APERTURE / GARAGE
NA MX-5  ·  FM TURBO  ·  REV_5

Build Sheet — Flyin' Miata NA

Project Boost

Cracked manifold → hot-side rebuild · 260 whp at 15 psi → ~300 whp on a 3" turbo-back · FM manifold and charge side kept

building roadster…
drag to orbit · click a marker to jump to the part
260 → ~300
Wheel HP (measured → target)
~3.2k
Spool (rpm, target)
GT2554R → GTX2860R
Turbo Frame
3"
Turbo-back · new
00

The situation

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. Measured at 260 whp on 15 psi of pump premium — roughly 306 crank. That is not a tired turbo, it is a choked one: the 2554 compressor runs out of flow near 30 lb/min, which is exactly where 260 whp puts it. More boost buys heat, not power.

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: a real step in power that still spools like the 2554 — on the FM manifold and charge side you already own, with a new 3" turbo-back so the exhaust stops being the ceiling. Fuel side is already done.

Inventory · keep vs replace
  • Exhaust manifoldCRACKED → replace
  • GT2554R turbotapped out → replace
  • FM turbine outlet (2.5")the ceiling → Kraken adaptor
  • Downpipefabricate 3" from the V-band
  • Cat + cat-backKraken 3" system
  • FM front-mount intercooler✓ updated — keep
  • Charge piping / BOV✓ keep
  • Oil & coolant lines✓ keep (re-fit)
  • ID1050X injectors + big pump✓ fitted — done
  • Clutch (uprated)✓ fitted — done
  • FM Inconel manifold studs✓ have
  • EGR✓ likely deleted — plug the port
  • Engine managementretune required
01

The parts list

Recommended build. The turbo was chosen for its Garrett 5-bolt outlet — not because the FM outlet reuses it any more, but because a stock cast adaptor exists for that flange. That adaptor bolts to the turbo, so the exhaust side stops depending on whose manifold positions it. One piece gets fabricated: the downpipe, because nobody documents that joint.

⬇ Full parts list · 54 items (.xlsx) Every gasket, stud, nut, crush washer, line, fitting, clamp + consumable — with a direct link on each line.
Reconciled against what is already on the car: 19 items you don't buy, 7 deleted outright across two plan changes.
Fit verified against BorgWarner, Garrett, ATP, FM and Kraken source pages. $4,413 to buy.
01
Hot side · foundation · in hand

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. Its EGR fitting is 1999–00 style (M22×1.5); on a Hydra-equipped FM kit the EGR is normally already deleted, so that port most likely just takes a plug.

Cast stainlessStaggered runnersT25 flangeBolts to FM kit
Flyin' Miata — buy Compare
Bought$549 paid
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 rated 475 hp — on a 3" exhaust that headroom is finally usable. It wins on its 5-bolt turbine outlet: Kraken cast a Ni-Resist adaptor for exactly that flange, so the 3" system hangs off a stock part bolted to the turbo, not off manifold geometry. Order by part number — the same turbo ships in V-band and T3 housings, and either one leaves the adaptor with nothing to bolt to.

5-bolt → Kraken adaptor475 hp compBall bearingIWG3" in / 2" out
ATP Turbo — buy Full-Race Compare
ATP$1,713 +$116 actuator
03
Adaptor + one fabricated piece

Kraken V-band adaptor + 3" downpipe

Cast Ni-Resist elbow: GT 5-bolt in, 3" V-band out · then 3" to the cat, made on the car

The FM turbine outlet is 2.5" only — no 3" version exists — so it comes off. In its place the Kraken elbow bolts to the turbo’s 5-bolt flange and presents a rotatable 3" V-band. That fit is documented and manifold-independent. What nobody documents is a Kraken downpipe on a turbo the FM manifold positions — so that one piece gets fabricated: 3" from the V-band to the cat, flex section, your O2 bung welded in. Order the adaptor as TOP MOUNT; low mount will not fit.

Bolts to the turboTop mountKit: clamp + InconelFab: V-band → cat
Kraken — adaptor kit
Kraken + shop$231 + ~$350 fab
04
Back half · the ceiling comes off

Kraken 3" exhaust + 200-cell cat

304L, V-band joints, Simons silencing · configured for a custom 3" V-band downpipe

This is the joint that is documented: Kraken list “custom downpipes” ending in a 3" V-band as a supported fitment, so their exhaust meets the fabricated pipe by design. Configure it as chassis NA, downpipe fitment 3" V-band / custom, flange 3", and add the cat — the base ships as a decat. Optional resonator if drone matters; it can’t be retrofitted later.

Documented fit3" V-band joints200-cell catDuties paid
Kraken — exhaust (kit builder)
Kraken$660 + $308 cat
✓
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
02

Spool & power

At 15 psi your 2554 already makes 260 whp — at the far right of its compressor map, through a 2.5" exhaust that is nearly done too. This swap changes where the number comes from, and the 3" system lets it keep climbing. Illustrative wheel-hp curves; only the 260 is measured.

