KMiata is proud to announce a brand new engine swap option for the NA and NB Miata chassis. It’s still a K series, but with some significant changes that will bring broad appeal to lots more Miata racers and enthusiasts at a lower price point. We think this swap is a huge game-changer for the Miata world.
First, here’s some background on this project. Our current KMiata swap relies on the popular older K series engines. These 2.4L and 2.0L K24A and K20A variants were manufactured from 2002-2008 and were thoroughly embraced by the Honda engine swap community immediately. No four cylinder engine had ever done what the K series could do, which is why it still stands head and shoulders above all other four cylinder engine options on the market.
In 2009, Honda threw a wrench into the K series platform with the release of the new K24Z3 engines found in the 2009+ TSX and 2008+ Accords (K24Z3). Instead of individual exhaust ports, Honda cut costs and met emissions regulations by designing the R40 cylinder head with an integrated exhaust manifold and a single exit exhaust port.
The aftermarket community saw this limitation to exhaust flow and lost interest. Additionally, these new engines used completely different wiring harnesses, ECUs, and an incompatible crank sensor, which made them nearly impossible to use for engine swaps.
While our current K24A2 swap is a fantastic option, we were hearing the same consistent feedback from a portion of our customer base:
- 200whp is plenty for my street/track Miata
- My racing class requires me to run a stock Miata subframe
- I need to detune my K24A2 for my desired racing class
- The K24A2 swap is beautiful, but a bit outside my budget
This feedback caused us to give a second look at the K24Z3 engine about 18 months ago. The engines are inexpensive, widely available, strong, reliable, should be good for over 200whp with a tune. Additionally, the main reason our original K24A2 swap uses an aftermarket subframe is to clear a 4-2-1 race header. The K24Z3 single exhaust port could give us the tighter exhaust packaging needed to run a stock, unmodified subframe.
In order for a K24Z3 swap to be done, our entire swap kit would need to be reworked from the ground up. The goal was to create a K swap that could be completed by the customer for $2000 less than a K24A2 swap and checked all of our performance boxes.
The ideal engine to use for this swap is the K24Z3 engine found in the 2009-2014 Acura TSX. They are cheap and plentiful. Our test engine was only $350 delivered from a local salvage yard.
The K24Z3 found in the Honda Accord can also be used, but does have a slightly lower compression ratio (10.7:1 vs. 11:1). Because of this, we opted to stick with the TSX variant.
The other compatible option is the K24Z7 engine from the 2012-2015 Civic Si. However, we’ve found these engines to be quite a bit more expensive and they have identical specs to the K24Z3 from the TSX.
As stated above, the K24Z3 features a higher compression ratio (11:1) than the K24A2 (10.5:1). It has iVTEC on the intake cam only, and also includes a 50 degree VTC gear from the factory, so there’s no need to upgrade it like we do for the K24A2 engines. The intake ports are a bit larger, the valves are larger, and the cam lobes are larger. The connecting rods are also visibly thicker and stronger than all older K24 engines.
Here’s a chart that breaks down the specs of the K24Z3 versus the K24A2:
A key benefit of the K24Z3 swap is that the stock Miata subframe can be retained. Not only does this lower costs, but it also simplifies installation and makes the swap legal for popular classes like NASA ST4 and TT4.
Since we now have CNC machining capabilities in our shop, we decided to design billet aluminum mounts instead of welded steel mounts. Additionally, we incorporated mount pads from the GM LS engines to accommodate customers looking to build full street cars with an OEM level of vibration damping.
We used Energy poly mount pads in our test vehicle designed for track use, but an OEM style rubber mount also fits. The lower mount point bolts to the subframe in the factory mount location for a clean, easy install that requires no subframe modification.
Adapter Plate, Flywheel, and Clutch
Our proven K series to Miata transmission adapter plate and flywheel are re-used for the K24Z3 swap, which pairs with any 1.8L Miata clutch kit.
One part of the existing clutch setup proved to be a hurdle for our exhaust routing: the factory Miata slave cylinder. Since the K series exhaust port is on the passenger side of the vehicle, the slave cylinder occupies most of the space needed for exhaust routing. Any pipe over 2” won’t fit.
Our solution to this problem was to eliminate the slave cylinder completely and design our own hydraulic release bearing from scratch. The end result replaces the factory release bearing, guide tube, clutch fork, and slave cylinder for a clean, reliable solution that frees up space in the transmission tunnel and also gives an exceptional clutch pedal feel.
Left side is the 5-speed piece, and the 6-speed piece is on the right:
Here’s the 5-speed unit installed:
This bearing is paired with a ¾” Wilwood master cylinder and braided stainless clutch line and bleed line which is all included in the kit.
The K24Z3 exhaust system includes a 3 inch downpipe attached to the head, with an oval section under the transmission so the exhaust can be routed in the factory Miata location on the driver side of the tunnel and PPF. A complete stainless exhaust system with resonator and muffler is also available. Our prototype is pictured here, and production pieces will have a few less welds:
Ground clearance is only reduced by ½” just behind the front wheels at the crossover pipe thanks to the oval section.
