This has been brewing for a while for me, and with the recent purchase of a complete RX8 powertrain I'm now getting SERIOUS about this. The hope of this thread is to collect some necessary tech info as well as share the progress of the project.

Here's the background - i own a 94 Miata that was purchased for my kids (16 and 18 now) to drive to school. the car handles great but def needs another 100 hp to make it interesting. i've considered the small block v8 transplant, and even a RX7 drivetrain, but the RX8 just makes sooo much more sense. NA, light weight, revvy, Mazda origin, etc. And best of all, there are very few in existence. About a month ago i ran across a '04 RX8 engine and 6-speed trans and bought it for crazy cheap. Question 1: how do i identify what engine exactly this is? Is it a "13B-MSP high power" version referenced in the electrical diagrams? there are numbers on the block, how do i xref.

I'm now in research mode trying to identify any serious deal-breakers/pitfalls. The most significant of which is powertrain control (ie, engine electronics). I've spent quite a bit of time digging into the various systems and have some issues i'm still concerned about. My primary intention (else dealbreaker) is to end up with a perfectly running RX8 powertrain in a Miata. The secondary goal is to eliminate any troublecodes. fortunately, things are looking promising!

So, here goes:

I plan to use both the RX8 engine and trans together (vs using the Miata trans). I will need to fabricate a new driveshaft and tunnel/bracket. I'm hoping the shifter location will put the engine and trans in a suitable spot. Since the Miata final drive is a bit taller than the RX8, i'll lose some low end grunt but the lighter car weight should more than make up for it.

I also plan to use an RX instrument cluster. That will simplify speedo, gauge, and tach functions as well as provide the MIL and security idiot lights.

Re the immobilizer function, i have two options: reflash the ECU by Racing Beat to remove immobilizer functionality, or install an entire MIS setup (key, coil, RKE module, and matching ECU). question: will the reflash effectively defeat the immobilizer system in the absence of the other components?

Also, i am concerned about the other vehicle systems that interface via CAN with the ECU such as ABS, EPS, TCM, DTC, TPMS. The instrument cluster will be connected, but none of the others. Will the absense of any of these components compromise/derate engine performance? Or will their absense simply just tell the ECU that those options are not installed?

Related to the above question, is the ECU specific to any set of options (like ABS, or DTC)? In other words, is there such a thing as a "base model" RX8 that doesnt have ABS, DTC, TPMS, or TCM? That leaves the EPS (power steering) interface to sort out, and if it's a 'read-only' controller then the ECU may not even know it's absent.

thanks in advance. this should be a fun project.

 

Heh. I've pursued this same path. Everything is basically suspended on my end due to life circumstances, but some of the points you will need to consider, and questions to your answers.

1) If the engine and transmission were from the same car, then as it was a 6sp MT, it is the more common 6-port engine. If you have the intake system on the engine, then you can also verify that way, as the 6 port has a greater complexity and 2 more runners than the 4 port.

2) There is actually a more reasonable option that pairs with the engine electronics. If you get an RX-8 ECU and wiring harness completely, it will run the engine, be viable for the accessport, the keys can be coded to the ECU like any RX-8 and work with the immobilizer, will enable full ODB2 compatibility, allow full ABS (assuming the sensors and wiring are run, TPMS (same consideration), and if you get a Touring or GT trim ECU it will give you TCS and DSC as well. If you don't want TCS / DSC, I highly recommend getting a Sport trim ECU, since the ECU won't look for them. If the ECU is looking for them and can't find them, it will be a perpetual irritant, and limp mode could be triggered.


When I was considering this project, by far the cheapest option was not to buy each component individually, but rather buying a complete RX-8 for ~$3-4k and removing everything you need from the same car. The car won't have things the ECU doesn't expect, the ECU will already be looking for everything that is there, and you can still part out the rest of the car for a bunch of your money back. Buying individually can run you >$4k and you can run into compatibility issues.


Mechanical considerations:
A) You already see the need for a custom driveshaft

B) The engine and transmission will NOT simply "be in the right space". You will have to do a bunch of fabrication work to get the position right. The biggest problem is that the Renesis e-shaft exists from the middle of the block, where as the original 1.8L exits from the bottom of the block. This means that if the engine at the same height at the oil pan, the transmission is easily several inches higher. The fabrication work needed is greatest for the NA Miata, a bit less for the NB, quite a bit less for the NC (nearly exact fit with the NC). The transmission tunnel for the NA usually needs to be raised to accomodate.
Another option here is getting the Flyin Miata tubular subframe. It will eliminate alot of fabrication by allowing you to mount the engine exactly where you need to. Firewall may still need modification.

C) The cooling system is going to be a bit of trouble. There isn't enough space in an NA for the RX-8's stock cooling system, especially with the engine probably being a bit farther forward. You will likely have to go completely custom in terms of pipe routing and radiator mounting. Same with adding at least 1 oil cooler, which should be easily exposed to lots of air flow. If you go with the dual OEM oil coolers that an 8 has, then you will still have to run custom lines. I would recommend a single larger oil cooler. And you will have to get creative for where you put it.

D) The NA 1.8L rear end is entirely strong enough to handle the torque, but might not be able to handle higher heat production. You will be entering unsure territory there. May want to consider swapping with an NC rear-end that is known to work fine.

E) There are fuel system differences, however you should be able to use the Miata's tank and use the RX-8's pump, lines, etc... Another point in favor of buying a full salvaged RX-8.


I'll have to look at my notes for more points.

 

More:

F) The exhaust from the NA 1.8L engine exists on the driver's side, the Renesis exhaust exists on the passenger side. The OEM manifold MIGHT fit, depending on where and how you mount the engine, however you will probably have to cross the exhaust to the other side of the driveshaft. Fortunately, the OEM manifold can easily be improved on, and the best header designs for the RX-8 require a long tube design with a much further back collector. Since each header runner is much smaller in diameter than the rest of the exhaust, this can work to your advantage to run the narrower runners "flat" to avoid ground clearance problems before joining them in the collector on the "correct" side of the driveshaft.