Build Sheet — Flyin' Miata NA
Cracked manifold → full hot-side upgrade · 270 hp → a comfortable, fast-spooling 350
NA chassis on a 2000s-era Flyin' Miata turbo kit — Garrett GT2554R (a.k.a. GT25R), the small ball-bearing unit that was the workhorse of that kit. Good for ~270 crank / ~230 whp, quick to light, but tapped out up top.
The exhaust manifold cracked. On a 20-year-old log that's not a repair — it's the cue to upgrade the whole hot side while it's apart. The GT2554R is also the ceiling on power, so it goes with it.
Goal: 350 hp that still spools like the 2554 — bolted onto the existing FM kit so there's no full-kit re-buy. New pump + injectors are already in.
Recommended build. Everything shares the FM kit's T25 turbine flange, so manifold, turbo and downpipe all interchange — and the intercooler, piping and lines carry straight over.
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; T25 flange mates the EFR.
The spool king in this class. The Gamma-Ti turbine wheel is far lighter than Garrett's Inconel, so response is as quick or quicker than the 2554 while flowing 325–350 whp. Runs cooler than a Disco Potato at the same power. Internal wastegate = no external gate plumbing.
Steps the old 2.5" up to 3" to let the bigger turbine breathe without choking top-end — but keeps a smooth merge so you don't bleed the low-end spool you're paying for. Add a high-flow cat if it stays street-legal.
Matches the 3" downpipe end-to-end so the turbo isn't fighting a 2.5" restriction downstream. Straight-through muffler keeps it civil on the street while a 350hp Miata wants the flow. Pick the note you can live with.
Your updated FM core and charge piping carry over untouched — it was never the bottleneck, the little 2554 was. Same for the BOV, oil feed/drain and coolant lines: re-fit them to the new turbo and manifold. This is exactly why the hot-side-only path works and you don't re-buy a kit.
Why this swap keeps the character: the EFR lights within a whisker of the old 2554, then keeps pulling to 350 where the little turbo fell over. Illustrative wheel-hp curves.
Curves are illustrative, not dyno-measured — real numbers depend on boost, fuel and tune. The shape is the point: near-identical spool, a much taller and flatter top end. That's the whole reason to jump frames instead of just cranking boost on the tired 2554.
All three hit 350 hp on the T25 manifold and reuse the FM kit. They differ on spool, heat and wallet.
| Turbo | Power | Spool / response | Heat | Flange | Est. price |
|---|---|---|---|---|---|
| BorgWarner EFR 6258 Pick | 325–350 whp | ★★★★★ Ti wheel — quickest |
★★★★★ coolest |
T25 · IWG | $1,550–1,750 |
| Garrett G25-550 | 300–550 cap | ★★★★★ modern billet, strong |
★★★★★ moderate |
T25 .49 · V-band | $1,450–1,650 |
| Garrett GT2860RS — "Disco Potato" | up to ~350 whp | ★★★★★ proven, older |
★★★★★ runs hot |
T25 · IWG | $1,050–1,250 |
Why the EFR wins: at this size the 15-year-newer Gamma-Ti turbine spins up faster and dumps less heat for the same power — 350 that still feels like the fast little 2554. The G25-550 is the value-modern pick with headroom to 400+. The Disco Potato is the proven budget route — it'll do 350 but you'll feel the heat on track. All are a straight swap on the same manifold + downpipe.
New turbo + injectors means a fresh calibration. If the FM kit runs Hydra/MegaSquirt, get it on a dyno with a tuner. Do not run 350 hp on the old 2554 map — that's how pistons die.
Uprated clutch already fitted — good, since the stock NA unit gives up around 250 lb-ft and 350 hp would slip it. One less line item; just confirm it's bedded in before you lean on the boost.
Opposite of the Garrett rule. BorgWarner integrate the orifice into the EFR bearing housing: "an external orifice SHOULD NOT BE ADDED." Your restrictor is the 0.040" one inside FM's 27-12467 swivel fitting — it leaves with the old turbo, so just don't carry it over. The EFR arrives with its own −4AN inlet fitting; your FM feed line already ends in a −4AN flare and screws straight on.
Don't buy a solenoid. "A boost control solenoid valve (BCSV) is included with every EFR turbo," mounted on the compressor cover. Drive it at 32 Hz or less, coil is 23 Ω. Start low, ramp to 350 once the tune is clean. The stock "medium boost" canister targets 12–18 psi, so expect to chase creep near 20.
FM Inconel manifold studs already in hand — good call, they're what survives the heat cycles. Still grab fresh gaskets: turbo-to-manifold, manifold-to-head, downpipe. Cheap insurance against a second cracked-flange saga.
Run the numbers at 0.55 BSFC and 85% duty: 550cc = 324 hp, 650cc = 383 hp. So 650cc is the floor for 350 at the crank — and ~750cc if your 350 is at the wheels. Settle crank-vs-wheel before you buy injectors. The 255+ lph pump is fine either way.