I'm sure the title brought you here. but put simply, that's what this is.
The Backstory:
a Friend of mine, late 2022 messaged me with a crazy idea. Run a MSM in STU class. Could we do it?
I looked through it, and though it'd be an underdog, the challenge intrigued me.
Somehow he came up with a 12k mile 100% stock MSM and dropped it off for me to start.

Goals were to make as much power as possible with the restrictions (stock from throttle body to head outlet, stock turbo in the stock location)
And to add lightness.

I started out by pulling the OE intake and intercooler tubing (all steel, blech) and fabbing up a setup starting at 1.5" piping, growing to a 2" intercooler, and finishing off with a 2.5" to throttle body, in the hope that the smoothest path will be the best mix of velocity and volume.

Then fabbed up a turbo inlet to replace the stock rubber boot, including a return path for the BPV that should keep the turbo spun up the best possible while off throttle, and hopefully not blow back through the MAF (my opinion why the factory MSM setup is terrible on boost recovery)

I sourced a TurboSmart BPV similar to the FM Little Enchillada, but with the same Inlet/outlet size. Then plumbed that to the turbo inlet.

For ECU, class rules state(d) that if the ecu could not be tuned, it could have a piggyback and/or "electronic components" inbetween OEM sensors and the ECU, but ONLY using factory connections (ie, a patch harness). that the piggyback must be "supplemental" to the OEM ecu. and that one must be able to unplug the piggyback and have the car run on the OEM ECU.
So, we chose a MAXXEcu Race to wire inbetween the OEM Ecu and the car. I bought a patch harness and got to work wiring it up.

Boost control was Open, and I wanted some sort of failsafe from running lean (OEM has no wideband so couldn't race with a wideband connected to the maxxecu), so we picked up an innovate SCG-1 wideband/boostcontrol/gauge and got that wired and plumbed.

Before this, said friend had an STU winning STI, and had leftover parts from that, namely a perrin AOS. Knowing he was going to be replacing the OEM strut tower with a super fancy unit he found, I built a mount and plumbed the perrin unit in the factory separator place. (If you didn't know, MSM has a "built in" AOS in the strut tower)

Then suspension/chassis parts started showing up.
Remote Reservoir Penskes,
Supermiata 863 Bronze bushing kit
SmallFourtuneRacing Front Swaybar
OS Giken Diff
QRT Carbon seats harnesses and mounts
Coolerworkx shifter

after all that was in, it was time for some tuning.
To start out I tuned it with the built in MAP sensor and Wideband (cause I definitely don't want to be buying internals for this thing)
and got fueling mostly dialed in. then swapped it to the factory MAP/MAF hybrid (MAXX lets you blend tables easily, which is really nice) and got that dialed in.

Ended up on an inertia dyno at 220hp and 235tq. and a but puckering 97% injector duty cycle.

All that done, I passed the car back to him for suspension tweaking and refinement.

He kept at it, tweaking a few things with suspension. Aftermarket upper rear arms for better camber gain, offset front upper bushings for the same, differing spring rates and tophats. etc. As well as a rollbar when he realized that this car could kill him if things go way wrong.

Shortly after "finishing" the car and getting it back to him, my dad passed away and life got busy, not able to keep up with him and the car much.

A little earlier this year said friend messaged me offering me the car at a price I just couldn't say no to.


How I gave it to him in 2023

How it is now

Here Is a link to my Album where some extra pics of the build (and will continue to be the album of this car). More pics will be there than what I post here. Feel free to ask about whatever you see.

 

So now it's mine.... and STU is going away (into AST from what I hear, meaning I'd be fighting with STR prepped ND miatas... definitely overclassed)

But driving it... I feel this power is very nice for the tires and suspension setup (maybe a bit earlier spool...) but I don't like the lack of safeties required by class rules.
So... XB is where it'll be run, and I'll be able to do whatever I want with it (mainly adding safties, probably fueling changes making it safe at this power).

