ed1380

Currently building my engine for boost and while doing research on what the drivetrain can handle, I came across a splined bushing that's installed between reverse and 5th

From what I've read the stock bushing is smooth on the inside and the splines can twist and then you "can't rebuild the trans". The fix is a splined bushing. Ok that makes sense.

What doesn't make sense is how the heck are spec miata guys twisting the shaft in 5th and no one in here with boost and almost twice the power has had that issue.

ed1380

130 views and noone knows? I'd ask my mechanic but he only works on stuff with more than 150hp

curly

iTrader: (25)

We don’t really work on 5-speeds. Do anything you’d like to them, but no one’s rebuilding them, only swapping for 6-speeds.

ed1380

fair enough

bbundy

If you are talking NA/NB 5 speed with boost there are so many failure modes with them with much added torque that fixing that fifth gear spline thing is a total waste of time. I have broke well over a dozen 5 speeds and never had an issue with the shaft twist in 5th. even the picture you show has all the teeth sheared off the input gear has nothing to do with the shaft twist in that section you highlighted. If anything shaft twist would help cushion tooth impact loads.

jedd

I have raced spec miata for over twenty years and a 5 speed transmission lasts about a season for me before it needs a rebuild. When I have grenaded them its always 3-4 shift in a turn and the chassis is under some twist. I have heard the trans is actually flexing/twisting also but I cant confirm this. A high hp turbo car shifting in straight line on street tires stresses the trans in a different way i guess.

Gee Emm

Quoted for emphasis.

ed1380

I agree. From what I've seen it's usually the countershaft gears that go (like in the picture). And I think I have a way to help with the case flexing in that area. We'll see if it works. I mean if splines twisting was an issue then it would be happening with 200-300hp turbo builds. but...

On the other hand this has to be a real issue that actually happens otherwise mazda wouldn't be adding an extra part to their catalog in 2017 for a 10-20 year old car. https://mazdaracers.com/topic/6038-s...-shaft-spacer/

Hence my confusion and this post to pick the brains of people smarter than me

ed1380

if the chassis is flexing then the subframes are out of alignment and the trans is getting twisted via the ppf. it makes sense

bbundy

In 4th gear there is no drive force going any gears and there is no torque going through the secondary shaft. So Im confused what could grenade when you go to 4th? I have sheared off all the teeth on second, third, and when it fails in 5th its the input gear that fails not actually the fifth gear.

Also the 5 speed box has 3 bearings on the main shaft with the bearing plate in the middle. The shafts are pretty well supported from flex. the 6 speed dosn't have that center plate. I suspect case flex is a much bigger issue with the 6 speed than the 5 speed. The 6 speed gets it better strength from having much larger face width on the gears and larger diameter shafts. From what I remember the 6 speed has 12 lbs more steel in the rotating gears and shafts. I think the weakness in the 5 speed is simply small gears and shafts. It's light weight.

rjacobs

really?

Walther motorsports has made a big business of rebuilding 5spds...

Mazda Motorsports recently released a gear set for the 5spds...

Lots of places rebuilding 5spds...

Now 6 spds, I havent found a place yet that will rebuild them as I believe parts availability is sketchy at best and not economical at worst...

curly

iTrader: (25)

He was asking a forum, not Walther Motorsports.

Icedawg

That is a very good question. I have had that twisting happen in one of my Spec Miata 5 speeds. I managed to pound it apart, have some machine work done on it, and rebuilt it. (I do rebuild the 5 speeds for SM and first gen RX7 racing). It lasted the rest of the season, and then stripped the teeth off 3rd the next season.

I think the issue is heat; the SM trans, constantly run at max engine rpm in a race, simply gets too hot and can't dissipate the heat often enough during a lap. I noticed the reverse gear and 5/R selector hub on the main shaft was heavily "blued" on the one that twisted.

The SM guys try to avoid lifting at all, and have enough grip in the corners relative to actual speed, they can sometimes almost do that.
Racing a turbo Miata it will run up as high in rpm, but be shifted between gears a lot more often, probably run through a larger rpm range as a result, and cycled down in the corners from the very high straightaway speed, allowing a few more moments of cooling. A better duty cycle, in other words.

ed1380

Thanks. Heat was the answer I was secretly looking for. Im building a trans cooler and wanted to hear it from someone else to justify it 😅 Trying to decide if I want to install nozzles to spray down 3rd and the transfer gear. The lower gears are bathed in oil while the upper ones just sit and cook.

I saw a post of a turbo miata racecar killing 3rd gear in 2 trans in 2 hours. And they were Walter built.

Icedawg

Emilio's team has a post on here from a few years back on a transmission and a diff cooler for an endurance car. Very spiffy.
I don't think you need the sprayer you are thinking of, as long as the oil is cooled it will spray and transfer from the counter shaft and adhere like it is supposed to.

codrus

Sure, but nobody with a turbo bothers using a rebuilt 5-speed because adding 20% strength to a transmission that's 50% too weak for the torque of a turbo motor is expensive and pointless.

--Ian

ed1380

I remember playing with the lucas display at autozone years ago. It didn't look like much oil transferred between the gears. Sure it's enough for lubrication but how much heat does it carry away

sixshooter

iTrader: (12)

Oil manufacturers try to keep their engine oils from "roping" like that Lucas display does because it results in power losses and causes frothing at high speeds. But it's clever marketing to put a fidget toy on every parts counter in the country and probably increased their sales significantly.

It would be fun to watch the toy demonstrations at 4,000 RPM. I'm sure the regular oil coats every surface well at that speed.

rjacobs

The rebuilt 5spds with the new mazda motorsports gears are rumored to hold more power than the 6spds...

But thats a 2000 dollar gear set on top of somebody doing a rebuild... so price wise its hard to justify when 6spds are 1000 bucks or less all day long.

ed1380

I've heard the same frothing thing and stopped using lucas. I think it was on the bob oil guy forums

Im sure it coats it enough to lubricate at the meshing point, but I don't think its enough to carry away heat. I found a paper that tested it. Dip lubrication had lower temps than spray (or splash in our case)

it also had worse efficiency but nothing we can do about that short of going dry sump 🤣

I think the best way to tell would be to look at the broken gearboxes. Is it the lower (bathed) gears that break. Or the upper (splashed) gears that break