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Relative power levels for N/A (Normally Aspirated) BP4W/BP6D (1.8L) engine builds
Everything on this page assumes a healthy engine to begin with. Defined as less than 5% leakdown, within 5% of OEM compression with a tester.
If your car has 100k miles, don't spend a penny on upgrades untuil you have done a hot leakdown and compresion test.

Decide on a few things up front and it'll be easier to plan and budget for your engine build

- If racing, what class?
- Does it need to be emissions legal in any state?
- Minimum octane the engine will ever run on?
- Must it run in a street car, (A/C, OEM alternator, col start driveability etc) ?
- Actual budget for everything connected to engine/power making hardware?
- Tuned by a pro or semi-knowledgeable amateur or no tuning at all?
- Expertise available for tuning/set up challenges of an IRTB set up?
- Does the car need to be quiet?
- Do you have a shop in mind that is familiar with race BP engine performance builds?

Cost no object, a very short lifespan (1 hr) 300whp engine that runs on exotic fuel is possible. In the real world 130-160whp is possible within a reasonable budget for most owners. Above that, a supercharger, turbocharger or K24 swap is a better dollar to horsepower choice. The steps below assume 91 octane unleaded pump gas. Add roughly 6% more if running E85 on stock compression and cams, 7-8% more if running either/or race cams and compression ratio above 10.3:1.

All power estimates based on Dynojet SAE corrected whp
~96whp  B6 stock
~105whp BP05 stock
~119whp BP4W stock, USDM 49 state
~115whp BP4W stock, USDM CA
~125whp BP4W JDM
~116whp BP6D USDM
~125whp BP6D  JDM

140whp is bolt-ons, 91 octane and a good tune.

150-165whp will require blueprinting and/or port/chamber work in the head. Bowl blend is most bang for buck, followed by deshrouding then port matching then deburr ports. Actual full porting for stock cams only adds a little power beyond the bowl blend with stock cams. With race cams or F/I (Forced Induction), full porting starts to make bigger percentage gains over stock type builds.

175whp requires cams or >12.5:1 compression or IRTB.

185whp cams or compression and porting. Or IRTB's with a blueprinted stock engine and E85.

195whp porting, compression and cams.

205whp+ cams, compression, porting, E85 and possibly IRTB

225whp+ everything above and would be a very short lived, 9500rpm engine. Perhaps 3-8 race hrs and its a $25K Miata engine. A very few such dry sump engines have been built for SCCA GT3.

We have just touched 180whp on stock cams with an NB2 motor but it was not cheap. Based on the data from that experiment, we know 190whp on stock cams is possible with IRTB's, more compression and E85. At that point however, you should just feed it the cams it wants. 

For most owners, a stock cam NB2 (BP6D head) build makes the most sense. Run between 10.0:1 and 10.3:1 so it will allow MBT (highest ignition advance effciency) on 91 octane, programmable ECU that your local tuner is familiar with, bolt on a non-USDM "squaretop" intake manifold, Racing Beat header, 2.5" exhaust with high flow metal core cat, bowl blend, forged rods & pistons, stiffer valve springs. That should net around 160whp and be safe to 8000rpm sustained, idle like a stocker and last 200 race hours. An engine like the one described was built in 2012, raced continuously through 2021, winning three championshipos in SuperSpecCup. That engine has about 300 race hours on it and needs a rebuild!

For more info on planning the build of your long block, check out our article on that https://supermiata.com/planningbpenginebuild.aspx

https://supermiata.com/racing-beat-header-miata.aspx







For most owners, a stock cam NB2 (BP6D head) build makes the most sense. Run between 10.0:1 and 10.3:1 so it will allow MBT (highest ignition advance effciency)  on 91 octane, programmable ECU that your local tuner is familiar with, bolt on a non-USDM "squaretop" intake manifold, Racing Beat header, 2.5" exhaust with high flow metal core cat, bowl blend, forged rods & pistons, stiffer valve springs. That should net around 160whp and be safe to 8000rpm sustained, idle like a stocker and last 200 race hours.








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