Planning Your BP Engine Build

We will skim across the basics in planning a performance rebuild of your BP (1.8) Miata engine. While the B6 (1.6) is actually more common, we don’t open B6’s around our shop. We swap BP’s in. That is the first, most cost effective power mod you can make to your NA6. IOW, it costs less and gains you more power to swap in a stock BP than to modify a B6 to reach the same power.

We also won’t cover specific dimensions, weights, finishes, proprietary build techniques etc. We and several other Miata specific vendors offer complete engines with magic® built in. For pro level builds, you either teach yourself or pay a pro  This is aimed at helping you select parts for your own build or plan an engine for a pro to build for you, not engine building 101.

94-97 BP05 head casting, NA8
99-00 BP4W head casting, NB1
01-05 BP6D “VVT” head casting, NB2

We like to build our race engines with all of the durability mods for “insurance” against over revs. This is both downshifting too soon in a braking zone or the 8800rpm “Money Shift”  Once you have the durability mods matched to intended usage, see what’s left over in the budget for power.

Durability. An SCCA Runoffs engine might only be expected to make max power/not burn oil for about 10-20hrs of track time. For HPDE, daily driver, NASA racer, autocross.. we like to use 100hrs as a starting point. That is full load, high rpm max duty cycle thrashing. Street driving is something like 50 street hrs to 1 track hr. A bone stock OEM rebuild will easily last 100hrs for HPDE, running the OEM redline assuming good oil/coolant temps. Over rev it to 8200 once or twice though and.. kaboom. Bump that 7000rpm redline with a programmable ECU or add F/I and the life span drops immediately unless you improve the engine and its support systems. Overall, adding rpm always reduces engine service life. This can be mitigated by building a more robust engine and improved support systems like reroute, oil cooler, better quality oil, etc. Nothing wrong with extra insurance if it is in the budget.

Forged rods – OEM rods like to deform past about 7800rpm. Most US brand name forged rods utilize Chinese forgings but are final machined, maganfluxed and otherwise massaged here. They are all pretty good. We use Manley’s which are just a hair below OEM weight. For higher budget <8300rpm N/A builds we like Carrillo A beams as they are exceptionally light. For high budget F/I builds or >8300rpm, the Carrillo H beams. 

Bearings – The OEM bearings are good. Race specific bearings are harder, which helps resist scuffing. We like ACL Race bearings for the 94-05 blocks but other brands have been proven to work well. The NB2 engines (01-05) use a wider thrust bearing. ACL only makes a thrust bearing for the 94-00 BP’s so find an alternate for that one. 

Pistons – The OEM pistons can be made to survive for short periods above 8000 but are limited to about 7500 if you expect them to last 100hrs. Stick with the Mazda pistons or a well known US brand for OEM rebuilds. Stay away from mystery brand pistons/rings. We like the Supertech 4032 alloy pistons paired with Wiseco (NPR) XX rings for high rpm or high power. Tightest cylinder to wall, best oil retention and longest service life. Other good piston brands are CP, JE, Wiseco, Mahle, Manley. The BP is designed around fairly high tension rings, 16-18 lbs. Lower tension or thinner rings reduce oil temps and make a wee bit more power but tend to develop blow by in the BP’s low rod ratio architecture. Piston thrust loads are simply too great for fancy, super thin, low tension rings optimized for dry sump or high rod ratio engines.

Oil pump – The stock oil pump uses sintered metal gears. Good wear characteristics and cheap to manufacture but low tensile strength. This becomes a problem when the cranks starts to flex with the oil pump being driven off it. All cranks flex minutely even at idle then more as RPM increases. Again that rod ratio comes into play, the power pulse is short and sharp. Add force (F/I) or RPM and it begins to distort the crank. The forged OEM crank becomes a worthless noodle at about 8600rpm but starts to dance well below that. This or big F/I power pulses are what explodes oil pumps. For N/A builds below 7000rpm, stock gears are fine. Low power <7000rpm <190whp F/I or N/A <7400rpm, you can get away with stock gears but have no “insurance”. For high power F/I or high RPM N/A builds, billet oil pump gears are a must.

Crank damper – The OEM damper is stamped steel in most cases, a few years are cast. Besides not being rebuildable (elastomer replacement), they tend to not be round and well, fall off. High power or high RPM, the damper needs to be changed. Both elastomer and fluid aftermarket BP dampers have been proven to work well. All else equal, fluid based dampers have the edge in engine protection. The damper is a key ingredient in keeping the oil pump main and big end bearings alive. Many a nice engine done in by stock dampers.

