(DRIVETRAIN) my S4 transmission n/a mounted in FB
 

This is simply a summary of my conversion to use a s4 Non-turbo tranny mounted in my car. IF this has been done before and been written up on this forum, simply ignore this....


First of all, acquire a n/a 6port tranny and swap the tail shaft and the speedo gear from the FB tranny. Thats basically it! ...and bolt it in. I just talked to someone this week, and had a parts car, and said I could have it free if I got it out! Win-Win!

Why would one do this? For the higher 5th gear. Running the 1st gen at 80mph with a stock tranny gives about 4000-4300RPM in 5th gear depending on what year tranny you have. With a FC tranny (depending on the year) gives you about 3500RPM in 5th gear at 80mph. I did this for the lower operating engine speed for potentially better MPG's. It does tend to "lug" the engine more, but at speeds beyond 60mph stock first gen cars feel like they should have a 6th gear or a higher 5th gear. Another benefit of the FC tranny, it has vents that 12a trannies do not have.

I have not yet gone on long drives to compare to previous highway MPG's, but with the turbo, anything to increase mpgs is welcome.

I had no idea it was that simple. Hmm, and I think my brother is parting out his na s5, are the tranny's the same? I feel you on the gas mileage, I was trying to be gentle with my car on the 100 mile trip up to school, and I still got horrible gas mileage. The na fc trannys arnt by any chance much stronger than 12a trannys, are they?

No, I don't think they are really significantly stronger. I don't know if n/a S5 is compatable. I think only s4 n/a trannies are.

I know on the TII trannys you have to modify the shifter housing and rod, but you are saying on the NAs you simply change the tailshafts for the length adjustment and your good to go, how cool, and easy is that!!!

Also FC tranny has a longer 1st gear and a little closer gears. they're also stronger and can hold a little more power than a FB/SA tranny. good write up I hope to make the change eventually...

I did this, and it worked great.

You also have to drill a hole in the shift rod about 3 mm down from the orignal hole on the s4 rod. The FB rod did not work.

Why swap the tailsection? I have put my S2/S3/S4 gearboxes next to eachother, and as far as I could see they were all the same. Same shifter location, length,... The 2nd gen box bolted straight in.
Of course there *might* be a difference between euro and USA spec (which is why I ask why you swapped the tail), since Euro S4's are different from yours in many other ways.
The TII boxes are more complicated to swap, but euro S4's just fit like that (using a S2/3 mount, which fits the S4 NA box).
Also: is it really needed to change the speedo gearing? Both speedo's have the same gearing (if I'm right it's like 375 turns for 1km/h) in Europe, I actually have a S4 speedo in my S2 car (and ran it with all the gearboxes by now, all were off just as bad, because of the tires, and rear end being way different then stock...)

and this is exactly why i plan to include it in the rebirth of my car. actually, i was planning to replace the guts with NEW Gen II pieces, but i may just go with getting a used Gen II tranny.

if it's not too much trouble, could you snap a couple photos of the vents you referred to?

great writeup, by the way. ;)

That is odd, my s4 tail housing removed, and installed the 1st gen tail housing with the 1st gen shift rod and worked fine?

That is odd, the tail housing is the same I think... But the shifter location is further back on the S4 transmission which is why you need to use the 1st gen one to make it fit without cutting into the floor board. I didn't look very closely to the S4 transmission, but I am unsure if the transmission mount is the same on the tail housing location.

The speedo gear is different on the S4 and 1st gen transmission I swapped. The worm gear is noticably different if you look closely to the ratio. I think you would want to swap them to get a acurrate reading..at least I did.

I can't now, its in the car, and I have the car stored many miles away currently :) But, they are on the belhousing section FYI. I think they are a good idea as I'm sure a lot of shifting will cause the flywheel/clutch surface to heat up like brakes do. More of a concern for road racing or hard driving in general.

