last year at a lemons race we got some pointers from a SPEC racer. He basically told us "Don't use 5th gear, it a weak overdrive gear and you will break it if you are shifting hard into the gear". This kinda sucks because in a few spots on the track we are starting to bounce the rev limiter in 4th.

Currently we have a NA6 car with a 1.8 all junk yard hp mods and KIA 4.778 gears. The final drive is pretty awesome because of how quick the car feels, but now we run out of trans. Were working to source some 4.44 gears (not having much luck, do RX8 gears fit NA/NB 7" rears?) to bring things down a bit (would love the NZ 4.625 KIA gears, but I digress.

So my questions are:
What makes 5th gear is fragile?
- Synchro?
- The Gear itself?
- Brass Bushing?

Is there some race proven method to fixing the fragility?
- I have read something about replacing the brass bushing with a needle bearing (professionally done of course) but IIRC this was on RX7 transmissions so I don't know if that can be applied to our transmissions.

While I'm there I would like to put in a closer ratio 5th gear from a 626, does this cause any additional reliability issues?

Im open to swapping to a different bolt up transmission like an RX7 but I'm not sure this will solve anything and may make my hearing issues worse.
As Always, I appreciate everyone's insight.

 

Never heard of 5th being a weak point.

In the 5 speed trans the weak points are the gear teeth, usually in 2nd and 3rd gears. This is with moderate power and closer to stock ring rear numbers.

 

Neither have I heard of 5th being a big issue. When I have been with teams running a miata in Lemons/Chump/WRL I don't think we have ever worried about 5th going out.

Way back when I hung out with Spec Miata drivers I don't overly remember 5th being an issue for them either. Yes they went through transmissions, but usually 2nd, and 3rd gear.

 

Huh, strange, guy who told me isn't just a racer also a builder. Maybe he was talking about the early 5 speeds that came with the 1.6 engines? But that makes me feel better about using 5th.

does anyone have any experience with swapping a 626 5th gear into our transmissions? It's .858 vs our .814

 

We use a '94 5 speed and 4.77 in our Chump Car and never had a problem with 5th gear. The 5th syncro can have issues if you shift quickly from 2nd to 5th, so maybe that was were the miscommunication came from?

 

I talked to the guy that gave me the original info, there was a miscommunication. The down side in talking to him is now I'm pretty sure 4th is on its way out....

 

We have a 4.3 and can actually achieve decent top speeds. You should try that out instead?

 

We have tried 4.30. we like the 4.778 gears better. more wheel tq

 

Some spec guys breaking 5th gear think its from hard bump drafting, reverse loading the gears and what not.

 

We found the same thing on the 4.30 doing some rough simulations on tracks we run at and the 4.30 had many more places were we would be between gears or adding extra shifts. This is compared to the 4.77 and the 4.10. We use the 4.77 and it works out perfect for most tracks. Our V-max is 120-125 and we only hit that at Sebring on the back stretch.

 

We're at 148hp, how much are you guys running with this setup?

 

The fix is to install a 250+ rwhp turbo. That'll shatter 3rd gear, you'll install a 6-speed to replace it, and then 5th gear will never fail again (it's the 1:1 gear).

--Ian

 

4.77s are faster unless you are running out of gear in 5th. 4.30 cars use 2-3-4. 4.77 cars are able to use 3-4-5 which are closer together. It's the same reason why 6-speeds are faster than 5-speeds (closer ratios = less RPM drop = higher average RPM = higher average power).

 

Last time we dynoed the car with a fresh motor ('94 BP) it was around 120-125 whp. Plans are to bump the power up this year to take advantage of the new Chump Car motor swap rules (Either a BP4W or VVT). At that time, we'll go back to the 4.1 to "save" some points.