K20 Swapping Mazda RX-8?
As you can tell from the title of this blog, I'm highly considering the K-Series motor. The better question is why though? Wouldn't it have been easier to just swap in another 13B Renesis? Absolutely. Would it have been more cost efficient? Probably on the front end. Would it have been cool? Not really, don't get upset my rotary elitist. Here's the deal, the RX-8 is what the NC Miata should have been with a hard top. That's in fact the whole reason behind me wanting an RX-8. I didn't want a Miata I wanted a true coupe. The irony is that its not even a coupe. Anyway, the RX-8 is an underrated chassis; it has great balance, a sharp turn-in, and a low seating position that makes it feel like an actual sports car and not just a converted street car.
Be warned, this is not an easy swap. It's not impossible but it is difficult
Before we dive deeper into all of this mess. It's important to know, there will be custom fab work and wiring. There will be headaches. You will want to give up at some point.You'll need to be serious with yourself and ask yourself a few questions before you start though. Questions, I asked myself as well.
- Is this my daily driver? If it is, get another daily
- Am I mentally ready to cut up a perfectly good car?
- Am I prepared to spend double than what I think I'll need budget-wise?
- Am I in this build for the long haul or do I just want to get it done as quickly as possible? If you want it done quick, look elsewhere
My responses
- Nope, this is not my daily. Check.
- Not really but, I will if I need to. I really would prefer a clean install.
- Is anyone really? I'm a normal dude with a normal job. So, not really.
- Patience, is not one of my virtues, however over-building and engineering is.
LS Swap it!
I didn't just want to throw an LS into this and be done with it. One, that's expensive and I am being cheap where I can. Two, that doesn't jive well with my JDM elitism. A lot my friends have been trying to convince me to just LS swap it. For a moment in time, I did consider that. I just couldn't get past the fact that, in reality; I could've just bought a C5 Corvette for the same amount of money that I would need to put into the car to make the LS work.
So, thats where the K Series comes into play. Sure, there have been quite a few swaps done out there and they all vary to some degree on completeness. Most we see out there have been hacked up pretty significantly and the others are all from Europe for some reason. None of which have been clearly documented in it's entirety. I know thats a tall ask from others and so; Thats where this blog comes into play. We're going to go through this together, maybe.
RX-8 Community and Support
There was one guy who was doing the same exact thing I'm doing now; documenting his build. However, sadly he sold the car and so it never got done. He still pays for the domain, kswappedrx8.com, which is a testament to this guys commitment to the community. For me, even if in the future I get tired of this build and sell it off, I'll be able to keep using this blog for other purposes for you all. Build after build. Anyway, I digress.
Is the K20 better than a K24?
TL;DR: The K20Z3 fits better in the RX-8 Chassis, than a K24 or even an LS. With caveats.
K20Z3 sitting on an engine stand
The engine that I may go with is a K20. To be specific the K20Z3. I chose this because well, they're popular, have a ton of aftermarket support, and some of them are cheap. Now you could ask, well why didn't you go with a K24? The K24 has more torque, can arguably make more power easier, and I could've gotten one out of a junkyard. You'd be right to all of those but, it comes down to simple physics. One thing I always admired about the rotary engine (13b,20b, etc.), is their ability to rev out very high. Close to 9,000 rpm. With a K24, its possible to do this as well but, the amount of money and wear and tear on that setup wouldn't make it worth it. On the other hand engines like the K20 series, are square! Meaning equal stroke and bore. Just off that alone that means that it'll be more stable at higher rpm for sustained periods. The K20Z3 can rev out to 8,000 rpm, factory. Same with the K20A2 and even some of the "R" K20A's can rev out higher than that. Factory! So, my thought process was, this is as close as I can get to the rotary's rpm range; without having to put in another rotary to scratch that high rpm limit itch.
Another reason I considering the K20, is simply because of the dimensions. When it came down to mounting the motor in the engine bay, I looked at a lot, and I mean a lot of images online. Most of the swaps were K24's for whatever reason and there was an awful lot of trimming, grinding, cutting, and smashing. The few K20's I saw, required less of that. That's not to say there were no cuts or grinds; as there certainly will be but, it is a physically smaller engine than the K24. In a build like this, everything that can be smaller should be taken into consideration.
Subframe Modification or Subframe Replacement; pick your poison
Looking at these motors through the lens of trying to fit them into a car that was never designed for it, takes a lot of thought. You're going to need to do something with the front subframe. The K24's when it comes to physcially mounting them look to need a bit more parts to make it work with either a notched subframe or a replacement tubular subframe. Exponentially, making things tighter in the engine bay; especially when it comes to firewall clearance. While the K20's should, in theory require less; they're smaller. This doesn't necessarily mean its easier, its just slightly smaller. However, this theory will be put to the test in future blogs.
What about the K20A3 and K20A2
What about them? Joking aside; these are also great alternatives. There are caveats though. The K20A3 out of the base RSX and EP3 Civic Si, are really just economy versions of their more powerful siblings. However, that's not to say that you shouldn't use them. If you can find a K20A3 in great condition and you're okay with the factory 160hp, for cheap; go for it! This build is already going to be expensive. Save money where you can. The K20A2 is a different story though. This is where I am making a compromise. The K20A2 out of the RSX Type-S, is probably one of the most sought after K20 in the US. They're not as plentiful as they used to be, and their prices reflect that. Finding one is hard and when you do find one; people know what they got.
