So I’ve done some research already and can’t seem to find the blunt straight forward answer I’m looking for so I’m going ask and I understand if this gets removed
so I have a 86 fc that has all the wiring for a rotory but in my area finding one seems to very hard almost impossible unless I choose a rx8 renesis engine and how hard of a swap would that be is my question

 

Well as long as the Rx8 has been out I'm not aware of it being done yet if that answers your question. There's been a lot of talk about it over the years and on the surface it seems like it would be a decent swap if you didn't have any plans to go turbo.
How hard? You'll have to make or get someone else to make custom engine mounts of some sort as they mount completely different, but that's probably the easy part. I'm not even sure about the electronics. Probably would be better to just use a standalone ECU than to attempt to retrofit the RX8 ECU into the FC. If that's even possible. I imagine the electronics side of the swap is why it hasn't been done.

 

I mean I’ve found a couple threads of people doing it but they haven’t gone into much detail about doing it

 

i haven't seen anyone do it yet, although it seems like a good swap to me. 70more hp, slightly less weight, potentially while being emissions legal, and you can put mufflers on it if you want.
here is the list, and this is just the stuff that we think we know, there are for sure some things we don't know about...

Getting it in the Car: you'd need some kind of mount fabbed up. both mount in the middle of the engine, so potentially quite simple.
Clutch/Trans: you can use the FC trans, and appropriate clutch. the Rx8 trans fits, but you need to make a trans mount, and maybe a driveshaft, and then the shifter needs to move too. there is some combination of stock parts to move the shifter, but i don't know what it is (the Aisin transmission was used in the S1 Rx8, and a bunch of other cars)
Cooling system: it actually kind of looks like the Rx8 radiator and fan would fit the FC pretty well. plan B is FC radiator some E fan, and custom hoses. Rx8 uses 35mm hoses and the Rx7's are 38mm
Oil Cooler: you'd need some custom AN lines. the Rx8 setup could be made to work, but its probably also going to need some custom fab, Rx8 is wider
Exhaust: stock manifold, custom midpipe to your choice of muffler, easy!
Intake: its possible the stock stuff fits, but it would end up where the battery usually lives, aftermarket intake would probably help here.
Fuel: the Rx8 is returnless with a two speed pump. you would just remove the return line and the turbo and S5 cars have the two speed setup already

so the hard part, the Electronics. it does not look too bad, the stock engine wiring is fairly contained on the engine. it does need the powers and grounds hooked up. the stock ECU is flashable, so i think you can turn off a lot of the codes for the body modules, and OBD 2 stuff. still you can expect to spend a big chunk of the time with the wiring diagrams.

 

thank you this was actually very helpful and has most of the info I was looking for

 

plan B with the mounting might be that since the 13B-MSP is meant to sit lower and further back in the chassis, to just mount it lower and further back in the FC chassis, it might make the Rx8 shifter fit better, and since you need to make mounts anyways that part doesn't change much.

 

I'm reasonably certain that the Rx8 transmission is about 4" shorter, which is a pretty big difference. I'm thinking the even if the MSP engine is designed to mount further back, it would probably collide with the stock FC firewall in that arrangement. Unless you either cut and redesign it or "clearance" it with a big hammer.

I could be wrong though, my research into this swap never really got past the planning stages.

EDIT: You know what? I think I'm wrong about that from this side profile image.



I also see that the oil pan has a super low profile (I knew it was smaller, but not THAT much smaller). Looks like it could potentially work with the Rx8 transmission and stock shifter location. Changing the oil filter may be a pain though.

 

thank you for being active in this I really appreciate it I’m probably going this route and if anything it would probably be easiest to lengthen the drive shaft if anything
I’m in no rush and the price for a engine and tranny with engine harness is so cheap I know the renesis aren’t the best rotory but I think if I can get this done it would be very nice and reliable because I’m not going for power just want a rotory lol

 

I mean just doing a quick eBay search I can get a engine, tranny, harness,ecu from 1500 to 3000 which I also understand if I do 90% of the work myself it would still be another 1000$ to 2000$ in miscellaneous parts and materials
which best case I have a running fc with a rx8 drivetrain for 2500$ worse case 5000$ either way not awful in my opinion

 

the transmission in those pics is the 5 speed Rx8 trans, which is really similar to the T2 (and FD) transmission, you could most likely put a T2 tail housing right on it, and then that part would bolt in...
the 5 speed only came on the low power 4 port engines though, 192hp, so only 30 more than stock.

 

when it comes to fc engines, I learned to always tear them down to at least replace the soft seals. They dry up and cause smoke. Seen it happen to 5 of 5 engines.

Might want to factor this cost in. Either drop it in and gamble, or tear it apart and rebuild it anyways.. my 2cents