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Relative power levels for N/A (Normally Aspirated) BP (1.8L) engine builds

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

- If racing, what class?
- Street only?
- 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?
- 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 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.

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

160-165whp will require blueprinting and a min or port/chamber work in the head. Bowl blend is most bang for buck, followed by deshrouding then port matching then debur 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 lots of compression. 

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

195whp porting, compression and cams.

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

225whp+ everything above and would be a very short lived engine. Perhaps 5-10 race hrs and its a $20K Miata engine.

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

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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