1997 Mazda MX-5 Miata: DIY NB2 Coil On Plug Option
November 23, 2012
If you're installing an '01-'05 Miata engine (aka NB2) into an earlier chassis, one of the things you'll have to deal with are the coils. The NB2's VVT hardware precludes mounting the earlier coils in the usual spot at the back of the head. You can adapt the stock NB2 coil-near-plug hardware to the earlier harness, but these coils are generally considered the weakest of the NA/NB coils (themselves not much to write home about), so if you're planning on boosting you may find that you'll need some kind of other solution.
Beyond the jump is some kind of other solution.
The VVT hardware on NB2 engines -- the silver doohickey with the tube poking out of it -- lives where NA/NB1 coils wants to reside.
As we await the arrival of bits that support a happy spinny thing in the exhaust, I'm sorting out the ancillaries of our Keegan-built long-rod BP VVT longblock very recently delivered to project car HQ. One of those ancillaries is the ignition.
A common coil upgrade in Miataland is a coil-on-plug conversion using Toyota coils. These are abundant, stronger than stock Miata coils and relatively simple to adapt to NA and NB1 Miatas (note: this discussion assumes you have some form of ignition dwell control). Using Toyota coil-on-plugs on NB2 engines isn't nearly so straightforward -- another feature unique to the NB2 is that the holes in its valve cover are smaller than those of earlier Miata engines. As a result, early Miata spark plug wires won't fit in NB2 plug holes, so use of Toyota coils in an NB2 requires machining the bosses off the valve cover and opening up its plug holes.
The coils involved with the test fit here are from an '08-'09 Honda CBR1000RR; Denso p/n 129700-5150.
That sounds like a lot of work, so here's a possibility that doesn't require any modifications to the NB2 valve cover -- the coils from a Honda CBR motorcycle. These "pencil" coils are a popular retrofit in many other cars for their abundance and high spark energy (as they would need to be in order to successfully light off a high BMEP engine at ~11,000 rpm). CBR coils are commonly used in ridonkulously high boost retrofits (think 800+ whp 4-cyls). And since I had these in my garage and don't have NB2 coils, well, there's another reason to try them.
Like any coil-on-plug conversion you'll need to source mating connector shells, pins and wiring to fashion a coil wiring harness. CBR coils are inductive without an integral igniter, so nab a 4-channel igniter off an Audi A4 (alternatively some standalone ECUs have igniters built in) while you're at it and away you go.
CBR coils have a small diameter clip inside the boot, meaning they won't snap over the end of standard spark plug terminals (above left). Instead you have to use a spark plug with a removable terminal. Solution: Denso Iridiums have a screw-off terminal that exposes a threaded post (above right) identical to those used in motorcycles. Another bonus is that Denso Iridiums are excellent plugs.
The assembled length of the plug and coil is about 118.7 mm from seating washer to where the coil boot seats against the NB2 head. This measurement will shrink a bit once the plug's seating washer is compressed.
In the NB2, the depth from the top edge of the valve cover's plug hole to where the plug sits in the head is 120.5 mm, so we're looking at about 2 mm compression of the CBR coil's soft rubber sealing boot (near the top of the coil). From test-fitting the bits, 2 mm appears to be no problem as the coil's boot is slightly tapered, so it gives it a nice firm set. To prevent the coils from walking upwards out of place I'll use those threaded bosses in the valve cover near each plug hole (see lead photo) as an anchoring point for some kind of retaining ear.
So it's not yet a done deal, but so far CBR coils are looking promising as an elegant coil solution for highly boosted NB2s.
Jason Kavanagh, Engineering Editor