1.8 into 1.6 body

 

Parts you need:

- 1.8 engine with intake manifold and exhaust manifold
- if possible: 1.8 motor mounts, 1.8 alternator from a 94-97

When I did this swap (in 2000), this was all diy stuff, but in the mean time, Flyin' Miata have kits available to do this swap.

Parts to transfer from the 1.6 to the 1.8 motor:

- complete thermostat housing (requires removing the timing belt, unless you cut the black metal cover)
- oil pressure sender
- temp sensor at the back of the engine
- engine wiring loom (because you use the 1.6 throttle body)
- CAS (only for 1999+ engines as they don't have one, the 94-97 engines already have a CAS)
- starter (all starters fit, if necessary remove the rear bracket from the starter, it's not needed)

You need a 94-97 alternator on your 1.8 motor.  If you do not have one, you can use your 1.6 alternator but you must also transfer the 1.6 crank pulley, 1.6 waterpump pulley and belt over to the 1.8.  This only works if your 1.6 has a long nose crank though (91-on).  The short nose crank pulley (90) does not fit the 1.8.  Also the 1999+ alternator does not fit the 90-97 electrical system. If you go looking for an alternator,  you need one with a ribbed pulley and a sticker on it saying it has a built in voltage regulator.

motor mounts

It's best to use the 1.8 motor mounts but if you can't find them (they were on backorder when I did the swap) you'll have to modify the 1.6 motor mounts.  One way is to simply widen and slot the holes.  Other way: weld a piece of metal to them and drill new holes.  That's what I did, but I forgot that the 1.8 motor is 22mm longer than the 1.6 motor.  I kept the single rear hole in the motor mount as a reference and modified the 2 front holes.  Bad idea, since the engine sits too far to the front in the car now.  It does fit, but the a/c compressor only barely clears the swaybar.  Best to modify the rear holes thus (or get 1.8 motor mounts)!

Ignition

If you're putting a 1999+ engine in, transfer the 1.6 CAS over to the 1.8.   Since the CAS is located on the other side of the engine, the engine wiring loom will be too short.  Lengthen the 4 wires to the CAS.

You have 2 options for the coil pack.  You'll have to do some modifications either way.

1.  Use 1.6 coil pack with the 1.6 ignition system. The bracket is not a straigt bolt on on the 1.8 engine. It needs to be (rather heavily) modified (I welded new mounting brackets on it). It's a pain to modify (cut, weld, clean up, paint etc).  On 1999+ engines, the CAS hits the coil pack and you'll have to do even more modifications to it.  You could also use a 1.8 bracket and put the 1.6 coils in them, but you'd still have to do some mods.
The 1.6 spark plug wires are too short.  Use 1.8 wires.  Install the spark plug wires in the 1.6 order (3214).  IOW, passenger side coil fires cylinders 2 and 3 (the order doesn't matter since the wires are mounted in series through the coil pack, so you can put them in 2-3 or 3-2 sequence).  The other coil fires cylinders 1 and 4 (again, the order doesn't matter, 1-4 or 4-1).

2.  Use 1.8 coil pack with the 1.6 ignition system. This is a straigt bolt on but there is a 'but'. The late (after 1995) 4-pin coils don't have a tach driver in them so it will work but you won't have a tach signal. You need to make your own tach driver (easy). If you use the old style 1994-6/1995 3-pin coils, they have tach drivers built in. You than only need to modify the wiring a bit. Take a look here on how to use the 1.8 coil pack in a 1.6 car.  Install the spark plug wires in the 1.8 order (4123).  IOW, passenger side coil fires cylinders 1 and 4 (1-4 or 4-1).  The other coil fires cyl 2 and 3 (2-3 or 3-2). I used the 1.6 coil pack, but thinking about it, it would have been less work if I had used the late 1.8 coil pack and made a seperate tach driver. I'm gonna do it that way next time :) Use 1.8 coil pack and modify the electrical system.

throttle body

Transfer 1.6 wiring to 1.8 engine. Use 1.8 injectors of course. The 1.6 wiring fits fine. It's a pain to make the throttle body fit on the 1.8 intake, but it can be done. Widen the 2 bottom holes. It sounds scary, but just drill straight trough the throttle body.
Make an aluminum adapter plate in between intake and throttle body. I used 3mm thick aluminum (cuts easy). Thicker is better, but than you'll need longer studs / bolts. Cut it to the size of the 1.8 gasket. It'll appear to be too big for the 1.6 throttle body, but if you take a closer look, you'll see that it doesn't matter. Drill a tiny hole in the plate, about 2-3mm diameter. Drill this hole on the bottom, where it covers the oval hole in the 1.6 throttle body. I drilled a 1mm hole, but it is a bit too small (idle is now at 1000rpm). Grind the TPS mount, it'll interfere with the adapter (see pic).
Throttle cable is too long. Easy way, buy 1.8 cable. Fiddly way, make a new bracket. The egr pipe can stay, but you may remove the egr valve on the intake and plug its hole (the 1.6 ecu cannot control the egr valve so you need to block the passage if you remove it). I tapped it with 3/8 and put a plug in it.

Edit: when I did a 2nd engine swap a while later, I didn't bother drilling the tiny hole. Everything worked fine.

Transfer oil pressure sender from 1.6 to 1.8 (1.8 pressure sender is not compatible with 1.6 gauge). It sits next to the oil filter. 1.6 is BIG, 1.8 is very small (and crap). Lengthen wires to the CAS and fit 1.6 CAS onto 1.8 engine (if you're using a 99 engine). It fits on the exhaust side on the 1.8 (simply remove block off plate on the cam cover, it's a straight fit).  

exhaust system

If you use the complete 1.8 exhaust manifold and downpipe, you'll need to get a 1.8 catalytic converter too (the flanges are different).
You can use the 1.8 exhaust manifold and the 1.6 downpipe, but then you'll need to weld a flange for the O2 sensor on the 1.8 manifold (the 1.8 has it in the downpipe).  Also, the heat shield will need to be modified to clear the lambda sensor.

Good luck!