Recommended build. The turbo was chosen for two flanges, not for its spec sheet: a T25 inlet that meets the FM manifold and a Garrett 5-bolt outlet that meets the FM turbine outlet casting. That second flange is what lets the downpipe, cat and cat-back stay on the car.
01
Hot side · foundation
Flyin' Miata cast manifold
FM 1.8 (1994–2005) cast stainless, T25 flange
Direct replacement for the cracked unit. CFD-redesigned cast stainless — staggered runners for 24% more flow, Cerakoted and rated 1,800°F. Cast is the crack-resistant kind (welded eBay tubular is what fails). Drops onto your head, clears the FM intercooler piping; the T25 flange mates the Garrett turbine inlet. On a 1994–97 car it needs FM’s NA8 EGR pipe — its EGR fitting is 1999–00 style.
Cast stainlessStaggered runnersT25 flangeBolts to FM kit
FM$549
02
The heart · chosen for its flanges
Garrett GTX2860R Gen II
.64 A/R · T25 inlet · IWG · 5-bolt outlet · P/N 856800-5003S
Dual ball bearing, Gen II billet compressor good for 475 hp against a target well under that — so it works easy. The reason it wins is the 5-bolt turbine outlet: it is what your FM turbine outlet casting is machined for, so the downpipe, cat and cat-back all stay on the car. Order by part number — the same turbo also ships with V-band and T3 housings, and either one breaks the whole plan.
Bolts to FM outlet475 hp compBall bearingIWG3" in / 2" out
ATP$1,713 +$116 actuator
03
Already on the car · no spend
Your FM turbine outlet + downpipe
FM cast turbine outlet → FM stainless NA8 downpipe, 2.5"
Both stay. The turbine outlet bolts to the Garrett 5-bolt flange exactly as it bolted to the 2554, and the downpipe bolts to the outlet. FM use no gaskets on these joints, so the flange faces must be clean and flat. The nine studs and nuts are twenty years old and have been through a cracked manifold — replace those.
Reused$02.5"O2 bungs already in it
Reused$0 +$50 studs
04
Kept · and the honest limit
Existing 2.5" cat-back + cat
Stays on the car — and sets the ceiling
This is the trade you accepted. The FM turbine outlet is 2.5" ID, so the whole turbo-back is 2.5". That supports roughly 350 hp but sits at the top of its range, which is why the honest target here is ~320. A 3" cat-back cannot fix it — the 2.5" outlet is upstream. Inspect the cat while it is apart; a collapsed 20-year-old cat reads as a boost fault later.
Reused$02.5" = the ceilingUpgradeable later
Reused$0
✓
Already handled · no spend
FM updated front-mount intercooler
Kept from the current kit — already sized past 350 hp
Your updated FM core and charge piping carry over untouched — it was never the bottleneck, the little 2554 was. Same for the BOV, the oil feed and drain, and the coolant lines: all of it is Garrett-to-Garrett, so it re-fits as it came off. One exception — the Garrett compressor inlet is 3" and the FM inlet hose is 2.5", so budget a 3"→2.5" reducer.
ReusedPiping reusedBOV reusedLines reused+$20 reducer