Valve Springs – The OEM NA8 springs have higher seat pressure than the NB engines. The NB’s start to float valves around 7800 with fresh springs. That float point drops over time as the springs slowly die. They are a wear item. SOP in race engine rebuilds is to toss the valve springs. The NB2 “VVT” stock intake cam has a very abrupt ramp which allows the valves to bounce violently past about 7800rpm. For F/I engines sticking to 7000rpm redline, a slight increase in seat pressure is a good idea as boost tends to blow the valves open a bit. For any engine expected to see 8000 rpm whether intentionally or not, we recommend a heavier spring like the Supertech “light double” spring kit. Above 8000 sustained, the “heavy double”.

Valves – The most common mistake we see made with BP valves is getting too aggressive with the valve seat width in the hopes of improving flow. Stick close to OEM seat width dimensions if you want it to last. Additional chamfer cuts past the seat improve flow without affecting durability. For sub 7200rpm N/A builds, the stock valves work fine. Significant improvements in flow can be had with back cut and/or oversize valves. Back cut valves work best in conjunction with deshrouding of the chamber. OEM valves need to be kept below about 1650° to keep from melting. This temp can easily be reached with a poorly tuned F/I or N/A build. Stainless valves bump that to about 1750° which is significant. They are also harder and will last longer under sustained high rpm use. The next step is Inconel exhaust valves which will survive 1900° without melting, mainly a concern in F/I builds. OEM valve seat material is hard enough.

Lifters – BP05 and B6 OEM lifters are hydraulic and generally short lived at anything above 7400rpm. The NB engines have solid lifters with lash adjusted using varying size shims on top. The NB lifters are happy to the low 8000’s but only with low lift cams, which… won’t make power in the low 8000’s. For any engine that will either have more than .400 lift or see sustained >7500 rpm, we recommend SUB (Shim Under Bucket) lifters. They weigh a bunch less and will retain the lash cap even with the most extreme cam grinds. The reduced weight improves valve seat life and increases over rev protection. 

Crank – We have found that the OEM crank does not like to be lightened significantly, so we don’t. Kept below 8400 or so, it is nigh indestructible however. Balancing is always a good idea. If you plan to race the engine and run it above 8000rpm sustained, balance the entire rotating assembly (clutch/flywheel/crank/damper) together. For BP’s that will see either a sustained 8500rpm or 350whp, we recommend a billet crank. For a wet sump engine, stick with the OEM Bull nose design. Knife edging the crank removes too much material and can lead to rapid big end and main bearing wear.

Power
Big Picture – The most cost effective N/A bolt ons are Intake piping, intake and exhaust manifolds, programmable ECU, fuel. ITB’s if you have the patience. A turbo or supercharger kit from a trusted Miata specific vendor is the most hp/$. Those are all the subject of countless other threads so we won’t get into them here. Most F/I kits require extensive support system mods to survive on track in hot weather with an advanced level driver. So the cost of the kit itself typically represents less than 50% of the final total if that describes you. If it is an autocrosser, street only car or only sees “touring speed” on track in cool weather occasionally, an F/I kit might be just the ticket.

Blueprinting – N/A power inside the block comes from virtually every mechanical interface or area that intake charge contacts. Thus why a pro level N/A race engine is not cheap. Blueprinting, by definition is simply building to a specific tolerance within OEM guidelines. In practice however, there is a long list of trick, tweaks, processes and non-OEM dimensions a pro engine builder will utilize to reduce friction and increase power. Things that work great on an SBC, motorcycle or Honda engine may not work on the BP and vice versa. The low hanging fruit is a simple bowl blend and chamber deshroud, bit of compression, possibly oversize valves. Cams for the Miata tend to be relatively pricey so I wouldn’t put them in the cheap/easy category, but that’s where the big N/A power is.

Compression ratio (CR) – An N/A engine will always make its broadest powerband and be most efficient (BSFC) when it can be tuned to MBT (Mean Best Torque). In simple terms, that means timing advance for max torque at every load point without detonation (pinging). Too much CR for a given octane and it will “ping” before reaching MBT which, besides hurting power, destroys your engine. When in doubt, be conservative with CR. We find the NB engines hit MBT at about 10.3:1 on 91 octane. A few tenths higher for 92 or 93. E85 will tolerate 14.0:1. Plan your build around the lowest octane you will ever run, not the highest. 

Bore size – OEM is 83.0mm. The max for N/A is 85.5mm which requires a custom head gasket as the OEM just barely clears a 85.0. Low boost F/I ok up to 85.0mm. F/I high boost, we like to stick to 84.0mm to retain maximum cylinder wall thickness for bore stability. Anything 85.00 or bigger should get the block sonic tested. It is up to your pro engine builder to decide how much is enough. Power from bore increase is (generally) linearly proportional to bore delta. IOW, 83.00 to 85.00 is about 2.4% so a 150whp build becomes a 153.6whp build, or thereabouts. Personally, I love a 10.3:1 85.5 BP6D N/A build with a good flowing head for a 2100# HPDE build. Kinda of a sweet spot. Another thing to consider with larger bore pistons and a higher compression is an increase in piston mass. More piston or rod weight increases bearing wear exponentially, and you probably plan on revving it higher. Big high compression pistons weight a lot more than stock size low compression pistons. Adjust oil pressure, viscosity, cooling, bearing clearance, damper, etc, to manage that extra rotating mass.

