cgunnr

Alright, for starters, some of you may be wondering why this thread exists. Well, mainly because it doesn't yet, and after going through the mega annoying process of doing this and being unable to find resources, I figured it'd be nice to do a writeup so if someone decides to do it in the future, they have the resource. I did this because I blew a water jacket in my s4 motor, and I was able to score a streetported s5 keg for cheap and an s5 intake for free. Plus, i thought it would be silly and fun to have the slightly higher s5 power output and (in my opinion) prettier intake manifold. Now, on to the list of adjustments and modifications you'll need to make.

Note: this is for NA motors. I have no clue on anything T2 related.

Motor Changes
-I decided to retain the mechanical S4 OMP, so I swapped front covers. Super simple, basically just take the S4 Cover off and throw the S5 Cover on. There are some changes you'll have to make on the intake mani if you run an S4 one but i'll get to that further down the thread.
Motor Installation
Super straightforward, basically everything is the same, as long as you have emissions deleted (which I think you need to do, but don't quote me on that). That said, there area few changes of note.
-The S5 Coolant temp sensor (the gauge one on the rear iron) uses a different connector style, and is a different thread size/pitch than the S4. That said, the sensors are mechanically identical as far as I can tell (I tested with a multimeter getting them up to temp on my friends s4 vs my s5), and the s4 cluster gauge itself tends to be better and more responsive than the s5 from what I hear. So you're able to just change the spade connector type on the s4 harness and hook it up as you would with the s5. This will allow the temp gauge to work as intended.
-The S5 EGR Port is located in a different place. On the s4, it's a valve located on the top of the center iron, whereas in the s5, it's a rectangular hole cast in the side of the center iron, with a matching port on the s5 intake mani. If you're running an S4 Intake Manifold instead of an S5, you'll have to block off this port with steelstick, or you'll have a hell of a leak and a very smoky engine bay.
Due to this, you will want to run an S4 Lower Intake Manifold Gasket if you run an S4 Manifold. I've just found that it seals better. Obviously run the S5 one if you run an S5 manifold.
-The S4 Primary Fuel Rail will work on the s5 block. No changes are required there. The S4 Injectors also seat properly. My car ran fine with this setup without having to do much tuning, so you should be fine on stock fuel/ecu, BUT i do have an old school Apex'i SAFC II that I used to bump up the fuel to make it a little happier with the port setup/higher comp rotors.
All S4 Accessories are compatible as far as i've been able to tell in my year of dailying this car.

If you're running an S4 intake manifold, congrats, you've done the swap! If you want to run an S5 Manifold though, get ready....

Intake Manifold Swap (S5)
Note: most of this also applies if you're doing an s4 block with s5 intake manifold swap <3
This one is not easy. Vacuum stuff hooks up totally differently, the IAC valve is in a totally different place, and aux ports work totally different. Not to mention different mounting points for fuel rails etc. I'll give a quick overview of each major change here.
-Vacuum Things are decently self explanatory, but the wack ones I had were the OMP vacuum spider and the cruise control. These can basically hook up to any of the nipples that suck air in, as far as I've been able to tell. Just put them in similar spots to where they go on the S4 mani.
-Aux Port Actuation can be done in a about a million ways. IIRC, aaron cake's website has a great guide on RPM based electronic actuation, and I think people are even making kits nowadays. Or, if you want to run an air pump, you can always just hook it up like you'd hook it up on the S5, and it will work as intended. I'm lazy, so I just wired mine open and sacrificed low end.
-Thankfully, Sensors like the IAT sensor work just fine with the s5 manifold. No changes are required there.
-You need an S5 Secondary fuel rail. While the s4 primary rail works, mazda decided to completely change the upper intake manifold design and that requires changes to the injector spacing and mounting points for the secondary rail. Unfortunately if your s5 manifold didn't include one, you'll have to source one. (it's not that hard don't worry)
- The IAC Valve sucks. It's on the other side of the manifold from its s4 counterpart, and how you set it up is totally up to you. Follow the s5 method of routing the vacuum hose around the manifold, and don't worry about rerouting the coolant hose unless you realy really care about idle quality on super cold days. Just unscrew it. It's just there to warm up the IAC valve and I really haven't had any issues without changing how it's routed. You are technically able to run an S4 or S5 IAC valve, since they're identical except for the electrical connector. Personally though, i'd recommend running an s5 one if you can shell out the cash for one plus the new connector for the harness. You have to do some funky electrical connector modification to make the s4 one clear the fuel rail, since the connector sits on the top right where the rail is, creating a clearance problem. You will also have to extend the wires on the harness connector, since it sits on the other side. You need like 6-8 inches of wire to do this, and it's a pretty simple cut/splice/heatshrink job. Good excuse to rewrap and put new sheathing on your harness too.

Throttle Body

-Running an S5 throttle body requires no modification to my knowledge, however you will have to OMP delete if you do this, since there's no way to actuate the rod for the s4 OMP, and the s5 one is controlled by the ECU, which you're not running if you're reading this guide. To run an S4 throttle body though, you'll need to make several modifications.

-First, while the throttle body bolts align, the throttle body for the s4 has vacuum passages in the front of the casting, that don't match up with anything on the s5 intake manifold. You'll have to block these off with steelstik or jb weld or something.
-Additionally, you're going to run into a clearance issue. The rotating cylinder thing for port actuation sticks out quite a bit and will prevent you from getting the TB all the way on to the manifold. To remedy this, you'll have to cut off a part of the throttle body. The casting for the TB has a little half rectangular half semicurcular block in the bottom, which you'll be able to identify by it hitting the cylinder on the manifold. This is literally just a weird casting thing as far as I can tell, and serves no real purpose, so I just took a hacksaw to it. If you have a dremel it'll probably make your life a lot easier. If you're a real one with no power tools though you CAN get through it with a dull hacksaw though lol.

-Finally, you'll have to bend the OMP rod. The runners on the s5 intake mani stick way farther forward than the s4, meaning you have to bend the rod so that it can reach around the runners to the throttle body. there's no set way to do this but i just used two pairs of vice grips to finesse it. Took me like 5 minutes.

Congrats, you've swapped your s4 to an s5 while retaining the original harness and ECU. Obviously this is a really random and niche guide, but I struggled quite a bit trying to figure this one out, so thought it was worth providing this as a resource. Let me know if you have any questions or struggles. I'm going through this from memories and notes so it's possible that i forgot something, but I tried to be as thorough as possible. have fun, and good luck.