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Stealth NA 1.8 NB1 power mods

Naturally aspirated - MX-5 Miata Forum - forum.miata.net
I'm competing in a race series that frowns upon performance mods of any kind. They aren't banned, strictly, but you get penalised for showing up with obvious power mods. Hence, I'm putting together a list of mods that keep the car looking stock while 'removing restrictions' to making power in the upper rev band so that I can reduce the number of cars passing us on the straights.

Here are the build plans I've ammassed so far. I'm looking for thoughts, suggestions, additions, and general advice on how these things will mesh together and if I should add/delete/adjust any aspects. As this is a budget build there will be NO dyno time - just perform these mods and race the car. Also, as it is not registered, there is no test driving, and so the mods need to work out of the box in race conditions.

The car is a Australian spec 1998 NB1 with the BP-4W engine, 9.5:1 compression, solid lifter cams and VICS. It has 180000klms on the odo but runs great as is.

Mods list:

1: Exhaust: 4-2-1 stainless headers, long primaries, 2.25" collector, heat wrapped, stock heat shield retained, decat 2.25" straight through pipe with one muffler. Should I to go 2.5" tubes in case I want to turbo in future?

2: ECU. Replace stock ECU with JDM BP5A for higher rev limit and to suit head shave and higher compression, along with required fuel return system and JDM FPR. With this ECU do any of the other mods become not worthwhile or incompatible?

3: Up to 1mm head shave, 5X Racing adjustable timing wheel to dial in more timing advance. Have been told that running 1mm shave and full 15 deg advance on 'stock' ECU inadvisable but with JDM ECU? Should I keep it safe and run less advance and/or less compression? Will use adjustable timing gears to correct for ignition retard due to shave, plus adjust throttle stop screw as has been discussed.

4: Addition of Mazdaspeed BP-5A intake camshaft. Have looked at ExhIntake cam swap but finding a good used exhaust cam is proving difficult. Presume Mazdaspeed cam is the best value alternative given stock JDM ECU?

5: Mild home porting of head to clean up stock castings, port match the gaskets.

6: Home made Randall style cold air induction retaining stock air box and AFM, with K&N air filter inside stock air box. I see NA Miatas benefit from RX7 AFM but curious if the stock NB AFM is also a restriction at 6000+ RPM?

7: Coolant reroute kit

8: De-power steering rack conversion

9: 4.3:1 diff ratio swap out (All Aus models are 4.1:1)

Thanks in advance for your input guys.


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