Hello I’m sure many of my questions could be answered by digging endlessly through forums but I would like to make a post about my situation specifically.

I have a complete s5 NA car with engine and sketchy wiring
i have a complete s4 NA car with an engine with rusted out coolant passages but all good wiring.

can I put the s5 engine into the s4 car without using any of the sketchy s5 wiring and ecu.

could I get away with using the s4 front plate and mechanical omp and keep the s5 internals and housings and such. Will all of the s4 parts bolt onto the s5 engine?

I know many if not almost all of the electronics for s5s do not fit into s4 harnesses

if I put the front cover of the s4 engine onto the s5 and use the s5 flywheel and counterbalance
can I just bolt on all s4 parts like the air box, intake, exhaust, fuel injectors and rail, radiator and be able to plug it all in and go or do I have to do some big electronic swapping.

the car does not need to pass emissions I just need it to run and drive

 

Did you post this recently on the subreddit? I thought I saw someone in a similar situation.

Everything will bolt up as you describe. Yes, flywheel and counterbalance need to match the rotating assembly, so you need those to stay with the S5 engine when you swap it. You will probably make about the same power as a stock S4, since much of the S5 power gain is in the intakes, but if you aren't concerned about this then that plan sounds fine. Then you can just use all the S4 electronics and you're good to go. Radiators don't need to be swapped, but since you have two I would just pick whichever looks like it's in better shape.

Keep in mind that the front cover has an o-ring underneath, and this o-ring changes from S4 to S5. If this o-ring pops out or is pinched during installation, pressurized oil can escape from that passage and leak down into the pan. This will not cause an external leak, but will cause a loss of oil pressure that can cause you to wipe out the main bearings. The o-ring you need should match the engine, but check the casting when you're in there to find out: https://mazdatrix.com/86-92-front-cover-o-ring/

Also, the engine needs to be facing up (with the main pulley towards the sky, I mean) when swapping the front cover. This is to avoid the torrington bearing falling down on the shaft and getting crushed when torquing the hub main bolt down. Do a search and you'll see the destruction that causes. After you've swapped the covers and torqued down the main hub bolt, ensure that you can still turn the engine by hand.

Lastly, S5 engines have a higher compression ratio than S4. I don't think the ECU will care since these cars run pretty rich, but hopefully someone else can chime in and confirm that since I have never actually tried this myself.

Good luck Hope it all works out.

 

Yes that was me on the subreddit, thank you for the information and tips they will undoubtedly be useful. I will be doing a complete hard and soft seal replacement on the engine before I Frankenstein it into the car.

hopefully I can get this done in a reasonable amount of time, I’m getting a little tired of seeing this car in my yard as an ornament

I’ll try to keep you updated!

 

Sounds good One thing though, someone in a different thread on a similar topic mentioned that the air injection passage on the S5 is a bit different than the S4. This should only matter for emissions, but try out the S4 lower intake and S5 lower intake on the S5 block and see which is a better fit. There is one stud on the lower manifold that is different between the two, but apparently you can just remove it and use the other studs and it's fine. Then you can use the S4 upper with either.

Most of the time people are going the other way (trying to retrofit S5 parts onto S4) but here's a thread where they discuss it and talk about the differences: https://www.rx7club.com/2nd-generati...e-help-985502/

 

I have grafted an S5 engine into an S4 car before.

It can be a nightmare if not done properly.

I was not afforded the parts to do it properly.

I can't offer much in the way of help because I've been dilligently trying to forget ever having done it; mostly using alcohol and trying to learn other more useful things to drown-out the horrid memories.

Having done it, here's what I would do:

Use the S4 engine computer, wiring, and run premix.

Use the S5 intake, run the aux port actuators with RPM switches from summit or jegs and an electric air pump from a newer GM vehicle with a couple air solenoids. This is easier than the wiring nightmare that was running an S5 ECU in conjunction with an S4 body harness.

 

Just IMO, define what is sketchy about the wiring and fix it. You will create significant issues doing the swap instead of repair.