Turbo Miata reliability question

At about what point torque-wise can the 5-speed be expected to have its grenade pin pulled?

This question is not in relation to turbo Miatas, just the transmission. There will be no Miata involved.

I've seen bad things happen at around 400 whp, but the trans did have ~80,000 miles on it by then. 250 whp doesn't seem to bother it a bit.

Sorry I only have 2 data points, but I'd feel pretty comfortable up until 300 or so. YMMV.

Keith will have a good answer...

My 1991 turbo Miata has 165k on it and I've made over 40 drag passes. Last 5 I had slicks. Its also had a turbo for about 4 year now. So far no issues with the trans.

We have a 250 whp Miata that's often autocrossed on R-comps, and we've already replace the transmission once after second gear fell apart.

Even with 275/35R15 Hoosiers, my trans seems to be pretty happy. I've got 140,000 miles on it so far, not quite 10,000 of that is under boost.

YMMV indeed!

Blown up one too many 1g RX-7 transmissions?

Yes, with only 160-odd ft-lb. Second-gen units last a little longer but they still give up the ghost after a while. I haven't actually exploded one yet, because I change them out when the bearings start to make scary noises or the shifter starts shuffling back and forth with varying load. Happy bearings make for happy gears, unhappy bearings make for walking home.

I'm looking to make that more like 200-210ft-lb, and I had a suspicion that the Miata trans was stronger because of its more moderate transfer gear ratio. Looks like I may be onto something

You probably spin a 1000rpm or so more with a rotary and you are dealing with a minimum 5 year older trans. Just something to consider.

On the flip side of that, Mazda does seem to constantly improve parts even if they are seemingly similar.

I rallycross and most of the time I'm churning 3000-6000rpm. I don't rev all that high except at the dragstrip, and drag duty is rather light on transmissions in my experience. No shock loading there, no load reversals. No turning Redline Shockproof Heavy (the red stuff) into purplish-brown sludge with chunks in it after one weekend.

(shifty looks) I don't know how much I can... divulge... but I'm considering an engine swap, and it's not a rotary or DOHC engine, but it will be using a Mazda trans for simplicity. So in either case, WHP means nothing to me, just torque figures...

What's the car weigh? That's a factor.

At above 250 rwhp, the transmission is at risk with serious abuse, but most people don't have a problem until you're closer to 300. Drag racing - which does throw a fair shock load through the trans at launch - seems to be a contributor, especially with sticky tires. The failure mode is ripping the teeth off the gears without any advance warning.

The 6-speeds seem to be stronger than the 5-speeds, and are a direct replacement. I don't think the gearing is as well chosen, but it's a much closer ratio box.

How much torque does a 250whp Miata engine make?

I just got done bridge porting my next (last) 13B, so I'm still just thinkin'.

About the same - 250 ft-lb or so.

Keith wrote: At above 250 rwhp, the transmission is at risk with serious abuse, but most people don't have a problem until you're closer to 300. Drag racing - which does throw a fair shock load through the trans at launch - seems to be a contributor, especially with sticky tires. The failure mode is ripping the teeth off the gears without any advance warning. The 6-speeds seem to be stronger than the 5-speeds, and are a direct replacement. I don't think the gearing is as well chosen, but it's a much closer ratio box.

I thought the Miata you guys brought to MIS out here in Michigan for the super tuner challenge or whatnot was originally a mazdaspeed that had a 5 speed swapped in because it was stronger? I could be remembering wrong though.

Joey

It had a 5-speed swapped into it because of the gearing. We needed to hit 150 mph with that car, and the combination of tires, rear end and transmission was just perfect. Plus they shift better than the 6, so you're less likely to hit the wrong gate.

That was a pretty short event, so the chances of the transmission grenading were low.

Keith wrote: It had a 5-speed swapped into it because of the gearing. We needed to hit 150 mph with that car, and the combination of tires, rear end and transmission was just perfect. Plus they shift better than the 6, so you're less likely to hit the wrong gate. That was a pretty short event, so the chances of the transmission grenading were low.

Ah. I guess I had my story mixed up!

Joey

MadScientistMatt wrote: We have a 250 whp Miata that's often autocrossed on R-comps, and we've already replace the transmission once after second gear fell apart.

I did the same thing with my friend's turbo miata. Second gear gave up while out on course

I raced Mazda's for over 20 years. A well-built(ie ISC or similar shop) will last for many years of 8000 rpm shifts and abusive downshifts. Usually the synchros fail and parts begin to move around the trans. YMMV