Long term hopes:
Make a header that keeps the OE turbo in OE location, hopefully improving spool times.
Make the OEM ECU happy with the piggyback setup.
Swap Engine with built BP4W I have (10:1 compression, VVT head, VICS Manifold)

Very Long term:
If built engine blows and I want to keep it.... Rotary (IDK if single rotor SC or 2 rotor)
If built engine blows and I'm done with it.... LS3 swap (or probably LT swap at that time) and clean it up for someone else to play with



First things on the agenda are: Rewire the piggyback and regear the rearend to 3.636 (AutoX always being between gears just sucks)

It now has 14,200 miles, and instead of the QRTs has some kirkeys.

I will try and keep this updated with what I'm doing on the car, and how it goes.

 

Welcome to XB! There's a chance I'll make it down for Octoberfast next year and see you there.

I'm also hoping to find a 3.6 here at some point. Seems like the best setup before going into trans/diff swaps. Sounds like injectors would be a worthwhile upgrade.

 

Welcome, and cool idea for a build!

Is there something in that class that requires the MSM turbo be kept? I'd highly vote for ditching that old snail for something else, seems like a lot of effort to build a new manifold around the old IHI when something like a Pulsar GTX clone is available on the cheap.

 

Nope, XB let's pretty much anything goes.
It's more a personal choice at the moment. I like the idea of the challenge. and there's one option for a larger setup that might be fun to play with.

I'm sure eventually I'll drop it for something else (probably something with a belt if I do).

At this point I don't want to drop too much into "go for broke Power" I want it to continue to be a streetable car (even though it'll likely be trailered to events)

 

Fair enough! Just keep in mind that turbo is notoriously a turd, slow spool, no power, lots of heat and cracking, etc. Curious if that improves with a different manifold though, I'm all in for experimentation as long as your expectations are reasonable.

As a wannabe rotary guy, I also dig the 13b idea way down the road. Hope you stick with this car and we get some updates!

 

Oh totally, it's not a good turbo or system I agree... though I am curious as to what it COULD make with ideal setups (and for those who HAVE to have a setup that still works OEM controlled)
BNR makes a TD05 retrofit (taking oem hotside housing and machining for TD05 chra and replacing the coldside completely) which would be an "OEM" type upgrade for the setup that should give a bit more power and still MIGHT sneak under the radar of fairly picky emissions places. (though running out of injector is really the next problem, agreed)

I personally like supercharger theory for autox better than turbo theory. Having predictable and repeatable pressure in the manifold is best for predictable driving at the limit.

 

I've ordered replacement cmc connectors/pins for my maxxecu, so waiting on that I've been going through the diagrams and it drove me crazy. I had to get some wrench time in.

So, while it's down I decided to swap plugs. and install under plug TC's to measure individual CHT's at each jug.

braided covers barely fit with the plug wires

Routed them through the firewall near the fuel cable, and then into the passenger footwell where they'll connect to the maxx behind the glovebox


Mini K connectors BARELY fit through this hole


I also plan on drilling and probing the OEM Log Manifold and stabbing it with individual cylinder temps as close to the head I can (due to the log design likely to mix signals)

Because the maxx has builtin 8 channel tc inputs I decided that part of my plan for this car, is to test and prove ideas and theories that I've never seen data for. and make that data available to whomever wants it.
One of those things I've never seen data for is the Coolant Reroute. My plan for data collection there is to get this back running with mostly OEM stuff (shy of the intake/intercooler setup) and then log what CHT and individual EGT's are before reroute.
Then install a reroute, don't change anything else, and log. (friend has a hub dyno, so I can load this up consistently and for a LONG time to get reliable data)

Patiently (or not so) waiting on the pins, so I can get deeper into the rewire on this.

 

Wow, that would be awesome data to have! I'm not sure how many people have ever collected thorough data on the subject. Always keen on more numbers/charts though.