And with the slave cylinder deleted our 3 inch downpipe fits with no issue at all.
The intake manifold used for this swap has been in the works for two and a half years and is something we’re really proud of.
For full details, check out the KMiata RWD Intake Manifold product page. Long story short, it’s packed with features not available in any other manifold. It’s a reliable one piece OEM style cast design with geometry designed specifically for a RWD mounted K series engine. Unlike all other manifolds used for RWD swaps, this manifold clears the stock idler pulley and factory belt routing. Use a 1360mm 7 rib belt with the stock tensioner and you’re good to go.
It’s also the only manifold that fits on the older PRB and RBB heads as well as the newer R40 heads found on the K24Z3. Just use a new OEM manifold gasket and supplied hardware, no adapters are needed.
A K series idle air control valve port is also located on the bottom, so the engine idles like stock regardless of temperature. The kit also includes the 74mm cast throttle body pictured.
Oil Pan and Pump
In order to fit a K series in the stock Miata subframe, a new oil pan design was needed. Final product photos of the aluminum pan are coming soon. This new pan uses the same style sump with a K20 oil pump, the KMiata oil pump adapter, and it’s own pickup tube. Since the K20 oil pump doesn’t bolt directly to the K24Z3 block, our oil pump kit also comes with a modified OEM windage tray and bracket, as well as the necessary parts to relocate the oil filter to the same location on the block as a K24A engine.
The factory K24Z3 electronics are very different than the older K series engines. Older ECUs, wiring harnesses, and crank sensors are all incompatible. The 2009+ TSX uses CAN bus, has a much more complex immobilizer system, and can only be tuned by reflashing the ECU though the OBD2 port. Because of this, we decided to look at other simpler options with the goal of having cross-compatibility of all of our KMiata wiring harnesses and electronics.
The main hurdle to running this new engine on the older 02-04 RSX harness and Kpro is the crank position sensor. The K24Z3 has a trigger wheel built onto the crankshaft on the flywheel side, while the older engines have the crank sensor in the timing chain cover behind the crank pulley. And unfortunately the timing chain covers are not interchangeable due to a different bolt pattern. For reference, here’s a shot of the K24Z3 cover on the left and the K24A2 cover on the right. Notice the black crank sensor plug on the K24A2 cover.
Our solution? Design and machine our own timing chain cover that has the K24Z3 bolt pattern, but has a provision for the old style crank sensor:
Initially all swap kits will include this machined billet cover, but we are already designing a cast aluminum cover that will drastically reduce machine time and look OEM.
And yes, we will be offering this chain cover separately, which opens up the possibility to use K24Z3 blocks in all sorts of swap platforms.
Power Output and Delivery
So the big question is what kind of power does the K24Z3 make? We had a target of 200whp, as this was the magic number everyone was looking for.
After 40 minutes on the dyno, our goal was achieved.
206whp, 165wtq, 7800 RPM rev limiter. Torque is available everywhere throughout the RPM range, and the car is a blast to drive. Since tuning, we’ve spent a few days on track with multiple drivers and the feedback has been overwhelmingly positive. It runs and drives like a stock Miata, but with another 90 whp! Reliability has been identical to a stock car as well. Not bad for a $350 junkyard engine.
It remains to be seen what additional N/A tuning options are available to gain more power. The obvious answer is to swap on a K20 cylinder head, but then the stock subframe creates some packaging constraints. Thanks to our new-found exhaust clearance with no slave cylinder, we just may have to develop a real header that fits with the stock subframe.
The other obvious benefit of this engine is that it can be turbocharged very easily. We plan on doing some turbo testing in upcoming months. Thanks to our stout BMW transmission options, making 500+ whp from a stock block K24Z3 reliably should be pretty straightforward.
Availability and Pricing
Introductory pricing on the K24Z3 swap kit is $4295.00 and includes the following items:
-K series to Miata transmission adapter plate
-K series to Miata flywheel
-Machined engine mounts
-Engine mount pads
-Baffled aluminum oil pan with pickup tube
-KMiata K20 oil pump adapter
-Hydraulic clutch release bearing with clutch lines (5 or 6-speed)
-Wilwood ¾” master cylinder kit
-KMiata RWD intake manifold
-74mm throttle body
-Wiring conversion harness
-K24Z3 upper coolant neck
-Slim flywheel dust cover
-Billet aluminum timing chain cover for K24A crank sensor
-3” downpipe with flex
-Optional upgrade: full 3” exhaust system with resonator and muffler (price to be announced shortly)
Customers will be responsible for adding all OEM Honda parts, an ECU, 1.8L clutch kit of choice, and miscellaneous bits like hoses and lines.
If you’re interested in receiving one of the first K24Z3 swap kits, be sure to subscribe to our email newsletter so you’re notified of the pre-order. And as always, email us with any questions at firstname.lastname@example.org. Thanks for reading!