Watterson02's Miata Turbo Build (Third Motor)

Watterson02 · Mazda Miata (NA8)

Mazda MX-5 Mk1 (NA)

The Third Motor� oops

Read the full write-up at The Third Motor� oops ↗

An NA8 Miata turbo build through three engines, documented on a forum

Watterson02 bought a $3500 NA8 Miata in 2019 and progressively turned it into an autocross and drag car, adding a turbo setup during the pandemic with forum-sourced parts. After blowing up a 5-speed, suspecting a bent rod, and then spinning a main bearing, the car sat until he sourced a third engine for a from-scratch rebuild with forged internals, now being machined and assembled by a local shop.

Specs

As stated in the original write-up.

Engine: NA8 1.8L, third short block being built with 83.5mm 8.5:1 Wiseco pistons, Manley rods, ARP fasteners
Turbo: Garrett GTX2560R, open box, with eBay cast/machined T3 exhaust manifold
Fuel & Ignition: Flowforce 640cc injectors, MS2 PNP ECU, AEM Eugo wideband (replaced an Innovate MTX-L)
Cooling: Cobalt radiator with Cobalt coolant reroute and 180 degree thermostat
Gearbox: 6-speed swap with a used FM clutch, after original 5-speed was destroyed; believed 4.3 final drive
Wheels & Tyres: 15x9.5 Konig Freeform wheels with 245 Maxxis VR-1 tyres; stock wheels used for daily driving
Oil system: Boundary 2-shim oil pump, Fluidamper harmonic damper
Valvetrain: Supertech heavy duty double valve springs, reused OEM valves and cams, OEM head gasket

Parts used

Parts named as used in the original write-up, grouped the way the Parts page groups them. Where one of the shops we index lists the same thing for this car, it is shown underneath with a link to the shop.

Engine 6

Wiseco 83.5mm 8.5:1 pistonsWiseco
Forged pistons for the third engine rebuild
In the shops
Wiseco Seamless Piston Ring Installer ↗Flyin' Miata· $37.99 USD
Wiseco Piston Ring Set ↗Flyin' Miata· $33.99 USD
Forged Turbo Pistons by Wiseco (1.8L NA/NB) ↗Flyin' Miata· $1,099.00 USD
Manley connecting rodsManley
Fitted with the new pistons in the third engine build
In the shops
MANLEY H BEAM RODS ↗MiataSpeed· $457.20 USD
Connecting Rod Big-End Bearing Set ↗MazSpec MX-5 Spares & Accessories· £74.99 GBP
Carrillo Super A Connecting Rods (NA/NB) ↗Flyin' Miata· $1,249.00 USD
Boundary 2-shim oil pumpBoundary
Installed in the third engine rebuild
In the shops
Boundary Engineering Upgraded Oil Pump ↗Flyin' Miata· $369.00 USD
Fluidamper harmonic damperFluidamper
Replacement damper for the third engine, after a wobbly stock unit was suspected to have spun a bearing on the second engine
In the shops
Internally Balanced Steel Harmonic Damper ↗Moss Miata· $590.99 USD
Internally Balanced Steel Harmonic Damper ↗Moss Miata· $579.99 USD
Tiger Harmonic Damper ↗Sunbeam Car Parts (Alpine, Rapier, Tiger)· £114.00
Supertech heavy duty double valve springsSupertech
Fitted to allow higher rev ceiling with stock valves and cams
In the shops
Supertech 16V Dual Valve Spring Kit - Black (16-21 WRX/STI) ↗JDMuscle — Mazda MX-5· $585.42 USD
Supertech 16V Dual Valve Spring Kit - Performance Replacement (02-21 WRX/STI) ↗JDMuscle — Mazda MX-5· $585.42 USD
ACT Heavy Duty Pressure Plate ↗MiataSpeed· $311.99 USD
ARP fastenersARP
Used throughout the third engine rebuild
In the shops
Rocker Assembly Fastener Kit, ARP, 6-cylinder ↗Good Parts· $24.00 USD
Rocker Assembly Fastener Kit, ARP, 4-cylinder ↗Good Parts· $16.00 USD

Fuel & Ignition 4

Flowforce 640cc injectorsFlowforce
Used injectors fitted for the turbo build
In the shops
725cc/min EV-14 Fuel Injectors ↗Flyin' Miata· $429.00 USD
Bosch 550cc Injectors, Set of 4 (NC Turbo) ↗Flyin' Miata· $325.00 USD
MS2 PNP ECU
Standalone ECU used to tune the turbo setup
AEM Eugo wideband sensorAEM
Replaced a failure-prone Innovate MTX-L without rewiring, stopped repeated sensor failures
In the shops
AEM X-Series Wideband AFR Sensor and Controller Gauge ↗Moss Miata· $259.99 USD
AEM X-Series Wideband Gauge Kit ↗Flyin' Miata· $259.00 USD
NGK Direct-Fit Wideband Air/Fuel Ratio Sensor (92-00 Civic) ↗JDMuscle — Mazda MX-5· $339.00 USD
Garrett GTX2560R turboGarrett
Open box turbo bought from a forum member for the turbo build

