This article is written for all the people out there who are considering swapping in either the Honda K20A2 or the K24Z3 into the Miata. The specific engine we will be looking for the Miata will be the BP6D engine which came in the final NB and is a 1.8 liter motor with VVT. 

The BP6D engine was a final evolution of the B6 engine from Mazda which traced its roots back to a 80’s design which could be boosted. There is a a general cost benefit ratio around the 160whp mark where any additional money invested in the BP6D engine could be better directed to an engine swap for both more power and more reliability. 

The K20A2 was high on my list for a long time, however, the more I dug into it, the more I feel that it does not represent good value compared to the K24Z3. The big advantage of this motor is the higher flowing exhaust flow on the head and better rod ratio and shorter stroke for revving the engine out. Unfortunately miata engine bays are cramped and some of the advantages of that higher flowing exhaust get compromised without the very best in header design.

This weakness is magnified in right hand drive applications. Between the oil filter, the steering column, subframe and overall tightness, there is just not a lot of room in there for good exhaust packaging for the K20A2. Due to its desirability in traditional Honda applications, this engine also costs significantly more than the K24Z3. 

One interesting build to explore if you are still interested in diving into the K20A2 route is the build by 949Racing called Deviate. Initially motivated by a screaming 9,000rpm redline and an engine build from Hytech, the team went to extraordinary lengths in solving oiling issues using an external pump and a myriad of other solutions to make it work. In the end, the car was sold and they moved over to the K24Z3 platform for their endurance racing dreams. 

Upon analysis, it looks like the K24Z3 is actually a scaled up version of the BP6D, with a far superior rotating assembly, head design and valve-train. A stock and healthy K24Z3 worth around $500USD will outperform a $10,000US built BP6D with no weight penalty. 

K24Z3

Capacity 2354.1cc

Bore 87mm

Stroke 99mm

rod 152mm

rod / stroke ratio: 1.535

BP6D

Capacity 1839.6cc

Bore 83.5

Stroke 85mm

rod 132.9

rod / stroke ratio: 1.56

Two crucial differences to note is that the VTC angle is 50 degrees on the K24Z3 vs 44 on the BP6D. This allows for a wider spread of torque across the rev range. Secondly the application of VTEC valve lift technology allows for a second aggressive cam profile to kick in at 5800 rpm that helps the engine pull harder to redline. Combined, this puts the K24Z3 in a totally different class than the BP6D. 

The major criticism of this engine is the log exhaust manifold, but thanks to headflow analysis provided by 4piston racing, we can see that it punches above its weight, all the way to a 7,800rpm redline, especially when compared to the BP6D. 

Stock K20A Intake 296 CFM

Stock K20A Exhaust 199 CFM

Ratio: 1.48

Stock K24Z3 R40 Intake 308 CFM

Stock K24Z3 R40 Exhaust 182 CFM

Ratio: 1.69

Stock BP6D Intake 210 CFM

Stock BP6D Exhaust 157 CFM

Ratio: 1.33

So keeping score, the K24Z3 engine is simply better than the BP6D. 10% higher compression, 33% more displacement, 30% more head flow, 8% higher redline, and 20% more torque all through the rev range. Fully dressed the K24Z3 weighs 158kg, which is exactly the same weight as the BP6W at 157.8kg. That’s not the end of the story. 4 Piston Racing teased a new cylinder head program that takes the R40 head an improves it beyond a stock K20A2 head. 

Stock K24Z3 R40 Intake 308 CFM

Stock K24Z3 R40 Exhaust 182 CFM

Ratio: 1.69

4P Pro R40 Intake 360 CFM

4P Pro R40 Exhaust 221 CFM

Ratio: 1.62

Using their CNC head, with a billet camshaft, Saenz connecting rods and 12.8 Wiseco pistons they achieved 340hp at around 8,000rpm at the crank with peak torque of 231 lbft at 7,800rpm. This might be an extreme example, but it really demonstrates what is possible with the K24AZ3 platform. 

The overall summary of my analysis is that for around 160whp, the BP6D with minor bolt ons will provide great value for money. However beyond that, you would be chasing diminishing returns, and it would make sense to move on to the K24Z3 platform instead.

To cap off this article, here is are some torque figures of the 3 engines, the absolute values are not as important as the shapes of the torque curves that will determine how the engine feels.