Ok a few months ago I bought a S5 t2 and basically just found out it has an S4 t2 engine in it. The electronics are not communicating at all understandably. It has all S5 turbo II wiring and the S5 turbo II ecu but only the engine is s4 turbo II. I bought this car so I can learn more about rotaries and I am learning a lot so far, as I just dropped the engine out of it. So these are the options I can come up with

1. Get an s5 turbo engine
2. go with a standalone ecu

option 1 seems like its going to be real expensive somewhere around/above $5,000 and I wont learn anything by buying another engine and dropping it in.

option 2 seems still expensive but not quite as much as getting a new engine. Since I have the engine out already and the compression is somewhat low (~65 in one and ~75 in the other) I figured it would be a good idea to rebuild it and get some more rotary experience. Planning to go with a Haltech Elite 550 and get a patch harness from Banzai racing to use the existing wiring harness (I have removed the wiring harness and I am testing continuity to make sure all the wires are proper) but if there is a different/more correct way to do it then I am open to suggestions. Trying not to have to get a whole new wiring harness since I know that mine is ok but if necessary then so be it. Also I know I'll need to wire up a wideband sensor, and intake temp sensor for the ecu, but are there any other sensors I'll need to make it all work properly?

Thanks, just trying to get a better understanding of what I can do in my situation.

 

so what parts are S4? if its just the engine its not too bad to get the S5 stuff working, if its the whole thing like the intakes and such, its more involved, but still possible.

 

As far as i can tell its the engine, lower intake manifold, upper intake manifold, and turbo. Everything else is true s5. The theory at the moment a previous owner blew up the original engine, sourced an s4 engine and thought it would it work but it never did.

 

It should work with a s5 car, s4 keg w/ all s5 electronics.

I believe it’s that simple. Just remember to account for the difference in spark plug location, but the s4 t2 keg is lower compression than s5 anyways.

I ran a s5 t2 keg in a s4 car with all s4 electronics for years, so a bit opposite from you.

You should specify your exact setup. I’m always surprised nobody ever posts pictures or videos, it’s 2022.

 

so the things that are different are the TPS, you need the S5 throttle body and TPS.
the Metering pump. you need the S5 one plugged in, but it won't bolt to the engine in a useable way unless you change the front cover for an S5 one. the thing can just be bolted to the fender, and you can premix/use the S4 pump though
injectors. the S5 injectors are different, easy to swap in.

the other tricky part is that mixing S4 and S5 can make stuff like the intercooler not fit and such.

plan B, maybe you put an ad up and see if someone has all of the S5 junk from the top of a JDM engine. you then end up with S4 engine, but all S5 stuff on top, which works

 

Could you elaborate a little more on spark plug location? Also my car is at a shop rn and we are having some disputes about things somewhat related to the rx7 but that's a whole other story on its own but here's some pictures of what I pulled out. Seems to me like all s4 stuff besides maybe the Trans?

 

Found an s5 turbo complete intake manifold (upper and lower), throttle body, and TPS all together. Also want to make sure I am understanding the omp correctly. The s4 is mechanical while the s5 is electrical, so I am plugging the s5 omp into the connector and just mounting it somewhere on the car but not really running any lines to it as I am just using it to trick the ECU into thinking it is working and doing something, while in reality the s4 OMP still on my engine is working through a passive system still injecting oil with the injectors right? Regardless I am going to premix, I have a whole bunch of Idemitsu waiting to get used up and I believe is 0.5oz per gallon with the omp, and 1oz per gallon without the omp, so I should premix at 0.5oz per gallon correct?

 

Just for more information the whole reason this is all occuring is a result from not getting any spark on the car, and from digging through that and calling various rotary shops and through a bunch of questions and stuff finally it was discovered that this engine is an s4 engine. Here's a link to my old thread about trying to get it to start.
https://www.rx7club.com/2nd-generati...spark-1154080/
Just want to understand: The electronics for the s5 wont communicate correctly with the s4 stuff because some of the s4 parts are mechanical, therefore not connected to the ecu, and the ecu won't allow for spark to occur because of that? So by keeping only the s4 engine, because it is so similar to the s5 engine, and putting the s5 UIM, LIM, throttle body, OMP and TPS on, plugging it all up to the S5 harness on the car already, the ecu will be happy and let the car start up right?

 

yep, the ECU needs to see the metering pump working, but since you can't bolt it to the engine without tearing the engine down, its just easier to put it out of the way somewhere

 

no, whatever is causing it not to spark is still there. you should be able to hook up the crank angle sensor and spin it by hand (ideally with a spare, or you pull the one from the engine), and it should spark.

if you give the ECU power and an RPM signal you should get spark

 

Regarding spark plug location,

s4 and s5 housings have different spacing between the leading and trailing spark plug. That’s it, but I think it does affect split timing so just something to keep in mind.