So why is the K20A2 a compromise in this build of mine. Well, in reality, if I am really wanting this to be a top-tier build. I should be using a K20A2 head on a K20Z3 block. That's the ideal setup for a K-Swapped RX-8. Here's why. THe K20Z3 has two sensors on the back of the head, these two sensor correlate with i-VTEC. The K20Z3 has i-VTEC on both intake and exhaust. Not necessarily a bad thing, in fact it is a good thing. Its more modern than the K20A2 single i-VTEC. However, it means there is going to be less space to work with against the firewall when the K20Z3 is installed. Additionally, there is a water outlet on the back of the K20Z3 that will require a plate to fit it in the RX-8. That will just add complexity and take away from working space when its all said and done. What should be done ideally is using the K20A2 head, which has the water outlet coming out of the intake side; drastically making things easier. As well as only having one sensor come out the rear. So, my compromise is this, I got the K20Z3 for FREE. That's right FREE. Snatched it up off of FB Marketplace. However, the block had a crack in it. So I found another block for $260, again on FB Marketplace. Considering I got this engine for free and trying to find a K20A2 head is really difficult. The decision to use the K20Z3 head was made. Will I regret this decision? Possibly. Have I thrown away money? Additionally, I may not even opt for a K Swap in the end.
It's a K20Z3
Pretty tight clearances, for the firewall
Why the K24A2 isn't a great choice
So why isn’t the K24A2 the right choice for this RX-8 build of mine? Well, at first glance, it looks like a no-brainer. It’s got more displacement, more torque, and it shares a lot of compatibility with other K-series stuff. But once you start thinking about how it fits into the RX-8 specifically, the compromises start to show.The first issue is size. The K24 block is taller than the K20 by a noticeable amount. That extra height might not sound like much on paper, but in a chassis like the RX-8 where engine bay space is already tight, it becomes a headache. Hood clearance gets dicey real fast. You either have to lower the engine, which messes with your subframe and driveline angles, or you have to start shaving the hood or running a low-profile intake manifold, which adds even more complexity.
Then there’s the weight. It’s not like the K24 is some boat anchor, but it’s heavier than the K20. The RX-8’s biggest strength is how balanced it feels, and adding more weight up front starts to shift that. One of the reasons the K20Z3 appealed to me so much is because it keeps the front end light and responsive. It maintains that sharp, tossable feel the RX-8 is known for, while still giving you more reliability than the Renesis ever could.Also, the K24A2 isn’t quite the plug-and-play dream it seems. There’s extra work when it comes to fitment, coolant routing, and hood clearance. Sure, the torque bump is nice, but unless you’re trying to make big numbers, the K20Z3 gets you 90% of the way there with a cleaner install and fewer headaches. For what I’m trying to build, something reliable, fun, and relatively OEM-feeling; the K24 starts to feel like the wrong kind of overkill.Could it work? Definitely. People have done it. But for me, trying to keep things simple and focused, the K20Z3 makes more sense. It fits better, weighs less, and still revs like a maniac. And considering I got mine for free, it’s hard to argue with that.
Closing; next steps unknown
So where does that leave me? Honestly, somewhere between a decent plan and a mild identity crisis. The K20Z3 isn’t the perfect choice, and this swap isn’t going to be straightforward no matter how I go about it. The K24A2 is easier to find, offers more torque, and has plenty of aftermarket support, but it creates more problems than it solves in a chassis like the RX-8. The K20A2 would be ideal if it weren’t priced like some rare collector’s item. But at a certain point, you have to stop chasing the perfect motor and start working with what you’ve got.
That’s where the compromises begin to stack up. The K20 swap, while exciting, isn’t exactly clean. There’s no off-the-shelf, plug-and-play kit for this chassis. Engine mounts will likely be custom or adapted from something else. The wiring isn’t impossible, but it’s definitely tedious. Then you’ve got to think about the shifter placement, coolant routing, oil pan clearance, throttle cable or DBW integration, fuel system — none of it’s handed to you in a nice little box. You’ll probably have to trim the firewall. You’ll definitely be fiddling with the subframe. And that’s not even getting into the little surprises you won’t see coming until the engine is halfway in. It’s doable, but it’s not a bolt-it-in-on-Saturday kind of swap.
And yeah, in the back of my mind, the idea of just going LFX is still there. It makes sense. It’s modern, reliable, comes with factory AC support, and swap kits actually exist. It has torque, it sounds good, and it would probably get me driving sooner. But it’s heavier, and it changes the character of the car. The RX-8 was never about brute power; it was about feel. And while the LFX is tempting, part of me still wants to chase that high-revving, lightweight setup that feels closer to what the RX-8 should have been.
So for now, I’m still leaning toward the K20. It’s a mess, but it’s my mess. If it works, I’ll end up with a reliable, fun-to-drive RX-8 that doesn’t rely on apex seals or constant maintenance. If it doesn’t, well, I’ll learn something; and maybe I’ll finally give in to the LFX voices whispering from the back of my brain. Either way, the journey is the point.
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