Great question! For now I'm torn between the FMII clutch and the SM clutch. Both organic. The FM holds a good bit more torque so I'm leaning towards that, although research has shown me that they've had some issues over the years. I did ask them about that at FM and they did recently re-deisgn the clutch disk to address some small percentage of people who were unable to get enough adjustment to disengage the clutch.

Flywheel wise I have an older 1.8 FM flywheel lying around I was going to use for a 323GTX project, but it's probably gonna go in the car. The car is currently on the ACT XT Organic 1.6 clutch, which is rated for 300 ft lbs. I've slipped it a few times so I'd like something with a bit more tq capabilities than that.

I'd rather not go back in once I get the 6 speed in, hopefully that lasts quite a few years for me at least. I also am considering going with a GTX2860 down the road (over the winter perhaps?) instead of my oldskool gt2860 eventually so it needs to hold at least 300wtq. Which is just outside of the SM clutch limits.

 

The new hotness in clutches seems to be Uniclutch. I don't know if they have a miata six speed application yet, but it would be worth it to look them up.

 

Almost the same plan/situation I'm in. Hoping to go to GTX2860, likely over winter. I left the FM stage 1 in when I swapped to the 6 speed because I didn't want another job or to spend the money buying a new clutch when mine was still fine for the power level.

I'm leaning towards the FM stage 2 personally, but it's a ways down the road still.

I'll have to look into Uniclutch and some of the other options as well.

 

I'll toss in a quick vouch for the SM sport organic. The pedal stiffness difference compared to stock is laughable and it modulates very well. It feels just like OEM but maybe 10-15% stiffer. Mine has held up to 20k miles of 230hp and still looks new. It'll handle up to 290 Tq, allegedly. oh, and it's also orange.
Edit: I glossed over your 300 tq limitation, Carry on

 

Trust me, I'd chose SM over FM for this kinda part nearly every time. But in this case that 290ft lbs won't cut it, my XT was rated at around that and would slip from time to time. If only SM had a stage2 or something that would hold a bit more tq. My car made 280wtq on the dyno, and on cold nights when boost would creep to 20+psi it would slip on a hard shift.

 

I make about 300ft/lbs on my ACT, no slippage so far. I'm sure if I beat the **** out of it, it'd complain, but so far nothing. It'd a HD plate with the ceramic disc. I've driven the XT plate with ceramic and it's really tough to modulate on the street, but holds more. There's only so much torque you'll be able to hold with a fixed flywheel size and reasonable clutch pedal effort. I'd say the XT is the limit of clutch effort. Ceramic will be more resilient to constant slips and shifts of a track car, too many of those on a organic disc and it'll start to slip when hot.

 

I'm a fan of ACT pressure plates. They are very popular in the honda community over the years and hold a good amount of torque. As curly pointed out their stiff PP with a full face disc is the best of both worlds in terms of holding power and engagement. Personally a stiff clutch pedal doesn't bother me at all. I find the SM clutch I have quite easy/soft compared to the ACT's I've used over the years on hondas. I forget which PP I'm running on my Honda now, but pretty sure it's the stiffer ACT one (can't remember if it's the XT or HD) with an OEM disc and it holds the power no problem.

I know on some cars there's some concern about "crank walk" (the extra pressure exerted laterally on the crank over time causes it to wear the thrust washers). Not sure if that's an issue with Miatas. It was a common problem for 2G DSMs back in the day. I think a good precaution is disabling requiring depressing the clutch pedal on starting the car.

 

That's true for pretty much any modified clutch, the thought is you wanna have the engine running with oil pressure before pressing the clutch.

The HD is the "light" pressure plate, XT is the heavier pedal effort. I'd recommend the HD with ceramic disc. Great bite for a track car without a heavy pedal. To be clear, it's still much heavier than stock. The XT is just a LOT heavier than stock. ACTs have been great, I will say we've had a few quality control issues with them lately at the shop, simply not disengaging, and ACT has warrantied them. I wouldn't worry too much about it in a Miata, since they're so easy to R&R.

 

The Uniclutch gets rid of the heavy pedal, since it is a twin-disc.

 

I'm digging this convo, thanks for the input everyone!

Uniclutch looks interesting, but it seems like a very universial solution (hence the name) with adapter kits to various platforms. I looked into it a bit and found some seriously mixed reviews. It's interesting in that it bolts to the flywheel but doesn't use it for clamping force. Probably best coupled with as light of a flywheel as you can find. Do you know of anyone using it for a Miata? From what I gathered the price is very reasonable for a twin disk.

That said it's unlikely I go with a twin disk of any type. I was heavily leaning towards Organic but Curly proposed the idea of a softer PP (vs my XT setup) and a ceramic disk. That's interesting and I hadn't considered that. Curly, how does engagement compare to a organic? As track focused as the car has become, it's still a street car at the end of the day which is why I was leaning towards organic on the disk side of things.