 

Still waiting on connectors and pins, so decided I might as well start in on the diff (as I didn't feel like deciding whether to pull the manifold for egt probe drilling or not)


when removing the fill plug for draining I noticed that my breather must be plugged... it's been in the garage for a while and is completely temperature equalized, but there was definitely a pressure differential. I'll solve that when reinstalling, probably put an extension hose on and mount a small breather filter to the top of the subframe.

A buddy of mine was way into RX's years ago, and still had some parts sitting around.
Parts like a FC TurboII rear diff.
Which happens to have a compatible housing with the Miata, and is thicker, with no notch for intentional weakening.
So I picked that up and cleaned it up a bit.



Notice that the arms don't curve backwards on the FC as they do on the Miata housing


also that the "beam" is closer to the pumpkin flange in the FC as compared to Miata


The pockets in the arms are much shallower in the FC.




So, hopefully this will prevent me breaking a housing prematurely. and I'll sill have the Miata housing as spare.

 

Today's Update, Diff disassembly.
Got the diff apart on the trans bench, and decided I might as well blow the OS Giken apart to clean, and see how it's tuned.


Only a few parts shoved in a small area

Seems whomever tuned this before removed (or didnt' install) any of the crossplate preload springs/bolts/nuts, so it's as aggressive as lock as possible.
With the mid spring setting on breakaway torque.
This explains quite a bit why it's pretty bangy while driving on the street and using some light throttle on long sweepers (onramps) and more than light throttle it grips up and goes slightly oversteery.
Not sure what I'll do about it.

Here you can see that it's a 1.5 way diff. But also the lack of crossplate preload springs/bolts.




Not sure what I'll do about the crossplate Preload... might just reinstall without and pretend I didn't see it (the car drives pretty damned good, so.... dont' want to screw that up)

Also, I noticed this weight on the pinion flange. Seems it is a TMD (Tuned Mass Damper), and just there for NVH. But I don't know for certain. Can I just press this off and be fine? or am I just confused as to what this is for?



Also, while switching to the 3.636:1 FD, I totally spaced the speed sensor change, and only now thought about what to do about it.
it's hard for me to spend the ~$200 (amayama) just to get the right gear on the end of a speed sensor, when I have a speed sensor that works.
So I took out the Speed sensor I have, removed the gear, and measured it out.
I've succesfully 3d printed helical gears in the past (power feed for my Z axis on the mill) and so I am confident I might be able to get a design working to just swap the gear for a few $ in filament, instead of having to buy a whole new speed sensor.
We'll see how that goes.

 

Well I'm an idiot. Apparently the FC Turbo diffs are 8" ring gear, making the housing larger.
The only thing "compatible" is the clutched limited slip differential, not the housing.

NON turbo rear housings are compatible, but it seems friend got rid of all his years ago.

SO, i'm back to just running the OE housing. unless I can find a cheap non turbo FC housing.
Cleaned it up and inspected the breather. Turns out the breather is functioning correctly, when pulling the fill plug I couldn't tell if it was negative or positive pressure. It had to be negative because the breather path is nice and clear.
I discovered the breather has a check valve in it, spring loaded.

Also got the OSGiken back together and installed the 3.636 ring gear on it. Just waiting on bearings/seals for the pinion now.

As for the speed sensor. I pulled the OE sensor, removed the gear, and drew something up with 10deg helix angle and 19T



Printed it out in PETG completely solid, mounted it on the sensor and tried it out by running the car in 6th at 4krpm for a couple minutes on the stands.


I think I may have gotten the OD of the gear a bit wrong, but with how quick and easy this was.... I'm tempted to just run it and check after the first few events (or a few hundred miles) Worst case I tweak the design and take an hour to reprint (and try and fish broken gear parts out of the housing.... LOL)

 

Love seeing more 3d printing content. I'm sure you're already aware, but there's a Miata 3d Printing page on facebook. Someone might have already drawn this up. Very cool though, hopefully it holds up.

