For Miata enthusiasts looking to boost their car’s performance, swapping the original 2.0L engine with a more powerful 2.5L engine is a popular and effective upgrade. This guide, drawing on insights from experienced mechanics and community forums like miata.net, will walk you through the process, highlighting what’s involved in this exciting engine swap for 2006-2015 Miatas. While discussions about ECU compatibility and interchangeability, such as those found in forums like site forum.miata.net regarding 2006-2015 ECU interchange, are crucial for complex modifications, this guide focuses primarily on the mechanical aspects of the 2.5L engine swap.
Performance Gains and Compatibility
The 2.5L engine offers a significant horsepower and torque increase compared to the stock 2.0L found in NC Miatas. While a stock 2.0L typically dynos around 140whp (wheel horsepower), a 2.5L swap can deliver around 170whp with bolt-on modifications. Further upgrades like street cams can push that to 180whp, race cams to 200whp, individual throttle bodies (ITBs) to 220whp, and high compression pistons with all the above modifications potentially reaching 240whp. Importantly, the 2.5L engine provides a broader torque curve, making even a 170hp 2.5L feel significantly stronger in the low to mid-range compared to a 2.0L with the same peak horsepower.
When it comes to compatibility, the good news is that the 2.5L engine, particularly the long block from 2009+ Mazda 3, Mazda 6, Ford Fusion, Mercury Milan, or any 2009+ 4-cylinder Duratec 2.5, shares many similarities with the 2.0L Miata engine. Motor mounts, transmission bolts, and the exhaust manifold are all direct bolt-ons, simplifying the swap process.
Key Differences and Required Modifications
While much is compatible, several key differences require attention during the 2.5L swap:
Oil Pan and Pickup
The 2.5L engine uses a different oil pan and oil pickup. Crucially, you must reuse your original 2.0L oil pickup and oil pan. Additionally, the 2.5L engine comes with balance shafts, which are generally recommended to be removed for performance applications. A balance shaft delete kit is a common solution, but another effective method is to tap the balance shaft holes for pipe thread and seal them with red Loctite. Welding is not recommended due to the cast material’s potential for cracking.
Crank Pulley
The crank pulley is another component that needs to be swapped. The 2.5L crank pulley has a different number of teeth compared to the Miata 2.0L pulley. You must reuse your 2.0L crank pulley. When reinstalling, always replace the washers and use a new crank bolt. Proper torque is critical: torque to 90 ft lbs, then turn an additional 90 degrees. A strong breaker bar is essential for this step.
Timing and TDC
Setting the timing requires finding Top Dead Center (TDC) for cylinder #1 before swapping the crank pulley. You can use a dial indicator or a specialized SST set. After finding TDC and setting the cams, temporarily tighten the crank pulley bolt to 25 ft lbs with the SSTs in place. It is vital to REMOVE all SSTs, including any crank locking bolts, before final tightening. Failure to do so can result in damage.
Electrical Components
Several electrical components differ between the 2.0L and 2.5L engines. Coil packs, spark plugs, and the wire harness “condenser” are all different. Reuse your 2.0L components for these items and discard the 2.5L equivalents.
PCV and Thermostat Housing
The PCV valve barb size is different on the 2.5L engine. Reuse your 2.0L PCV valve if you are not using a catch can. Similarly, reuse the 2.0L thermostat housing and coolant temperature sensor housing located at the back of the head, as these are also different on the 2.5L.
Cylinder Heads and Intake Manifold
The cylinder heads present a significant decision point. You can use either the 2.0L head or the 2.5L head. The 2.5L head offers larger ports and better flow on the intake side, which is beneficial for performance. However, the 2.5L head is not directly compatible with the 2.0L lower intake manifold. The 2.5L intake manifold is designed for front-wheel-drive applications and cannot be reused.
To use the 2.5L head, you will need to port-match your 2.0L lower intake manifold to the larger ports of the 2.5L head. This can be done using a die grinder with an aluminum-specific bit or even hand files. Aim to get within 1mm of the port size. The 2.0L gasket is slightly larger than the 2.0L manifold but slightly smaller than the 2.5L ports, making it a usable compromise for port matching. The 2.5L head does not use a traditional gasket but rather O-rings on its intake manifold.
For a more advanced approach, you could weld material onto your 2.0L RWD lower intake manifold to fully match the 2.5L head ports, then surface it and use RTV sealant. This is more labor-intensive but allows for a perfect port match. Even with port matching, gains from welding might be marginal as the runner size of the 2.0L manifold remains a limiting factor.
Additional Tips for Engine Swaps
For those performing the engine swap from the top, consider these helpful tips:
- Remove the cowl and strut tower mounts: This significantly simplifies engine removal and should only take about 10 minutes.
- Remove as many accessories as possible: Stripping down the engine beforehand makes handling easier.
- Disconnect the wiring harness: Remove the wiring harness from the fuse box and battery, leaving sensors connected to the engine.
- Clear valve cover components: Remove anything bolted to the valve cover, being careful with the thermostat housing at the back and removing the large water pipe.
- Unbolt the AC compressor: Unbolt the AC compressor from the block and move the AC lines above the engine. There’s no need to depressurize the AC system.
Addressing PCV Valve Cover Interference
One potential issue you might encounter is interference between the 2.5L valve cover’s PCV barb and the water pipe, as the barb is positioned further back on the 2.5L. Using your 2.0L valve cover can resolve this. Alternatively, if you are using a catch can system, this interference becomes a non-issue. This is also an opportune time to install an EGR block-off plate, especially for off-road applications, and eliminate the stock EGR valve.
Conclusion
Swapping a 2.5L engine into your 2006-2015 Miata is a rewarding upgrade that delivers a noticeable increase in power and torque. While it requires careful attention to detail and some component swapping, the process is largely straightforward thanks to the similarities between the engines. By following these guidelines and consulting resources like miata.net forums for community insights and detailed discussions on topics such as 2006-2015 ECU interchange, you can successfully enhance your Miata’s performance and driving experience. Remember to always prioritize safety and consult with experienced mechanics if you are unsure about any step in the process.