Ports – The BP ports are big enough for most of the cams people will commonly use. Flow can be cleaned up by smoothing transitions and applying a specific surface roughness. The BP’s low rod ratio rears it’s head again as a constraint in that the BP is very sensitive to port velocity. Hog the ports out so it makes magic numbers on the flow bench, lose velocity at mid lift and you are guaranteed to make less power than a stock head. Our CNC ports, for example, leave many areas of the OEM port untouched. The BP chamber responds well to deshrouding, specifically sized to your bore and head gasket. What this all means is again, go to a well known Miata specific vendor to get your porting and chamber work done. Uncle Joes Speed & Machine with a bunch of SBC engined race boats out back? Honda K24 experts accustomed to massive, high flowing ports and huge rod ratios? They may a or may not know how to optimize for the BP’s stodgy architecture. Find a BP specialist.

Valve size – Stock valve sizes work for any build. The greater the flow with say cams or F/I, the more will be gained by going oversize. Our CNC head has a +2mm oversize exhaust valve option. We have found that it is needed for max power output types of builds. Overkill for your 7400rpm 150whp pump gas HPDE build but just the ticket for your 8100rpm 180whp N/A or 300+whp F/I build.

Cams – The Miata aftermarket so far has provided precious few choices for NB heads. Of the many NB cams we have tested, we found the Tomei cams to work well over a useful RPM range. There may be other good cams available that we have not tested. The option also exists for custom cams. Do your homework before going down that road though. The NB heads have room for a .370 lift lobe, assuming OEM 36mm base circle, before requiring clearance to the cam box. Running a smaller base circle gets around this but then typically requires SUB (Shim Under Bucket) lifters and chamfered lifter bores. Anything much past about 250° @ .003 lift will raise the rpm of the torque peak and hurt power below 4000rpm. Not an issue for a 6 speed track car, but a pointed concern for a 5 speed daily driver. Grinds optimized for better flowing (Honda) heads or different rod ratios may actually lose power in a BP head. We have seen this a lot. IOW, homework.

Cam Gears – Purpose built aftermarket cams will be designed to work well with OEM cam indexing. Custom, one off cams tend to be based on more guesswork by the cam grinder so plan on gears to dial them in. In either case, there may be additional power or more beneficial powerband shaping available with adjustable gears, maybe not. There is often a few whp or shaping available even with stock cams if they can be adjusted a degree or so. Only way to tune cam timing is with dyno time so factor that into your budget.

Coatings – There are four basic types of coatings to discuss: TD (Thermal Dispersant) which shed heat, TR (Thermal Rejection) which insulate a part from heat, OS (Oil Shedding) which allow a part to shed oil faster and AF (Anti Friction) coating which reduce friction.

TR on the piston crown, chamber and valve heads will often allow another .5 compression for a given octane fuel. Just the crown coating will help reduce oil temps. Just the chamber and exhaust port coating helps reduce coolant temps. Fully coating the entire chamber and exhaust port pays significant dividends in power, particularly if using gas as fuel. Much smaller power gains from coating on E85 as it tends to run much cooler anyway.

AF coatings on cam journals, main bearings and piston skirts improve power and durability. Be cautious with AF coatings on cam lobes/lifter faces with steep ramped cams as they can “skid” on the low friction surface and actually do more damage than good.

OS coatings on underside of piston and on crank help sustain steady oil pressure and improve power a bit.

TD coatings don’t do much an aluminum parts but are effective on steel or cast iron parts. 

In general, we only coat the chamber/crown/valve heads/exhaust port in TR coatings for gas engines. Otherwise, no coatings. Our 2011 Thunderhill 25 hours engine had pretty much every coating possible everywhere. We ran a stock radiator and had to block part of it and the oil cooler off because it ran too cool otherwise. That engine got about 12.5mpg at race pace making 147whp.

Summary
I left out a lot because I reached the character limit for one post! I mentioned budget 6x so far because it is the beginning and end of such a discussion. Don’t expect a pro to be able to provide good answers to your questions without you being able to answer the budget question. Only then can you determine what balance or power and durability you can achieve.

“How much does XXX whp cost?” Final whp is the answer, not the question. Budget + intended usage = whp. So the question should be “The CFO will let me spend $XXXX for the long block, I already have XXX bolt ons, it’s an HPDE/daily driver no A/C on 93 octane in Texas… how much power can I afford?