1997 Mazda MX-5 Miata: The Next Level
October 06, 2011
Project-wise, it's been pretty quiet around this little green tictac lately. Let's see, I changed Project Miata's oil last week. Thrilling. Oh yeah, the a/c stopped working temporarily until I replaced a blown condenser fan fuse (the higher current draw of the new fans are likely the factor here, but so far so good with the new fuse, which has lasted 10x longer than the old one). So exciting.
Blah blah blah. Time to turn the wick up on this project thing. I've been talking to some peeps and we've hashed out a plan. It involves dragging the Miata's BP engine into the 21st century in keeping with the car's dual purpose intent.
The idea is to maximize the area under the curve and to do it as reliably as possible. Also, to keep it real it has to be done on pump gas, and around here that means 91 octane. An LSx swap is certainly one valid approach to this, but the wholesale changes involved would result in perpetual downtime and too many extra pounds in the nose for my liking. Besides, I'm of the mind that there's life yet in the Mazda BP engine.
Making power on 91 then becomes an exercise in maximizing flow and managing the knock threshold. Decisions cascade from those notions. There's no hard power target, but 300 hp would be excessive so let's size things up around that and see what happens.
We'll start with the version of BP with the most potential -- the one that came in Miatas built from 2001 to 2005. Compared to the one in our '97, these later BPs had a better head and packed variable valve timing on the intake cam. VVT allows overlap to be dialed in when intake pressure is high relative to exhaust pressure, and then dialed out as exhaust manifold pressure rises. This broadens the powerband and staves off detonation.
Originally the VVT BP engines had high compression, something like 10:1. This is incompatible with 91 octane when we're talking about the kind of boost that will make Oldham happy. Also, the stock rods tend to fold like a map when pushed hard.
Since new reciprocating components are needed anyway, we're taking this opportunity to add a twist -- a long connecting rod layout. The required piston and rod geometries to achieve this are being worked up by JE Pistons and Mil-Spec, respectively. Keegan Engineering will mastermind the head specifications and engine assembly, an area that is deceptively critical for performance. Similarly, we're working with the big brains at Apex Speed Technology on engine management and tuning.
Then there's the happy spinny thing to be mounted in the exhaust necessary to achieve the required flow. I have ideas around this, though details are TBD.
Lots more to come, as some of these bits are already trickling in. Should be a fun ride.
--Jason Kavanagh, Engineering Editor