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Reaching the 140whp power cap

copyright Supermiata 2021

How to make the power cap in Super Spec Cup

This information was compiled to give Super Spec Cup drivers a better idea of what steps must be taken to meet the 140whp power cap. Not meeting the power cap could mean your car is less competitive.

Glossary of terms:

NA6 chassis code 90-93 with 1.6L B6 engine

NA8 chassis code 94-97 with 1.8L BP05 engine. “BP05” casting mark on head

NB1 chassis code 99-00 with 1.8L BP4W engine. “BP4W” casting mark on head

NB2 chassis code 01-05 with 1.8L BP6D “VVT”  engine. “BP6D” casting mark on head. VVT = Variable Valve Timing

Complete engine: A fully dressed engine with all accessories (water pump, manifolds, ignition) and wiring harness

Long Block: The complete assembled engine (block and head) with all accessories removed

Short Block: Otherwise known as “bottom end”. This the assembled engine block without  the head or any accessories.

BP – All 1.8L Miata engines

Why 140whp?

 The power cap was set at 140whp with very few restrictions on methods used to achieve that level. This power level is achievable with even a used engine that may be down on compression a bit. This reduces costs to drivers as they are not required to perform any specific modifications or purchase spec parts in their quest. The series was planned during 2011-2013. At that time, used NB1 & NB2 engines could be purchased on ebay with relatively low mileage and good compression/leakdown numbers for $900-1200. Static compression test is a good gauge of the condition of the engine. Leakdown test allows a mechanic to pinpoint where the wear is, valves, rings, head gasket, etc. 

Where to find your race engine

 Fast forward to 2021 and those low mileage ebay engine are all but non-existent. Occasionally a healthy USDM (US sourced) ebay engine pops up but it is now the exception, not the rule. There are  occasionally JDM (Japan market) engines in good shape on ebay, but they are getting harder to find. Healthy is defined by compression test numbers above 180psi in all cylinders.

Mazda no longer offers new crate engines. An OEM rebuild from most generic shops will cost  $2200~$3000. Some specialty race engine shops will perform an OEM rebuild for about the same cost.

A quick google search turns up Treasurecoastmiata.com in FL with rebuilt short blocks for $1500 and rebuilt heads for $600. To mate these two in your garage would add another $200 or so in parts, plus your time. The point is that a few minutes spent searching the interweb will find rebuilt engines readily available.

What modifications are needed to make the power cap on 91 octane

 First, we will list the absolute minimum modifications to make the power cap.

Programmable ECU (Engine Control Unit) or computer. The OEM ECU calibration (non-programmable) will not function at all with the other necessary modifications. There is zero possibility of getting even near the power cap without a programmable ECU. Most SSC drivers are using a Megasquirt based ECU as there are many tuners in California than are familiar with it. Make sure you have a tuner lined up that is familiar with whatever brand ECU you settle on.

 Professional engine tuning. This is done on a “dyno” or engine dynamometer. The car is strapped to a set of rollers that provide resistance. The professional tuner plugs a lap top into your programmable ECU and adjusts fuel and ignition to optimize your custom tune for your engine and modifications. If the engine makes too much power for the cap, the tuner can reduce ignition advance or adjust cam timing (BP6D) to “de-tune” down to the power cap. It is common for a healthy BP with the normal modifications to make a little bit too much power. This is an ideal situation as the owner then has the luxury of adding some ignition advance back in a year or two later as the engine begins to wear and no longer makes the power cap.

Header. RB (Racing Beat) is the clear winner here but Raceland, JR (Jackson Racing), OBX headers also work, albeit making slightly less power than the RB. The JR header is high quality, fits perfectly and very robust. The OBX and Raceland may not fit well or last as long as the other two. The CX Racing header is completely non-functional. Do not use it even if it is free. The OEM exhaust manifold will only make the power cap on E85 and an expensive race prepped long block

2.375 or 2.5″ free flow exhaust. Contrary to what some folks might tell you, exhausts do not need to be loud to make power. In our testing an ear splitting straight pipe makes less power than a quiet RB street exhaust. Spec Miata 2.25″ exhausts can work but tend to be expensive, loud and will always make less power than a 2.5″ exhaust. A used or damaged exhaust with pinched tubes and internally broken muffler chambers will choke the engine. Make sure your exhaust is clear of obstructions. You will not need the restrictive catalytic converter. We also suggest not fussing over exhaust weight too much. You are likely to require ballast to meet the min weight so exhaust weight down low in the car is a good thing. Ultra light exhausts cost more and tend to be fragile.

CAI (Cold Air Intake). This is a critical component that we often see ignored. Here is a link to the correct configuration: view here.

The more the feed to your filter is directly from outside the car , the more power you will make.

DO NOT place the filter on the left (drivers) side of the engine

DO NOT place the filter where its primary source of air is the hot air leaving the radiator

DO NOT make coupler or pipe bends in take any tighter than necessary. Large radius bends are your friend

91 octane fuel. You will need at least 91 octane pump gas. Lower octane fuel can allow the engine to “ping” or knock which reduces power and also causes internal damage.

Coolant reroute. While it is possible to meet the power cap on a dyno without a coolant reroute, it will not actually run cool enough in the summer to make that power during a race.
Minimum of 180psi compression test in all cylinders.

Modifications that allow a less than perfect engine to make the power cap

Square top intake manifold. This is the OEM NB2 manifold from all markets outside of North America. So any used NB2 engine purchased from Europe or Japan will have this manifold. The USDM manifold has a rounded upper surface, the square top is a flat upper surface. Everything else being equal, the square top will make 4-7whp above 5500rpm over the USDM manifold.

