5spd gearbox and 272bhp at the wheels
 

Need some advice here. I live in a country where na/nb miata parts are rare. I bent a shifter fork yesterday and could not find a replacement in time for a timeattack event this sunday.
However, I do have a 5spd gearbox as spare but I have read that 5spd is a weaker box compared to the 6spd box.

If I go ahead and run with the 5spd box on 272bhp, am I definitely going to break the gearbox? Has anyone done similar setup for events?

I will be running it with fm2 clutch and dunlop d03g. There is not much allowance for drivetrain slippage.

If you drive it like you stole it there is a good chance that it will explode. There have been a few people on this site that have magical 5 speeds that take the abuse but more often then not you would have to baby it at that power level in order to not break it.

Im with Ryan G... the 5 speed isnt known to handle shitloads of power. BUT, its a luck of the draw thing. if you have the time and resources, and a way to get the car home from the track IF it DOES explode, then it may be worth it. Like Ryan said, some people have the magical 5 speeds... youll never know if you have one til you try. lol
If you do, just try to take it easy (easier said than done considering you are RACING) lol

Remember, its not horse power that breaks transmissions its torque. From what I've gathered 5 speeds start to live on borrowed time above 220 ftlbs. Though if miata parts are that hard to come by, I would still consider losing 1/2 a tenth every shift by being gentle.

Shift gently and it should at least last the weekend.

Thanks all. I will give it a shot.

Torque and fatigue under load I believe is what kills 5 speeds I believe. Much past 250 HP and the 5 speed will have a finite life. It could take a weekend or two at the track or it might go a full season or more.

Over 300 ft-lbs you get 1 or two events out of a 5 speed if youre lucky and you will strip second, strip third, or strip the secondary shaft drive gear by braking teeth off the gears after the fatigue at the root. It is not really hard shifting abuse that causes the issue it is cyclic loading caused by transmiting peak torque. Time spent at torque level to failure.

Bob

I went through two 5 speeds the first year or so of tracking my car. The car was putting down 251whp and 239wtq. I was aware of the weak transmission issue so my shifts were never hard. Both times I was already in gear and hit a small dip in the track, and that split second loss of traction caused me to lose 3rd gear. Running a 6 speed now for over a year, still alive.

Quick video of how it sounds when you rip out 3rd :vash:

In my case it wasn't even a momentary loss of traction -- I just put my foot down coming out of turn 11 at Laguna and wham.


--Ian

I've broken seven of the five speeds. Every one failed while fully in gear and accelerating through the peak torque region of the power curve. Second gear, third gear, and the secondary shaft drive gear on different occasions.

I've broke two 6 speeds. First one was some other failure where a shaft developed a whole lot of for/aft play and eventually just started whining and binding didn't really strip gears. I am not really sure what this failure was I haven’t taken it apart. Second one was stripping forth gear on the front straight at Laguna.

I now have a 6 speed out of a low milage Mazdaspeed in the car. Suprized how much better the shifting feels than other 6 speeds I’ve driven. Hopefully the shot peened or whatever gears will add longevity.

I've been running my 5 speed all last season. I guess its either, I have one of the magical 5 speeds, my driving style has helped or I'm just lucky up to now. Whatever the case, my 5 speed is on borrowed time. I plan to install my 6 speed soon.

I broke 4th in my 5 speed, I was full throttle entering the freeway.

4th is straight through 1:1 on the primary shaft. I didn’t think it went through any gears. When I have broken the gear that drives the secondary shaft 4th is the only one that still engages. Break second or third and you only loose that gear as long as not too many pieces of it end up stuck in the other gears. driven 100 miles home after braking second.

I didn't pull the thing down but I drained the oil and it had a tooth in it. 2nd and 4th were very noisy but still worked, I was able to drive home using 3rd and 5th which sounded fine. Then again this is going back a few years, after reading what you have said it sounds like I might have that backwards!

Here's mine - losing 3rd at Laguna Seca a few weeks ago. Gear fully engaged, foot already down, hitting the torque peak and it just stripped all the teeth. This was at only probably 170 ft-lbs or so since I had backed the boost down to conserve a slipping clutch.

Drove the car 430 miles home, it was just rather noisy ;)



Trans is out of the car now, getting a new 1.8 trans and new clutch and flywheel while I'm in there. When I drained the trans here's what I got:

https://www.miataturbo.net/attachmen...ine=1368741923

I'm guessing there isn't really anything to be done to build a stronger 5 spd?

The term is cost effectiveness

You can probably run an insanely high fd like 4.80 or something. That will probably reduce stress on each gear.

Of course there are fancy options like quaife and others available, all of which are considerably more expensive than a used 6sp.

Comes down to $$ vs end result. The 6sp swap usually wins that comparison

2 grand + install gets you a qualif gearset. But the gear ratios are kind of silly.

The gear ratios are in the quafe gearset are awsome in my opinion. The 6 speed has a gear for getting around the pits really loud a gear for getting around the pits quiet and 4 usable gears for the track. The quaife with the wide ratio also has 4 very usable gears for the track and a second gear tall enough to be sutable for autocross in a car with enough power to warrent a quafe gearset. even with a 3.63 rear end the second gear with the 6 speed is too short I dont want to gear it to use 3rd in an autocross because I would be rev limited for top speed at about every track I go too. The gear ratios in the 6 speed are what is silly. the 5 speed ratios are better and the quaif is better still.

You go to some fast auto-xes or my gear ratio calculator is wrong wide ratio + 4.10 is like 80 something on man meat tires in 2nd, the 6 speed with a 3.63 is like 70. Err wait, I have a higher redline.