Cooling 2

Gates water pump and timing beltGates
Fitted during the third engine rebuild
In the shops
Gates Water Pump & Alternator Belt ↗Moss Miata· $33.99 USD
Gates Water Pump & Alternator Belt ↗Moss Miata· $31.75 USD
Gates Racing Timing Belt ↗Moss Miata· $59.99 USD
Cobalt radiator and coolant reroute kitCobalt
Installed for cooling, incompatible with AC which caused a puncture later JB welded
In the shops
Coolant Reroute Kit, Cobalt ↗Moss Europe (UK)· £343.50 GBP
Coolant Reroute Spacer Kit, Cobalt ↗Moss Europe (UK)· £117.76 GBP
Coolant Reroute Kit, with EGR tube, Cobalt ↗Moss Europe (UK)· £389.00 GBP

Gearbox & Drivetrain 2

6-speed gearbox
Swapped in after the original 5-speed failed
FM clutchFlyin' Miata
Used clutch fitted with the 6-speed swap

Suspension & Steering 1

Tein coiloversTein
Taken from a previous Miata and fitted to this car
In the shops
Coilover, flex-z, Tein ↗Moss Europe (UK)· £1,608.00 GBP
Coilover, flex-z, Tein ↗Moss Europe (UK)· £1,703.75 GBP
Coilover, flex-z, Tein ↗Moss Europe (UK)· £1,421.00 GBP

Wheels & Tyres 5

Advanti Storm S1 16x9 wheelsAdvanti
Early wheel upgrade, later sold
Federal 595 215/40/16 tyresFederal
Fitted with the Advanti wheels
In the shops
215/45R15 85V Toyo Proxes TR1 Tyre ↗C Bradley Motorsport· £124.50 GBP
BFG Rival S 1.5 tyres
Fitted on stock wheels for autocross
Konig Freeform 15x9.5 wheelsKonig
Fitted for autocross after selling the Advantis
In the shops
Wheel, Dial In, grey, 15x9, ET35, Konig ↗Moss Europe (UK)· £239.26 GBP
Wheel, Dial In, black, 15x9, ET35, Konig ↗Moss Europe (UK)· £250.75 GBP
Wheel, Dekagram, black, 15x9, ET35, Konig ↗Moss Europe (UK)· £261.76 GBP
Maxxis VR-1 245 tyresMaxxis
Mounted on the Konig Freeform wheels for autocross

Body & Trim 2

Spare trunk lid with spoiler
Found and fitted for appearance
In the shops
Boot/Trunk Lid Spoiler ↗British Auto USA· $1,661.06 USD
Trunk Spoiler - KG Works Style by RSpeed ↗Moss Miata· $184.25 USD
Trunk Lid Cable ↗bpnorthwest.com· $6.28 USD
Replacement passenger door
Fitted and paint matched after a collision

Other 2

Eggbay cast exhaust manifoldeBay seller
Machined cast manifold used to mount the turbo
Exhintake exhaust cam swap
Fitted to adjust cam specification, timing accuracy later questioned due to drilling error

Photos

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How it was done

Starting condition and early mods

Bought in 2019 for $3500 with 99k miles, a torn soft top, dead speakers and dry rotted tyres. Owner fixed the speakers, fitted a new top, added 16x9 Advanti Storm wheels, and later swapped in a rollbar and coilovers from a previous Miata. A collision led to a replaced door, and a spare trunk lid with spoiler was fitted.

Electronics and cooling upgrades

Installed a MS2 PNP ECU and learned tuning basics. Replaced a failure-prone Innovate MTX-L wideband with an AEM Eugo, solving repeated sensor failures. Fitted a Cobalt radiator and coolant reroute with a 160 degree thermostat, later changed to 180 degrees; the AC-incompatible radiator was installed anyway and eventually got punctured and JB welded.

Turbo build and drivetrain failures

During Covid, built a turbo setup using used Flowforce injectors, an eBay cast manifold and an open-box Garrett 2560R, sourced from forum members, plus exhaust and intercooler parts. The 5-speed failed and was replaced with a 6-speed and used FM clutch. A missed shift to 8350 RPM at Bristol Motor Speedway likely bent a rod, which later caused a rod to let go.

Second engine and further damage

Swapped in a $500 engine from a friend's parts car. Continued autocrossing and daily driving, fitted Konig Freeform wheels with Maxxis tyres. Hit a deer, popping the fender back into shape. A wobbly harmonic balancer was suspected to have caused a spun main bearing, ending this engine's life; the car was parked and its clear coat began peeling.

Third engine rebuild

Sourced a running $500 NA8 engine, disassembled it and confirmed bearing surfaces within spec. Ordered forged internals (Wiseco pistons, Manley rods, Boundary oil pump, Fluidamper, Supertech springs, ARP fasteners, Gates belt and OEM gaskets) and planned to have the block cleaned, head refreshed and bearings specified and assembled by Center Street Machine.

Head machining decisions and ongoing turbo issues

Pulled the head himself in a driveway. Decided against larger valves and SUBs but had the short side radius smoothed and valve guides inspected. Later fitted an Exhintake exhaust cam swap, but suspected the cam timing was off due to an inaccurately drilled dowel hole, and continued troubleshooting poor turbo spool linked to a T25 turbo flange mismatched to the T3 manifold flange.

A summary in our own words. The full account, with all its detail, is in the original write-up: read it at The Third Motor� oops ↗

Worth knowing

Do not install an AC-incompatible radiator while leaving the AC hardware in place, as parts can rub and puncture it
Check for a bent rod after an over-rev rather than continuing to drive the engine
A wobbly harmonic damper should be addressed promptly, as it may be linked to a spun main bearing
Cover a parked car to prevent clear coat damage from sun exposure
Mismatched turbo and manifold flange sizes (T25 to T3) can restrict exhaust flow and hurt spool and top end power