Went from 3.909 to 4.30! And rubber to PU!
 

Over the christmas break I swapped in a GTUS rear diff into my vert. My vert stock came with the slow 3.909 ring and pinion in the open differential. After the swap I now believe that I should have done it much earlier. The rotary seems to be in the power much more with these gears. Fourth gear is now good enough to drive around all day in. The acceleration is better enough to justify spending $400 like I did or even up to $600 I'd say considering you would need to spend at least that much to find this kind of performance increase elsewhere. If you do ALOT of highway driving you might want a 3.9 or 4.1 but even on the highway the 4.3 makes 5th almost usable. With the 4.3 I was pushing about 3700 at 80mph (with the correct speedo gear). Now I might not recommend this swap going from a 4.1 to 4.3. Going from the 4.1 it would not be such a large improvement and would maybe not be worth the premium price. It is also a fairly intensive swap which will make you swear 7 or 8 thousand times.

During this swap I also changed out all of the bushings to the energy suspension PU ones. This was 10 times the job of swapping the differential. The rear control arm bushings are in there tighter than the pilot bearing only wishes it could be.

After installing the bushings and the differential I had the car aligned. It now feels like a new car. I could not believe how big of a difference the bushings made. You honestly need to experience it yourself. For all the work it took to swap in the diff and new bushings; I feel it was just barely worth it.

P.S. If anyone from MAZDA is reading this: How "F"in hard would it have been to make a slot so you could get the front mounting bolt out of the rear control arm? If there is only one design flaw on this car, I vote for this massive oversight.

Yo,


Samps, welcome to the 4.30 club! *grin* I'm pulling about 4000rpm in 5th at 80mph. I think the GTUs has a special 5th...but I'm still getting crap for gas mileage.

I was curious about the ride quality with the PU bushings...is it overly harsh? I don't want to turn my car into a track car just yet...*grin*

Good write up!

Kevin
1989 GTUs "16mph!"

I think you meant "MPG" ;)

4.30 gears are fun aren’t they? Yep that is the next step in chassis for me is to replace all the bushings with Polly. Glad you like it.

Congrats on the gear, Samps. Good move.

I asked in a recent thread as to why more people don't go for the gear and it is like everyone is dumbfounded by it. As you saw, it doesn't take an enormous amount of time or money. Especially when you consider how much money people spend on other POS items that add little to no performance gain.

https://www.rx7club.com/forum/showth...hreadid=143741

Oh, and the reason you get crappy MPG Kevin is you can't keep your foot out of the accelerator! It's a drug!

Glad your happy SAMPS!!! I would of liked to experiance the 4:30 gear set I sold you but I moved to the dark side and will be running in excess of 400 whp of boosted fun now :D so that rear had to go. I'll be doing the Poly swap soon also.

And YES the GTUs has a revised and lower gear ratio 5th gear, all other gears are the same ratio and it is a N/A tranny, NOT turbo.

~Mike........

4.30's ROCK!!!

;) Best drug there is period.

Yo,


Oops! I meant 16mpg! 16mph was my trap speed in the 1/4. *wink*

Yep, I admit it...I have leadfootitis! I can't keep my right foot from depressing the gas pedal. If the car isn't at 8000rpm...it just isn't happy. *grin*

Kevin
1989 GTUs "Shift before the VDI crossover? Nah!"

how is the lsd in the GTUs rear after 13 years of wear and tear? does it still work or did you rebuild it somehow?

I don't believe you can rebuild it. It seems fine to me but I have never driven a car with a known working LSD before so I really have nothing to compare it to. It does seem to "want" to go around corners easier. With the open diff sometimes it felt as if the car was fighting me to steer around a tight corner but now it just goes smoothly around with less effort. I'm not sure if that is due to the bushings or maybe the car was out of alignment before, but there is definitely an improvement.

As far as my speedo goes; there must be a different 5th gear for the real GTUS. And my MPG is not very good, but who bought this car for the gas mileage?

16 mpg at $1.50 a gallon; where else can you get 15 minutes of adrenaline for a buck fifty...

If this thread made you want a 4.3 GTUS rear diff, good luck finding one. I searched for about 6 months.

They are rebuildable

Whoops! Rarest, the bushings did not make the ride to harsh. To tell you the truth I think it made the ride better. There is a real "solid" feel to the car now, like all of the nuts and bolts were just tightened up a little. The results may be different on a coupe, but I think it is an excellent performance bargain for the money. But it is one of the hardest jobs I've done so far, if not the hardest. If you don't have another car to drive around for probably a week, then you might want to reconsider.

The differential swap can be done over a weekend.
The bushings can be done in maybe 5 days.
The bushings and differential can be done in a 10 day period including two weekends.
^
This is how long things took me factoring in: breaks, work, sometimes you just feel like sittings on the couch, I have no shop press, your fingers start to hurt, etc.. But for the average joe with a good collection of tools, this is a good estimate.

It would also be alot easier if you have a friend help. I used two jacks as my friends, and they were not to nice.

The front bushing on the rear control arm is the toughest one to get out. After about 6 hours of burning and cutting. We figured out the easiest way. Get a torch, heat up the steel sleeve inside the rubber bushing. Once the sleeve gets hot enough it will melt the rubber touching it and slide right out. Then you can just cut the remaining bushing with a hacksaw.


I also added a competition front differential mount. I have no idea if this made any difference since I changed so much at once.

Um, why not just change the gear in the OEM rear diff instead of buying a GTU-S rear diff?

You just need to shift into 6th gear ;) Wonder if the miata trans will bolt up.

thats what i've been wondering this entire time. i know aftermarket ring and pinion sets, i wonder why people don't seem to get em.

congrats on the new rear BTW

the price of an aftermarket ring and pinion set and the needed bearings and spacers, along with the special tools required for the job make the swap using aftermarket parts a little expensive. GTUS rears are not the most common part laying around most junk yards, but they do exist. If you can find the right deal, using the GTUS rear save a good bit of money and effort. It's basically a bolt in deal. I was planning on buying a ring and pinion set if I couldn't find the GTUS diff, but I happened to come by one at just the right time. Either way, the swap is something all serious rotorites should consider.

neato.
i hope to put better gearing in my new TII rear

what about the RX-8 tranny? its a 6 speed, anyone know the gearing in the diff? my Road and track doesn't say........

Miata trannies will bolt up, but you'll blow the thing up as its barely strong enough for the 120rwhp that slug has stock. From what I've read on them the gears are pretty weak.

Guys,

As for the PU bushings. I can't express the difference they made in my car. I like the PU (competition) transmission, engine, and rear diff. mount. They take all the rubber band feeling out of the drivetrain and eliminate shaking of the tranny/engine. It made such a big difference in the feel of my '87 that I am going to put them on my '89 with only 43000 miles on it. Car feels much more direct and sporty. I did not notice much, if any increase in noise / vibration. I rec. doing the front control arms too.

Todd

how long did it take you to do the change over.

Yo,


I assume the bushing kits come with instructions? Or is it DIY?

Thanks!

Kevin
1989 GTUs "Energy?"

its not hard to finger it out, we did seths, you have to pull the whole rear suspension out, but hey i bet you wanted to wax it anyways

mike

Yo,


Mike, that assumes /it hasn't already been waxed/.

C'mon now, I went to an Medical supply store and bought a bunch of those long ass Q-tips. They worked fine!!

*snicker*

Kevin
1989 GTUs "Word!"