Supermiata recipe for affordable N/A BP power
Supermiata recipe for affordable N/A BP power
We have published most of what is in the FAQ over the last 15 years but it’s broken up and dispersed over many forums, facebook pages and groups. So here it is distilled into one page. A bit of background on the BP engine first.
The 94-05 Miata 1.8 engine traces its roots back to 1987 when Mazda first offered a twin cam B6 (1.6L) variant in a FWD hatchback. Later a turbo, forged crank and oil squirters for the pistons were added. The BP Miata engine didn’t get the turbo but it kept the forged crank, rod length and oil squirters. It’s a tough, if under achieving little motor.
94-97 BP05 head casting. Hydraulic lifters , low lift OEM cams
99-00 BP4W head casting. Solid lifters, medium lift OEM cams. Improved port flow over BP05
01-05 BP6D head casting. Solid lifters, variable intake cam timing “VVT”, medium lift OEM cams. Same port flow as BP4W
We’ll focus on lower cost parts and upgrades for few reasons.
1. At the time of writing this, there are two well proven complete CARB legal kits available that will make a reliable 180whp on pump gas. One supercharged, one turbo charged. Both run $4000-$5000 for everything. Hard to beat that dollar/HP ratio.
2. For more N/A power, Kpower now offers a Honda K24Z3 swap kit that will cost the typical owner about $7000 to complete. That’s about 200whp on pump gas but not CARB legal.
A spending cap of about $5000 on a fresh BP engine will get you around 150whp. The BP is not a high efficiency engine capable of high N/A output, but is robust and cheap. More investment than that and it starts to make more sense to go forced induction or K24 swap.
Here the recommended steps listed by whp to dollar ratio. With few exceptions, none of these modifications will be CARB or emissions legal. If it’s a street car, check your local laws before opening the tool box.
A fresh rebuilt engine with zero modifications will make more power than a worn out engine with the entire catalog of bolt ons. Start with a fresh engine! Here is our FAQ on rebuild options
Planning your BP engine rebuild
Another FAQ we published for a local racing series that you might find useful Reaching the SuperSpec Cup 140whp power cap
Cold Air Intake
The single biggest gain on a stock 94-05 will be removing the restrictive OEM airbox. Depending on how you configure your intake pipes and air flow meter, a BP can gain as much as 8whp from this alone. Here’s is our article on building an intake. Supermiata DIY intake
Next is a programmable ECU and professional tuning. One of the more expensive external mods but the one that will make the most difference in power. Gains at all rev ranges and higher redline. Even more gains as you bolt on parts that improve intake and exhaust flow. If an ECU isn’t an option, advancing the base timing about 4° will provide useful gains but will also raise the idle speed. Without modifying the engine internally, the limits of the stock ECU are about 130whp regardless of bolt ons.
Next it’s a header-back exhaust. Most of the gains from an exhaust will be past the torque peak, 5000rpm and above. 2.25″ tubing at minimum , 2.375 works well for street. 2.5″ for track cars where noise is less of an issue. A resonator and offset flow muffler will keep noise down. Don’t confuse power with noise. A pure straight pipe will actually make less power than a good high flow exhaust on a near stock long block. A 2.5″ 100-200 CPI metal core catalytic converter will have almost no effect on power so it’s not a bad idea even for an off highway/ trackday car.
Header should be next on the list. In our testing, Racing Beat and Maxim Works make are the two best on the market. The RB is a bunch less expensive though. Ebay headers are hit and miss. Some make nearly as much as the RB but none fit as well or nearly as durable. Some eay headers will make less power than OEM. The 99-00 CA cars have a cast manifold and will benefit from swapping in an OEM NB2 header with the matching EGR pipe. The non USDM OEM 4:2:1 NB2 header is worth a few hp but requires a downpipe that is really hard to find. Not really worth the trouble. One US company recently copied the RB header verbatim. We have not tested it but on paper at least, it appears to be a decent option to the genuine RB. All BP heads have the same exhaust flange but different EGR pipe locations.
The 94-97 BP05 head will only accept the 94-97 manifold. The 99-00 cars had VICS (Variable Inertial Charging System) which, when tuned by a programmable ECU will improve power a bit over OEM tuning. The 01-05 USDM cars had the VTCS ( Variable Tumble Control System) . The VTCS butterflies hurt power everywhere. Options are to remove them and weld up the shaft holes or just swap to a different manifold. The 01-05 non USDM/ Canada cars had a different intake manifold colloquially known as the ‘squaretop” which matches the 99-00 manifold up to about 4500rpm but makes several more whp above that. The BP4W and BP6D intake manifolds can be interchanged.
An internally stock BP with a few bolt ons will make the most power with about 28-32° ignition advance at higher rpm. OEM is roughly 24° max. As coolant and intake temps rise, the OEM ECU will reduce ignition advance. Taking steps to keep coolant temps below 200° in al weather and operating conditions will pay dividends in power. This temperature related power drop is where a lot of owners lose power from their magic dyno numbers and don’t even know it. While the BP is a strong engine, the 90-05 cooling system is woeful. Here is an article we wrote on optimizing the Miatas weak cooling system.
Supermiata cooling system performance
In summary, if it’s time to rebuild your BP and emissions compliance isn’t a concern for your off highway/ track day build, here is what we think is the best bang for the buck.
- Rebuild engine with aftermarket heavy duty valve springs for over rev protection and increase valve seat life. Otherwise stick with OEM rebuild. If it’s a BP05, switch to BP6D engine.
- Build cold air intake
- Switch to non USDM squaretop intake manifold
- RB header
- 2.375″ high flow exhaust with resonator and muffler
- Programmable ECU
- 1-2hrs of dyno time fine tuning fuel curves
- 91 octane fuel
- Aluminum radiator/ seal gaps in ducting/ reroute
This combo, with proper tuning will make about 145whp on 91 octane and cost around $5000assuming you do the work in your garage. A stock NB2 makes around 115whp. That’s about a 25% power increase which may not seem like much but that puts the power to weight about the same as a stock FRS/ BRZ. If you have regular access to E85, 155whp is possible with the same hardware.