I've got a 250rwhp/240 tq miata trackday car. I understand that 250ish torque is about where the 5-speed box breaks.

Q: Which gear seems to break? ...and I'll try to be gentle.
Q: It would be ideal if I could swap the .814 5th gearset (from the 5-sp box) for the .843 gearset (from the 6 speed box). A long shot, but does anyone know if the parts might interchange?

 

Are you trying to switch from a 5 speed to 6? Or 6 to 5?

 

Everything about the 5 speed breaks.

He wants a 5 speed, and no the 5 speed gears do not fit a 6 speed case at all.

 

I have a 5 speed I would trade for your six speed

I'll even come do the swap for you!

 

All of them

Won't matter at your power levels

No chance, the boxes are from different manufacturers

 

track car + over 200tq = 5 speed is going to live a short, miserable life

 

Depends on the 5 speed. Sixshooter's has been holding up surprisingly well and he is not exactly gentle on the car. However, I am aware this is not the norm but it is also not unheard of either.

 

True, I even had mine in the '00 to 300whp but we all know it's just a matter of time.

 

OP, what is it that using a 5 speed will get you that using your current 6 speed and maybe changing your ring&pinion won't?

 

I was hoping to get more reliable shifting. ...by the sounds of it, it'll shift like butter right up to the point it blows.

Better gear ratio's would also be nice. W/6-sp at PIR (w/chicane), nearly every corner I'm either reving/lugging out of my sweet spot; shifting at the apex; a little too short and then a brief up-shift at the end of the straight.

Changing the rear gear might be an option. I could use a little taller rear gear. Presently, I use the top of 3rd only for the chicane. I would like a slightly taller 4th ratio for the back section of corners to keep from bumping the rev-limiter. I have a brief 5th -6th at the end of the back straight that is just a waste.

I'm going to tear down my current 6-sp to see if I can find a reason why it gets so difficult to shift after 20mins of track time.

 

I figured it was the shifting. The 5 speed is buttery smooth. Do you have a Miataroadster shifter for your 6 speed? I haven't driven one, but people here swear by it.

Also I can't recall any off the top of my head, but I'm sure there are multiple threads here about the 6 speed and which gear oil gives the best shifting performance on track, since it's the popular track gearbox.

 

I wouldn't tear it down until you've tried the MR shifter and the right oil.

 

If the MR shifter does not fix it you most likley Will find a bent shift fork, they are fragile and when they get bent or worn make shifting difficult.

 

Yes. I did try the amsoil and the miataroadster bushing. No shifters available until miataroadster makes a new batch mid summer.

I do think that I have a problem with my gearbox that is unique ...or there'd be more threads like mine. By the end of a race session (when it gets hot?), it won't shift smoothly... not even on the cool down lap. Hopefully it'll be apparent when I tear it down.

I'll try to get a spare 6-sp sourced out and then look for a 3.9 rear assy out of an anniversery ed.

 

Are you sure you don't want to trade me gear boxes? I live close by

 

By the sound of the forum advise it looks like I'd better hold onto this trans and try get it fixed!

Keep the boost down and shift smooth; hopefully, the 5-sp will get me thru the May double regional. I just have to be consistent to run with the front pack. Last event I took two positions, just to loose two back due to shifting errors. Doug Berger | Groups 1, 3, 6, 7 Main Race | 2015-4-ORSCCA-G1367R-14-2

I did find one msm 6sp in california. They wanted 1500 pluw shipping!

I'd better be nice to the local crowd in case I need help swapping gearboxes trackside! ... thx for your input!

 

You are way better off with a 6 speed. They are expensive too.

 

Is your exhaust completely shielded from your transmission from the engine to the tailhousing?

 

My 6 speed has recently started to get notchy, particularly when it's hot. It's started grinding 5th gear on quick shifts too. It's got higher mileage on it, around 160k and about 60k of that has been turbo'd. Let us know what you find when you pull it apart.

 

Sixshooter: I have just now done that, but haven't had an event to evaluate if any effect. My thought was to try running the 6-sp with shielding on the May 16 race; and if still a problem, then pull it and swap in the 5-sp for the May 17 race. Hoping to also install a temp sensor into the fill plug to get some accurate temp data.

I'm bummed that you are all so dubious about the 5-speed. I'll continue to work on locating a 6-speed locally. It might be problematic to find one before the next event. Work, life, etc. .... only so much time for this "fun" hobby.