E85. Refers to pump gas with ~85% ethanol content. This fuel produces more power than regular California pump gas (E10) but will require a different ECU map. Everything else being equal, E85 will make 7-10whp more than pump gas. This is achieved both by the additional hydrocarbons in the fuel, oxygen content and much greater octane. E85 has the added benefit of reducing coolant and oil temps compared to 91 octane.

E85 fuel filter & sock

E85 fuel cleans the ports, combustions chamber and fuel system. This cleaning affect can dislodge years of grime in the fuel system and clog the fuel filter. It is a good idea to replace the fuel filter a weekend or two after converting to E85 then keeping an extra filter in your race spares. The fuel sock is the fabric bag attached to the fuel pump inside the tank. E85 will also degrade the sock, sometimes causing pinholes. Even a tiny pinhole in the fuel sock that you can not see will cause fuel starvation at half tank. Normally, the OEM Miata tank will scavenge down to less than .5 gallon without fuel starve. Also keep an extra fuel sock in your race spares.

E85 injectors

Correct E85 fuel/air mixture requires a larger percentage of fuel. This will often require larger fuel injectors. At minimum, you will need OEM 270cc NB2 injectors but aftermarket 300-400cc injectors are insurance against fueling problems.

E85 fuel pressure

The 90-97 Miatas have an OEM fuel pressure of 43psi. The NB’s are closer to 60psi. The greater the fuel pressure, the more fuel an injector will supply. If you run E85 in an NA fuel system, you will probably need to increase the fuel pressure to 60psi to achieve sufficient fuel flow. The OEM NB fuel pressure regulator can be made to fit on the fuel rail where the OEM NA regulator sits. You may also install an aftermarket AFPR (Adjustable Fuel Pressure Regulator) to achieve the same increase.

Blueprinting. This is a process performed by experienced race engine builders during assembly. Careful adjustments to clearances, minor grinding and shaping to reduce friction or improve airflow.  A good blueprinted BP engine will make 8-12 more whp than an OEM rebuild. Blueprinting is expensive and not needed if the engine is healthy to begin with so we generally don’t recommend it unless you can afford it as extra insurance.

Porting, cams, bigger pistons. These types of modifications can make 10-40 more whp but are overkill for Super Spec Cup. Expensive and not needed to make the power cap if you have a healthy engine. Not recommended.

Options for your race engine

 The only known trouble spot of the BP at this power level is the OEM throttle body. Vibration and fatigue can cause the throttle shaft to fracture. This sends pieces of shaft and the tiny throttle shaft screws into the engine which can destroy it. Two known fixes: Purchase a Skunk2 or other aftermarket throttle body. The Skunk2 is imperfect and will require all fasteners to be loctited before use. The other option is stripping any oil from the OEM throttle plate and shaft then using a high strength epoxy over the center area of the shaft. This prevent the shaft from breaking but even if does, the epoxy will encase the small pieces that might otherwise go into the engine. Your OEM throttle body may never fail but of it does, your throttle sticks wide open (because the plate is gone) and you have nasty metal bits thrashing around where they shouldn’t be. Modifications to insure against that are a good idea.

Engine oil. Any oil will do, provided it’s changed regularly. Synthetic oil gives a bit better protection in case you accidentally over rev your engine. Synthetic oil will also allow your engines bearings to last a bit longer. But clean oil is more important than fancy oil. As your race car will have no catalytic converter, you can use a “race” oil with higher zinc (ZDDP) content for better wear characteristics. 10W40 a good place to start, but 20W50 will also work, just keeping in mind to fully warm the oil past 160° before applying full load.

Forged internals. We like to call these modifications “insurance”. A fresh OEM rebuild can be over revved to perhaps 8000rpm once or twice. Much more than that and will spin a rode bearing which destroys the engine. Or and over-rev may simply float the valves which permanently loses compression and power. Forged rods, pistons, heavy duty bearings, and heavy duty valve springs can allow dozens of 8000rpm over-revs without harm.

Oil cooler. A 140whp BP being revved to 7000rpm can reach 300° oil temps. If using conventional (non-synthetic) oil, the viscosity breaks down and reduce bearing protection. The BP is a very robust engine and will tolerate this abuse for a long time. Adding an oil cooler with minimum of -10 size feed lines, will drop max temperatures down. All else being equal, the engine will perform best and last longest when max temps are kept below 240°.

Overall cooling system health. The OEM radiator, coolant flow path and ducting are deeply inadequate for even a stock BP engine in HPDE. W2W racing will cause your BP to overheat, guaranteed. The subject requires more space than we have in this document so visit this tech page here.

The recipe

 We feel the best investment is fresh OEM rebuild, RB header, 2.375 or 2.5″ exhaust, Megasquirt based ECU, 91 octane pump gas, CAI, square top intake manifold. This combination will usually make 145-150whp.

How long will it last?

 Assuming an OEM rebuild as described in the previous paragraph, never overheated or over-revved, about 150 race hours before it will no longer make the power cap. A blueprinted, forged internal engine with square top and E85 will make upwards of 170whp when new. This allows many years of retuning to make the power cap. The oldest engine in Super Spec Cup as of 2021 is an 8 year old blueprinted, forged, E85 squaretop NB2 that has  maybe 400 race hours on it and still makes the power cap.

If you are fortunate enough to find one of the very rare sub 80k mile NB engines off ebay and the compression is good, snap it up!