When I started this project I selected the K24 over the K20 because, well, there is no replacement for displacement right? While that is true, the K20 contains components that make it a more performance-oriented power plant. Thankfully, Honda designs things in a very modular way which allows you to mix and match the best parts to create a best of both worlds.
There is a lot of debate on which cylinder head is the best flowing head. The K20 is largely regarded as the highest flowing cylinder head but the K24A2 I had from a 2006 Acura TSX is not a bad choice either. Fortunately (or unfortunately depending on how you look at it) the K24 cylinder head’s coolant outlets come off what is now the rear of the engine and this doesn’t allow for enough clearance from the firewall. The K20 head design has the coolant outlet exits from the left side of the head which made it a necessity for this swap. I sourced a K20Z1 head locally and fitted the 2006 K24A2 camshafts while retaining the 50 degree VTC gear.
Another area for improvement on the K24 is the oiling system. The K20 oil pump is a simpler and more performance-oriented design than the K24. Upgrading to the K20 oil pump is a common practice and allows for higher revving without cavitation. It’s a fairly simple process to adapt the K20 oil pump to a K24 block. KMiata has an OEM Type S Oil Pump Upgrade kit which contains everything you need. They also have a really useful video K24A2 Miata swap – Engine prep tutorial which makes the whole process even easier.
In addition to the upgraded oil pump I figured it would be essential to address lack of baffling in the oil pan. This was especially important since the engine is now mounted 90 degrees different than originally intended by Honda. There are quite a few different baffle systems available for the K-series but unfortunately most of them are designed for the aluminum K20 oil pan. Since I had already modified the subframe and built the motor mounts for K24 steel oil pan clearances, it would have been a lot of time, effort and money to switch to the K20 pan. Thankfully TracTuff came to the rescue with the TracTuff Oil Pan Baffle and the TracTuff Oil Pump Baffle (not pictured). These are compatible with oil pump and pan combo I had and would help keep oil from sloshing away from the sump during aggressive driving — especially under heavy braking where the oil has a tendency to slosh forward and up the front cover.
Selecting an exhaust manifold was fairly straight forward. There are only a couple of manifold designed for use in rear wheel drive configurations. KMiata sells a Race Header and a Street Header and after consulting them we decided the street headers was the best choice. I was a little concerned with how much real estate I had to work with and KMiata assured me the street header would provide better clearance. I still had to grind the chassis a little to get adequate clearance but the modification required was minimal.
The KMiata Street Header is a well-made and affordable solution but it was designed for a Miata not an RX8. The Miata exhaust travels down the driver’s side of the transmission tunnel where the RX8 uses the passenger side. KMiata provides a cross-over pipe which routes the exhaust between the oil pan and the transmission. Since this piece was not needed in the RX8 and I was not wanting to spend a ton of time and money on a custom exhaust I hacked up the cross-over pipe, added a flex-pipe and an RX8 flange to make an intermediate pipe which mated the header to the RX8 mid-pipe.
Continuing the theme of hard to find RWD compatible K-series parts — the intake manifold. The cheapest option is probably to to use an S2000 manifold with an adapter plate from Ballade Sports but I was having a difficult time finding an S2000 intake manifold for a decent price and a 20% off sale at Skunk2 convinced me to go with the Skunk2 Ultra Race Intake Manifold. It’s a two piece design so it can work with both FWD and RWD by reversing the plenum. Once reversed and installed I fitted a Skunk2 Pro 90mm Throttle Body which is cable actuated as opposed to the RX8’s factory throttle by wire. I figured this was a simpler approach but it would require me to swap out the RX8 accelerator pedal with one from an FD RX7 and fabricate a bracket to secure the throttle cable (not pictured).
One downside to this swap is that there is no real room to run any accessories. Even though there are plenty of options available for relocating the power steering pump, alternator and AC compressor, none of them seem to work in the RX8 chassis. I planned on retaining the RX8’s factory electric power steering rack so losing the power steering pump was fine, and as much as I want air conditioning, it’s a creature comfort I’ll have to live without. The Kmiata Idler Pulley allowed me to simply delete the power steering pump, AC compressor and idler pulley. It’s strange not having a tensioner on the belt but how it’s designed and the way you install you can actually get a fair amount of tension on the belt.
The last thing I threw in my shopping cart during the Skunk2 20% off sale was a Skunk2 Fuel Rail. Fuel rails are pretty basic items (at least for my needs) but the Skunk2 unit is a high quality piece that I’ve been really pleased with. Pictured here are the stock injectors but shortly after firing it up for the first time I discovered one of the was stuck open and dumping an excessive amount of fuel into cylinder #3. It was a good opportunity to pick up some Grams 1000cc Injectors to not only fix my problem but to ensure I would have adequate fuel down the road should I choose to add forced induction.
One of the last things to be fitted to the engine was a custom flywheel from ClutchMasters. Much to my surprise ordering a custom flywheel was much less daunting than I was anticipating. Clutchmasters has the crank and clutch bolt patterns for pretty much anything you could imagine and they only require a few measurements and specifications to build one for a custom application. It was shipped to my door in only a couple of weeks and it was only a couple hundred dollars more than an off the shelf unit.
The clutch I had was a fairly new ACT Heavy Duty clutch from my supercharged rotary setup. It’s good for 330 ft/lbs or torque which is more than enough for an NA setup. However, if I ever add boost it will definitely be a limiting factor.
I weighed the rotary engine with all the same parts and accessories attached and it weighed in at 285 lbs. I was really surprised to find a fully-dressed K24 was only 22 lbs more. With everything accounted for the next step was to squeeze this thing into the engine bay.