So here begins the story of fitting the k24 into the RX8. Spoiler alert, it was much more difficult than I ever could have imaged. The reason is that the k24 is a very tall and long engine. When you compare this to the pony keg size and shape of the Renesis you quickly realize the engine bay is not very conducive to fitting anything but a rotary engine.
Considering this is the second engine pull in three posts I won’t bore you with the details but with the engine out I could start measuring to estimate fitment. A friend of mine had access to a k24 and took some initial measurements for me. This allowed me to get a rough idea if it would fit and where the interferences would be. In theory it seemed tight but promising.
One of my requirements for engine choice was that it was similar in weight to the Renesis. This was a tall order because it’s such a small and compact engine but my hope was that the k24 was close enough to fit the bill. The Renesis weighed in at 282.5 lbs with all the necessary components for a functioning engine. Pretty light considering the only modification was a BHR lightweight flywheel. At this point it actually had me worried that the k24 was going to be significantly heavier considering how much larger it is.
Much to my surprise the initial weigh in of the k24 was not bad. It clocked in a 301 lbs and while its not a perfect apples-to-apples comparison with the outgoing engine it was close enough. It was missing a flywheel and clutch to be a perfect comparison but there was also a bunch of unnecessary parts that I would not be using and many of the OEM parts would be replaced with an aftermarket equivalent.
Fast forward to a completely assembled engine ready for install and it weighs in at 307.8 lbs. Add 4 lbs for the engine wiring harness, notice the Renesis weight does not include a 12 lb alternator and consider the BHR flywheel is 8 lbs lighter than stock and you get a comparative weight of 311.8 lbs (k24) to 302.5 lbs (Renesis). Even if these weights are off by 10-15 lbs it’s well within the weight criteria I was hoping for.
With the engine stripped down to the bare long block I could start the process of test fitting. Two things immediately stood out to me that were going to be problems. The crank case breather fitting and camshaft sensors protrude off what is now the rear of the engine and would be the first things to interfere with the firewall.
This picture pretty much sums up this project. The thing you think is your next hurdle is actually 3-4 hurdles down the road. I couldn’t even get the engine in position to see how the breather fitting and camshaft sensors fit against the firewall. The Renesis is so keg-like the cowl overhangs the engine by quite a lot making fitting a tall engine impossible without cutting.
I was lucky enough to have a bare k24 block at my disposal which made it much easier to pull in and out of the engine bay. By stacking the cylinder head and valve cover on the bare block I could get a rough idea how much of the cowl I needed to cut out.
I wanted to cut as little as possible to try an avoid breaching the firewall, however, the cowl and firewall join together as a single piece which made that goal impossible. I was able to get the engine into position while cutting relatively little but eventually would realize I needed to cut much more to allow for proper clearance for the breather, camshaft sensors and to easily get the valve cove on and off. As you can see here I’ve already cut off the nipple to the breather and it’s still right up against the firewall — more cutting was required. Eventually I was able to thread and fit a 90 degree fitting in it’s place but I bailed on that idea as more and more hoses and wires cramped the space. I ended up having a fitting welded on the side of the valve cover and routed it back to the intake through a catch can.
Long story short I had to cut a large chunk out of the cowl and recessed the firewall by about two inches. Thankfully there is not much behind the firewall that prevents you from doing this. Removing some insulation and pushing a wiring loom out of the way is all I needed to do. I didn’t have to remove the dash or cut anything behind the firewall to gain the extra real estate. The heat reflective tape was mostly to cover up the bare primer because I didn’t plan on paint matching the modified firewall. I also figured it was a good precaution since there is not usually a hot cylinder head that high up in the engine bay and the plastic HVAC unit sits directly behind the firewall.
With the top of the engine now clearing it was time to work on the bottom. With the sub-frame installed in the car there is no way to physically get a complete k24 into position. I was hoping there was a way around this by using another k24 oil pan or oil pump but upon inspection I pretty much lost all hope of that. The oil pump protrudes pretty much as far forward as it gets and all the way to the bottom of the oil pan. It’s in the exact wrong spot and I couldn’t see any other k-series or aftermarket parts helping in any way.
Time to start cutting (again). Before I started cutting I consulted a local speed shop 425 Motorsports and showed them what I was up against. My concern was that I would have to cut so much out of the sub-frame it would lose it’s structural integrity. They assured me if done properly it would retain the strength it needed and instructed me to mock up the clearances with poster board and keep the steering rack bolted to the sub-frame so it wouldn’t flex after I unbolted it from the chassis.
Much to my amazement it fit perfectly and all things considered I was pretty pleased with the result. This post doesn’t really do justice to what seemed like an eternity of cut, cut more, test fit, cut, test fit, cut more etc. This was really one of the most difficult parts of the project. The k24 just did not want to fit and my stubbornness in not wanting to cut more than I needed resulted in a painstaking fitment process. If you plan to take on this project yourself just do yourself a favor and grab the angle grinder and say to yourself “go big or go home”.