs. now, i don't know what they cost in the UK, and there will also be the issues of mounting it and figuring out how to keep your speedo functional. i know at least one FD with a N/A 20B, that runs/ran the Rx-8 tranny and while i'm not certain, i think it is/was actually an S1. obviously, S1s will probably be more available than S2s, but it sounds like the S2s are a good overall choice.

for the R-type trannies (T2, FD and Rx-8 5-speed), they are strong, but from what i understand, they don't like high revs. maybe the Rx-8 ones were improved in this regard -i don't know - but it's worth noting that they supposedly only came with the 4 port engines. another thing about the R-types is i think i remember reading somewhere that they render more driveline losses, too. that part, i'm not too sure about though (it may have been the T2/FD rear ends), so it would probably be worth looking closer into that tidbit. unfortunately, my memory is fallible.

here is a good thread that might provide some insights: https://www.rx7club.com/general-rota...guide-1153243/

EmboRotary

Problem with a T2 box for me is that they’re incredibly rare in the UK and the last one I saw for sale was £1200 ish? I also read a post a while back about the drivetrain losses you get with it, one user had a dyno sheet where he lost 25% at high RPMs. I’m working off the basis this engine will make ~300 FWHP, so 220 WHP sounds like a bit more of a sacrifice than I’m willing to make. Bear in mind this is a street car, so if I’m going to be putting up with the drivability of a PP, I want the HP benefits.

We got the 5 spd RX8 in the UK, but it was attached to the lower output 4 port engine, which also had a lower redline. Though, it does seem to sell for cheaper here.

mr2peak

S2000 trans. High revs no problem, one of the best feeling boxes ever made.

Or, newer MX5 trans.

RX7_Renesis

iTrader: (1)

I seem to recall that you can swap out the rear section of an RX8 tranny with an FB tranny to compensate for the change in shifter location.

This might be something I saw on RAD Potential's YouTube channel.

j9fd3s

iTrader: (3)

i think there is some way to move the shifter using other Aisin AZ6 shifter parts.

also not sure that you could use the miata gearset, the Rx8 uses the turbo sized shafts, and the miata is the NA sizes.
it may be possible to put the Rx8 bellhousing on the Miata trans, but the guts are almost completely different

EmboRotary

What makes you say that? I’m talking about only swapping the FB bellhousing and tailhousing onto the MX5 5 spd centre section. I haven’t bought a clutch/flywheel yet, so once I make a decision on the box I’ll buy an NA or Turbo package to suit.

diabolical1

iTrader: (3)

???
i don't think that's how it works ...
as i understand the Miata gearset swap, you're basically going to have to disassemble both trannies and swap the guts. i think you also have to grind off 0.5-inch (???) - i'll try to double check the number for you and post later - off the input shaft and give it a taper so it will fit the pilot bearing. my memory is a little fuzzy on that detail, but i do remember it's a pretty involved process and requires special tools.
it will be the N/A clutch. the Miata (up to the Gen 3 cars) used an M-type tranny, same as the Gen 1 and Gen 2 Rx-7s (not including the T2).

EmboRotary

Yeah it's been a while since I looked up the actual process, my memory's a bit fuzzy too haha. Looks like I'll probably be going with the MX5 gearset, seems the most convenient in terms of both upfront cost and replacement in the future.

Was kind of planning on the RX8 box, but at least I wont have to faff around with a GPS speedo now.

j_tso

iTrader: (2)

Input shaft modification shown from Mazdatrix

diabolical1

iTrader: (3)

here are a couple of threads:

https://www.rx7club.com/race-car-tec...sions-1142975/

https://www.rx7club.com/race-car-tec...mission-95788/

there's a thread over on RCC that detailed the process (with photos) and i'm trying to find it. he had to make a tool to pull the tranny apart and he showed what he did in the thread. i can't remember the member's name.

at any rate, disregard the first part of my last post, because i think it was just semantics and i misunderstood what you were actually saying. you were saying the same thing i did, but when i think of these trannies, i don't think of a bell housing, i think of a front housing ... and i know it's weird, but that's where i misunderstood you.

j9fd3s

iTrader: (3)

ah sorry, i thought you meant the 6 speed miata trans. the 1999-2005 miata SIX speed is related to the Rx8.

the 5 speed miata guts are totally proven, and its probably the easiest way to go

EmboRotary

Yeah I haven't yet found a thread that goes in depth into everything that needs doing. From a quick search around it seems like the gears just slot right in exactly like the FB shafts would, just need to trim the input shaft. I also work for an independent garage, not that we specialise in rebuilding gearboxes, but I'd assume we've got most of the necessary tools as long as it doesn't require Mazda specific stuff

Eydes2Rotor

iTrader: (3)

I vaguely remember us picking up a bit of power when we swapped in the 5 speed. That car got to be on the dyno a lot so it’s hard to remember, i will ask some of my old coworkers. I certainly think drivetrain loss with the 5 speed would be better.
As far as RPM related issues go, that car was regularly taching up to 9.5k, and it only ever saw track use. No issues with the 5 speed. Could’ve just been a lucky transmission though. What i can say with relative certainty is the 6 speeds are not reliable for high load applications