~300 whp target on 3" · ≈350 crank compressor headroom (~340 whp) — turbine housing is the next limit, not the exhaust 0100200300 WHEEL HP 3k4k5k6k7k ENGINE RPM 2554 lights ~2.8k GTX ~3.2k
GT2554R — measured 260 whp @ 15 psi, out of compressor GTX2860R Gen II on 3" — same boost, cooler air, exhaust no longer the limit (~300 whp, holds)

Only the 260 whp figure is measured; the rest is illustrative. The 2554 makes its 260 at the edge of its map — out of flow near 30 lb/min, pushing hot air, no knock margin left on pump premium — and the 2.5" exhaust was near its own limit behind it. The GTX2860R moves the same air at about 60% of capacity: cooler charge, lower shaft speed, room for timing. With the 3" system the curve holds past 6k instead of tapering. The grey line is roughly what the compressor has left; from here the .64 turbine housing, not the exhaust, is the next thing you would change.

03

Turbo, three ways

All three make the power and bolt to the T25 manifold. The outlet column decided it when the plan reused the FM exhaust; on a fabricated downpipe it matters less than it did — see the note below the table.

TurboPowerSpool / responseHeatTurbine in · outReal cost
Garrett GTX2860R Gen II Pick 475 hp comp ★★★★★
ball bearing, strong		 ★★★★★
moderate		 T25 · 5-bolt$1,829
BorgWarner EFR 6258 Rejected 325–350 whp ★★★★★
Ti wheel — quickest		 ★★★★★
coolest		 T25 · 92mm V-band$1,699 +$40 flange
Garrett GT2860RS — "Disco Potato" up to ~350 whp ★★★★★
proven, older		 ★★★★★
runs hot		 T25 · 5-bolt$1,050–1,250

The honest version, since the plan moved: the EFR was rejected when its 92 mm V-band outlet forced a new exhaust the plan didn’t want. Now the plan fabricates a downpipe anyway, so the EFR’s penalty shrinks to a $40 weld-on flange — and it ships with its own actuator and boost solenoid, which the Garrett does not. The Garrett stays as chosen: it’s a strong turbo with a stock adaptor for its flange, and this build is settled around it. But it is no longer the expensive option, and you should know that before the turbo is bought.

04

Don't forget

⚠ Order by part number

Ask for P/N 856800-5003S — the GTX2860R Gen II with the .64 A/R T25 housing, internal wastegate and the 5-bolt outlet. The same turbo ships in V-band and T3 housings; order one of those by name and the Kraken adaptor has nothing to bolt to.

⚠ Adaptor: TOP MOUNT · exhaust: custom V-band

Two settings that cannot be undone. Kraken’s adaptor comes in top- and low-mount castings and they are not interchangeable — yours is top mount. Their exhaust is configured to a downpipe fitment; choose 3" V-band / custom downpipes, flange 3", and add the cat. Configured kits are non-returnable.

⚠ Oil restrictor — KEEP IT

A Garrett ball-bearing turbo requires a metered feed. The 0.040" restrictor inside FM’s 27-12467 swivel fitting is correct and stays on the car. “Fit none” applied only to the EFR, which has the orifice in its bearing housing.

Boost control — buy a solenoid

Garrett supplies none, so a MAC 3-port is on the list at ~$60. The wastegate actuator is separate too — ATP-WGT-025, 14 psi, $116. The spring sets your fail-safe floor; the solenoid adds the rest.

EGR — most likely already deleted

EGR bleeds a little exhaust into the intake at part throttle for emissions; it’s closed at WOT and costs no power. FM’s own manual: on 94–05 FMII/FMIIR kits “EGR plugs for both sides are the default option” and “Hydra-equipped cars don’t use the EGR anyway.” So expect a plug in the old manifold’s #4-runner port and a plate on the intake. Confirm with a look, then the new manifold’s M22×1.5 port needs an M22×1.5 plug (~$8), not the $139 NA8 pipe. The pipe applies only if EGR is physically connected and your state tests for it.

⚠ Retune — mandatory

Do not run the old 2554 map. Give the tuner the real starting point — 260 whp at 15 psi on pump premium — plus: GTX2860R Gen II .64, MAC 3-port, ID1050X with ID’s own data loaded, 3" turbo-back. Hold 15 psi first, take the free gain, then climb.

✓ Fuel — done

ID1050X flow ~100 lb/hr each: about 48% duty at 350 crank on pump gas, capable of ~618 hp. A big pump is in. One free check left: the regulator must be boost-referenced 1:1.

Studs, gaskets, hand-offs

The FM outlet-hardware kit is no longer needed — the adaptor kit brings its own Inconel studs. Fresh manifold-to-head and turbo-to-manifold gaskets, yes. Hand the fabricator your O2 bung and tell them the pipe ends in a 3" V-band.

✓ Clutch — done

Uprated clutch already fitted. Confirm it is bedded in before you lean on the boost.

Hot-side build · parts only
$2,061
Turbo $1,713 + actuator $116 + Kraken adaptor $231. Manifold already bought. EGR is a plug (~$8) unless a look at the old manifold says otherwise. Exhaust side: Kraken $1,199 + fabrication ~$350. Add the dyno tune ($650) and consumables for $4,413 all-in.
⬇ Download the full build sheet (.xlsx) 11 tabs: "!! READ FIRST !!" — 20 gotchas, incl. which joints are documented and which one is fabricated · full list · "Already have" ·
one order sheet per vendor (ATP $1,830 · Kraken $1,199 · FM $72 · exhaust shop $350 · tuner $650 · turbo specialist $170 · fluids $143).
$4,413 all-in to buy — plus $139 only if EGR turns out to be retained.