So what you're saying is that I could pop out my GSL-SE transmission, swap over the tailshaft and speedo gear and drop in an S4 NA transmission? Nothing else needs to be done?

I have a FB trans, with an S4 tailshaft housing (was in the car when I bought it). The S4 mount uses a closer bolt hole spacing than the first gen. The shifter is about 2 inches further back on the S4 as well.
I simply custom made a crossmember for the S4 style mounts (3 total) and left it alone. It was barely bolted to the original crossmember when I got the car, and caused a vibration and numerous rattles inside the car.
But, yes, the FB/FC (S4 N/A) trans tailshaft housings are interchangable.

You need to swap the tail housing on the gearbox. Not the tailshaft.

in the first post you stated that stock fb's should feel like they should have a 6th gear.....finally! I thought I was the only person that thought this. Everyt time i got mine into 5th I could feel that she had so much more to give but the speed limit and my tranny just didn't agree with me

Correct- I presume that is why Mazda bumped up the gearing for 5th gear on the following years in '86 and beyond. Then finally in the Rx-8, they actually have a 6th gear.

And to clarify to all- I am saying the tail shaft housing, speedo gear on that shaft is swapped, not the tail shaft itself! Sorry to clarify later on.

Ok, update- I have went on a long drive to figure out if this setup helps for a FB> I went snowboarding in northern MN- Lutsen- really nice ski resort by midwest standards anyway along the northshore of Lake Superior- the boards for both me and my passenger just barely fit. It was a 200mile+ drive one way to the destination.

The results- with my "normal" driving 80mph average cruse speed with hard accerations everywhere, I achieved 17mpg on the way up. I haven't filled up since I returned to check the return mpg's. But its near empty at 234miles when I parked the car so its probably about the same-(I don't normally drive in winter- did so for the turbo pleasure and see if the 5th gear on a s4 n/a tranny will help the fuel economy.

In all honesty, this is not a great improvement. Simply put- 5th gear is just too high!!! I end up downshifting whenever there is a mild hill to avoid boosting at a low RPM were the power peak definitely is not at (2500-3300). A locked distrubuter does not give the best timing at these engine speeds evidently.

I have achieved 19mpg before with speeds of 60-70mph on a long drive. Seems like the optimal gearing for 5th gear is probably between .697 and .800. The engine vacuum is much less when the engine is lugging at a lower rpm and actually creeps into low boost range reading the boost gauge. It actually lugs the engine too much at 125mph with 6psi boost to accerate as higher with the stock 12a tranny 5th gear. I either need to up the boost controler or lower the gearing "slightly". :)

Hmmm...do you think this could have benefits for an N/A car? I imagine it wouldn't be great for driving in hilly areas with a turbo car.

Possibly- epecially a 6port 13b without a header and a highflow exhuast because it would still make good lowend power that the extra high 5th gear of a s4 n/a tranny would be able to use well without causing the engine to "bog" as much.

I should state that at 85-90mph, with a 12a, the engine speed is high enough with the s/4 n/a tranny to be better then spinning the engine as high with the stock 12a tranny.

how were you able to remove the housing from the 13b n/a tranny? im currently having some difficulties, but i already took off the 12a housing. ive followed couple manuals and yet nothing. help please =]

i think I'd rather just swap in miata gears. I've heard its a nice change.

I just removed the housing from a s5 tranny and tried using my 79' housing, but it was too small. So are you saying that I should change to a s4 and it would work fine? or is the 79-80 just smaller than the 81-85?(smaller as in the bolts holes do not match because the surface area is less)

I think the 79-80 is different from the 81-85.

If I got a 81-85 housing, would it fit the s5 tranny and also fit my 79 without me cutting anything up?

This is exactly what I am running. For a stock engined N/A car down in Texas, this is great. I was getting 25mpg driving from Bryan, TX to San Antonio not too long ago. I wasn't driving all that nice either. Never had a problem with lack of power up hills.