I'm not looking for some insane torque capacity either, the 6 speed is only going to be happy until close to 300wtq and I'm pretty much there already. My idea of upgrading to a GTX series turbo isn't necessarily for more peak power, or even RPM-spool correlations, but more for transient response after a gear shift. That's where the current oldskool 2860 leaves the most to be desired. And maybe turn it up to ~350ish on the street from time to time. Track use I still wanna target that 260-275whp range.

My current XT setup is a bit stiff on the pedal. I don't hate it, but it's definitely the stiffest clutch I've ever had and one of the stiffer I've felt. I'd appreciate a bit softer of a clutch. But I also felt @redursidae 's SM organic and the pedal felt too light to me, maybe something in between those.

The FM stage 2 clutch seems to fit the bill, but I've heard it has a strange leverage curve and you can feel it during engagement, so that does make me hesitate there. And their previous issues, which I've seen plenty of, though those are supposedly resolved with the new disk.

 

 

I also appreciate the conversation, as it's pretty much the same path I'm planning on heading down soon.

I don't have much to contribute, but I drove a buddies car that was making around 275 and he had the FM Stage 2. It was a couple years ago, but I remember it being pretty easy to get used to. I don't remember anything really weird about it, which to me says it was pretty easy to drive.

He originally put a SuperMiata clutch in there, had my buddy break it in when he was out of town, and removed it pretty shortly after due to not liking how it felt on the street.

 

All this dialogue reminds me about running back and forth between the SM and FM options when I first bought a clutch for my NA. I will say the SM clutch with the 4 puck ceramic/metallic disc was gnarly to drive on the street, and I'm not normally one to complain about drivability lol. I still occasionally stalled my car till the day I did the trans swap. No hate on SM at all, obviously that knife-edge engagement comes with the territory when you're talking 4-puck non-organic discs.

If the new FM stage 2 fixed the issues with their old clutches, it seems like it'd be a no brainer. I mean, you ride bikes right? Leg strength isn't an issue then

I'm sure it'd be more work than I realize, but if SM had a higher-rate PP paired with their organic disc, I think that would be a total winner. Just enough torque capacity to max out a 6 speed, but without the drivability hit of the 4-puck disc.

 

All this clutch talk because you finally found the torque? might be time for a thread title change

I haven't seen anybody use them in miata's, but Southbend clutches are fairly well thought of in the subaru world. and they rate all their clutches by ftlb. and have a range from 190-400 depending on "stage" and options. https://www.southbendclutch.com/clutches/12594/

Local here I have a clutch shop that focuses on semi's but will do one off custom jobs.
I have had them refriction disks for me before, including making up custom setups. One for my EG33 swapped XT6 was organic one side, ceramic other side, with marcel plates. held the extra torque of the EG33 with the same diameter and PressurePlate from an EJ18.
Maybe there's something local to you that would do similar.

now on to my technical blurb, feel free to ignore,
Clutch "Grip" is usually determined more by coefficient of friction and engagement surface area more than it is by pressure plate stiffness. this is why the ceramics and metalics are used in clutches designed for more torque.
BUT that high coefficient of friction usually ends in a really grabby juddery engagement.
There are some things clutch MFG's can do to help this.
Putting a feathering plate (marcel) between the frictions and the mounting disk is a common one. This plate is essentially a spring holding the frictions away from the mounting disk by a small amount. When the pressure plate starts to "pinch" the disk to the flywheel these springs start the engagement sooner, but compress and allow the pressure plate to move more between the initial contact, and the full engaged point.
Damping springs on the hub can help if designed well, but they are more used to damp the acceleration/deceleration of the flywheel and stop sudden clutch engagements from damaging the transmission than they are to help with smoothing engagement.
so for a best "all worlds" clutch that holds torque you would want a large diameter disk, with a high coefficient of friction, feathering plates, and a sprung hub.

 

There's an easy fix for thrust bearing concerns - just bypass the starter interlock and use your head when you touch the ignition key. You can do that by messing with the wiring to the clutch switch, or just clamping it closed all the time.

I made my own version of this piece FM sells with their uprated clutches out of some scrap metal. I haven't seen it in years because it has never had an issue.

Jeep even shipped their older manual transmission vehicles with an electrical pigtail to bypass the interlock switch. You disconnect the switch and plug in the pigtail, and then you can use your starter while in gear for off roading over obstacles. Since I don't always think, I've sometimes emulated this with my Miata.

 

Haha, not sure if you were being serious, but I'll respond anyway. Ya as curly and Ironhydroxide said, there's two main ways to increase grabbing power on a clutch. You either increase the force exerted by the PP on the clutch disc (which increases pedal effort), which pushes the clutch disc against the flywheel, or you increase the grab of the disc (like the different "bite" per say of brake pads). If you look at the clutch disc carefully, OEM discs are usually "full face" discs, as in the clutch material covers roughly the whole surface area (or most of it) of the disc. They also have springs between the outter portion that engages the PP/flywheel to the inner ring that rides on the input shaft of the tranny to dampen the engagement a bit. Both of these improve driveability/engagement/shock.

