20b Cosmo Manual Transmission Conversion
 

Hi Guys

I am about to acquire a 20b Cosmo on which Id like to do a manual conversion using an RX8 Six Speed Gearbox. Now As far as Im aware, the bell housing will bolt up directly.

The rest of the parts I plan to use are below:

RX8 6 Sp Box
Modified Prop made from the RX8 Prop Front Section and the Cosmo Prop Yoke for the Differential
Custom gearbox Mount using the exisiting underbody bracing holes to mount the mount (as it were!) to the chassis
FC TII Slave Cylinder
FC TII master Cylinder
FD Clutch
FD Pull-Push Type Clutch Convertor
FD Brake Pedal Assembly
FD Clutch Pedal Assembly


Now what I was wondering is...does anyone have a wiring diagram for the 20b Cosmo (or more particularly the part which shows the starting circuit) so that I can see which wires need bridging on the inhibitor switch?

Does anyone see any problems with any of the parts that Im using? Could I use RX8 Master and Slave Cylinders perhaps?

Thanks in advance

Jitesh

Jitesh you'll find that the clutch pedal and master cylinder from a Mazda 626 fit well. Likewise the brake servo and master cylinder as the stock abs pump gets in the way somewhat.
Regards Kevin

you need the rx8 clutch slave, if you're lucky it'll be on the trans, rx8 master should work too, slave is weird, master looks like the fd....

just leave the inhibitor switch plugged in. the ecu doesnt seem to mind starting in drive

Mike, doesn't the RX-8 master share a common reserviour with the brake master? I'm sure Jitesh will figure something out.

Oh and Jitesh, you might want to consider not using FD pull-type clutch components. Push type is a lot better in many peoples' opinions. More parts available, easier to work on etc.

Hi Kev

Thanks for that. Im sure ill be contacting you in due course for some more help on the project! Hows your new one coming along?
Just in the parts gathering stage right now!
Which series or chassi 626 do you mean...i like hunting scrap yards for gold!

you say leave the inhibitor switch plugged in...dyou mean cut the loom off the autobox and plug it in?

Jeff - i think ive listed a pull-push convertor :D

It's the model car with the intergeral rear spoiler i've used in the past 2.0 litre version. But i'm fairly sure all models are interchangeable. When you come to mount the gearbox have the bolts from the donor gearbox to hand as the Cosmo ones are shorter. If your running big power its worth making a bracket to mount where the auto starter motor was to utilise those mounts as well.
As for my new motor bodyworks nearly done needs a final flat and polish. Cage is in along with most of the wiring just waiting for the last bit of machining to be done on the engine then i can get the serious work done. All being well it'll be at Rotorstock probably not ready to run on the strip though.
Regards Kevin

The RX8 gearbox most probably will not be able to hold up with the power levels that a stock 20B puts out. I'd recommend an S4 TII transmission....

I'm looking at doing the same thing and I've been looking at a few of the manual conversions done in Japan and they're all using the TII transmissions... Much stronger than the 8 trans....

Looking good Kev! Cant wait to see it at Rotorstock....I was going to tryaand have the manual conversion knocked out by then but realistically its not going to happen....mainly cos i dont have any parts yet and im off on holiday tomoro!

Funkspectrum - Ive been following the progress of a few cars using rx8 boxes including a privateer drag car in the UK. I havent seen him have many issues with the box inc on hard launches. I do plan to mount the box securely and make both gearbox and diff braces to reduce any shock loading on the drivetrain.
My main reason for choosing the RX8 box is because i reckon ill have more chance of finding one that isnt tired...most S4 boxes will be well worn by now!
Also, no matter what box i use, im going to have to make some sort of shifter extension, and the more bushes you introduce in the shift mech, the more sloppy each shift will feel so im after something which wont feel totally shagged!

I've always used the fd Box there's still a few low millage ones about, and with a cheap ebay short shifter with a dogleg in the lever it fits more or less right in the cosmo auto shifter position.
Kevin

Interesting..I have an FD box (or 3 lol) lying around my garage....The aim was most defeinitely to have it coming thru the stock auto lever posn. I do want the install to look as factory as is possible!
Has your one had the manual treatment yet Kev?

yep, its like the fd.

the rx8 uses a t2 type clutch, fd pressure plate and t/o bearing wont work

The RX8 uses the same transmission as the S2000, Aisin transmission. They won't hold up to a turbocharged 20b. Some guy on this forum was stripping 3rd gear out just by stomping on the gas in 3rd gear pulls. However, the RX8 transmission bolts right up to the 20B, but you need to relocate the oil filter because it clips the slave cylinder.

JiteshTII,
what are you doing for your ECU to run the 20b with the manual trans?

and i dunno where you got that the Aisin Transmission was in the S2000, it doesn't say it anywhere on the page....

http://en.wikipedia.org/wiki/List_of..._transmissions

a manual transmission will work fine with ther factory ecu, you just disconnect the auto ecu from the main ecu, simple!
I know because this is all we did with mine. we simply disconnected everything that was attached to the auto, other than rewiring the inhibiter switch, we didnt have to touch anything else to gett he egnien to run fine. Speedo was a different situation.


I dont see and RX8 box lasting long, they are only a glorified mx5 box arent they?

I used an FD box, with a FC bell housing and a custom spacer (16mm) as I alsready had a brand new HD FC clutch befor I came across the FD box cheap.
This put the gear lever smack where the factory auto lever is.

I used a 626 pedal box, but you need to brace it in a couple of places as mine tore some of the factory metal in a couple of places due to the HD clutch.

rx-8 transmissions are made out of glass, you'll destroy it quickly.

if you want a 6 speed put in a T56.

Ive done this conversion but in a 13b model this is what i've used

fc gearbox
fd clutch pedal and master

its pretty straight foward the only bit i had a problem with was the firewall flexing so u just have to make sure u brace it well.I ended up just taking the dash out to do it,also the fc gearbox is much easier to get the speedo working all you need to do is simply take the connector half off the standard auto sensor and match it with the fc sensor

Try ausrotary.com
or 3rotor.com for more info

the rx-8 transmission will cry for mercy, once the 20B fires up and you go for a slow 20mph roll down the street, due to the low-rpm torque offered by the 20B.

the Rx-8 tranny is an MX-5/Miata transmission with an extra 6th gear shoe-horned into it. once you hit any kind of power beyond the transmissions own torque capacity it will pretty much break all the gears.

if the 8 used the same tranny as the S2k, then RX8 trannies would be strong as hell since there are many 10sec stock block and tranny s2k's

I've heard that the 929 5-speed tranny is the same as the TII tranny.

Since you like junk yards, you may find a 929 more available than a TII tranny's. The TII tranny's get taken quickly, from junk yards but many people don't know that the 929 tranny will work.

This is what I used on my Cosmo AT - 5 speed swap

FD clutch pedal
FD clutch master
FC slave
FC trans
FC clutch and pp
FC flywheel

This is a long thread but this guy was the first to drop a 20B in an RX-8. even with the cryo treated gears, they kept breaking. RX-8 tranny is the same as the Miata and S2000.

http://www.rx8club.com/showthread.ph...&highlight=20b