 

I've been waiting on parts a lot more than I expected to be,
But Todays update:
Got the diff reassembled, and installed the 3.636 gears.


Coast Side




Drive Side

I've also decided to reinforce the OE housing with 1/4" plates instead of continuing to try and find a Non Turbo FC housing.

and in more "boring" sense, I've done a fair amount of wiring work, but you'd not be able to tell with pictures. All wiring looks like spaghetti until the 90% mark.

 

Good stuff in here, can't wait to see what you do next. OSG and 3.63 is end goal stuff!

 

Same as Option for me, I'm going to sit here for a few minutes and drool over that diff.

 

I'd say you should start looking for the R&P sets soon then.
when I was looking most places had them NLA and those that didn't were going up in price quick.
I was lucky enough to find this set local from a guy in the local S2k Group.

 

Today I got off work early for the holiday, so I decided to make some use of the time and get the EGT probes in.

Luckily I have a spare manifold that I picked up during previous owners reign for 3d scanning (if you want the scan, just ping me)
I used that to decide where I could drill for the probes. I picked as close to the center of the ports as possible, and tried to find a place where I could get the holes without hitting the head, or interfering with the hex on the compression fittings.



Because I want the least amount of "crosstalk" and this is still a log manifold I went as close to (even angled into) the ports as I thought feasible.
Smoked a few drill bits, but that's to be expected when it's sand cast and drilling a hole on an angle.



Tapped the holes and got to modifying the heat shield



A step drill, some tin snips, and a burr made the trim job pretty easy. Didn't go as well as I WANTED, but it's good enough for now (future plans are to replace this manifold, so this heatshield is only long enough for this world as the manifold is.

Then to installing the probes.
You want them with the ends centered in the port, so you don't get wall cooling affecting your reading.
My method is, before installing, set the probes in the adapters flush with the "exit", and mark at the end of the nut,
Then install the adapters and push the probe all the way down till it bottoms in the port, mark the probe there.
Pull the probe out till both marks are in view, then mark "center".
set probe at that mark on the nut, and tighten.


Routed the wiring and now it's here. Just got to route inside the car to the passenger footwell, and terminate the cold junctions.

 

Took the long weekend holiday and finished up the wiring.
lots of check and double check that is monotonous but really important.


Mostly done, this takes up way too much room

when all the line items in the spreadsheet have been checked, guess it's done. Took it out to the car and plugged in to test, make sure I've not done something stupid like leave the main relay power out.

Looks a mess, but best to find problems now

Under the dash is really tight with the piggyback between the ECU and harness. but it fits.


well the test "passed" so I brought it back and cleaned it up with some braided loom for abrasion resistance and to make it not look like ***.

all tidied up


threaded the harness through the car in the dash (didn't take pics, idk why) and got it all plugged in, including the TC's
Fired it up and ran it for a heat cycle, with the changes the tune needs 100% redone so probably wont be firing it up much till I get the diff back in and can do some street tuning.

One hiccup, one under Plug TC is acting like it's disconnected. all other TC's aren't showing correctly.
Decided to just take the weekend win and step away to diagnose that issue later.

So on my plate for this week is:
Weld the diff housing reinforcement plates on.
Diagnose TC issues.

 

Found that the TC issues are that "automotive" TC's are UNgrounded, and "aircraft" TC's are grounded. and the MAXX is only setup for Automotive TC's.
So, all of my TC's won't work unless I do my own CAN-MAXX input (which I could do/have done for MS3). at this point I don't feel like breaking out an old project, so I'll be getting new exhaust TC's that (hopefully) work with the 1/8" NPT threads or compression fittings already in the manifold,
and as for the plugs I believe I'll try epoxying normal K type TC's to the base.... IDK how it'll go, but... why not try.


and for other more substantial news. Got the reinforcement plates welded on the housing.
i'm not a "welder" by trade or training. but I feel this, at least, will be better than the OE housing.
Feel free to criticize, I know I don't know everything and willing to improve.