As you move away from a "full face" and sprung disc, engagement generally speaking gets "worse," as in it's harder to slip it and it's more of an on/off switch. When you switch to a "puck" disc, the surface area instead of covering the majority of the face of the disc is now concentrated to several spots, or pucks. You have 6 puck sprung discs down to say a 3 puck unsprung disc. The fewer the pucks, the more pressure (with the same PP) that's exerted by the individual pucks, which increases its holding power (with the tradeoff being engagement). So generally speaking a 4 puck will behave more like an on/off switch than say a 6 puck.

Of course on top of this you have material selection which affects the grabbing/holding ability but that's my very basic explanation of pressure plates and discs. When you get into the "twin disc" or multi disc realm, this is where physics comes in and someone else can explain. All I know is they are obviously more expensive, complicated, and more like an on/off switch. In order to have a high holding torque without a very stiff pedal effort they use magic (science). I'm no expert on this topic but hopefully it helps someone and makes sense.

 

I guess I'll chime in with my FM/SM experiences. My car came with an FM stage one happy meal. Two previous owners, ~22k miles and the clutch wouldn't fully disengage. The flywheel and pressure plate were badly overheated and if you didn't lock it up fully before boost was building it just slipped. The MSM clutch delay valve probably made the issue worse. I finally got it adjusted but almost had to trim or remove the rubber bumper for that extra bit of travel. The damage was already done so I replaced it with an SM PP and 4 puck disc while I was installing a built MSM engine into my MSM.

I have previous experience with ACT almost 25 years ago. I installed a Miata PP and organic disc into an 1995 Escort GT that I turbocharged. Pedal pressure was high, almost too high for anybody other than me to drive normally but I can't remember which PP I selected and the normal release bearing wasn't a good fit for the PP fingers. A better bearing might have reduced pedal pressure. After my stage one FM clutch I didn't want another, would have needed a stage two and wasn't sure how heavy the pedal would feel.

The SM PP is easily modulated, the only thing I hate about it is that my 4 puck disc squeals loudly engaging 1st and reverse, often into 2nd as well only a shorter and quieter chirp. The disc does not want to slip, the longer you ask it to slip, the more heat builds up and it starts chattering. I usually only experience chattering while backing into my sloped driveway. It's almost 3 car lengths up to the door and I often have to pause for the door to fully open. Engaging again to move into the garage results in chatter and noise (sprung hub working). With my ever improving MS3 tune, I can now often fully engage it at creeping speeds just as I'm cresting the curved concrete gutter/curb slowly to avoid scraping the exhaust tip. Even if I have to pause because the door opener range sucks, there is less heat from slipping which allows normal engagement to get into the garage.

It hasn't slipped with 19-20 psi spikes as I tuned closed loop EBC (GT2560R, new FM manifold and outlet pipe, 2.5" Magnaflow exhaust) but getting moving in traffic was difficult at first, much of that was my tune not being dialed in. The engine stumbling a bit as the clutch locks up hard and fast wasn't a good combo. Now that I have corect idle VE, a closer transition to Fuel VE, and more accurate afterstart, warmup and aceleration enrichment has almost eliminated the stumble/bog. Might still have a tiny bit within the first 30-60 seconds after a cold start but otherwise the SM PP and 4 puck is daily driveable. I can't remember the rated torque capacity but remember recently seeing Ed or Emilio stating they have heard of a few running it past the rating without slipping.

I also agree it would be great if SM had a stage two PP with slightly higher clamping force and pedal pressure. I haven't driven a stock Miata for years but the SM PP feels like I'd expect from the factory, within the Goldilocks zone.

 

I have only heard good things. There are several NC guys on m.net with great feedback on the Uniclutch. I haven't seen anyone with feedback for an NA/NB yet, though.

All this talk about being able to hold power while still being easy to drive on the street sound like exactly why the Uniclutch was invented. It will hold any power you plan to make rather easily, yet have an easy pedal.

 

I’m skimming some of these responses, so sorry if this has been covered, but to answer your question, the engagement on the street in my ceramic/HD pp is easy. Takes a minute to get used to, but otherwise easily streetable. I wouldn’t want an organic disc on the track, it’s just not built for the constant shifting.

it’s when you put the ceramic disc with your XT pp that the finite pedal stroke required while also exerting a lotta force just becomes too difficult to modulate. I liken it to when I tried a ceramic disc on mobius’ twin disc, I think installed it ~30 times on the way to work. It’s much better than that, but you get the idea.

they make a 4 and 6 puck, they’re rated for the same torque and feel the same. One just has larger 4 pucks, the other smaller 6 pucks. Dunno if it’s a “old” and “new” design, but thought I’d mention.

 

Just a note on our Supermiata clutch - the torque figures published on the website are crank tq, not wtq. The puck disc would hold your power, but yeah, it's a puck disc. I had one in Sonic and it was fine on the street but occasionally did make noise on engagement, wasn't something I wanted to be in stop and go traffic in for sure.

As much as I'd love to do a higher-clamp-load clutch, it's development time/effort we can't spare right now unfortunately. Too many irons in the fire.