The MR20DD has an unusual arrangement where the exhaust ports for the center two cylinders are Siamese’ed. The non-direct injection version of the engine has 4 exhaust ports. Although this seems disastrous for performance, it is not as bad as it may seem, as these cylinders are as far as possible apart in the 4 stroke cycle. So we don’t have to worry about cross-contamination of the adjacent cylinder. We think Nissan did this to conserve exhaust heat to speed cat light-off during cold start, which reduces emissions. Although the large exhaust port volume of the center cylinders can be less than optimal for velocity and VE, the exhaust heat can help spool a turbo faster. We were also looking at the stock exhaust manifold and were thinking that it could easily be modified into an effective turbo manifold!
Another interesting feature is the super high volume EGR system. It is reminiscent of what can be found on some diesel engines without urea injection. It has a big metering valve and a water-cooled EGR gas cooler. It is the silver box on the left side of the engine in the pic above. We think this engine has high EGR flow for 2 reasons; to reduce NOX emissions due to the direct injection enabled high compression ratio (12:1) and to reduce pumping loss at part throttle by filling the cylinders with inert gas, reducing intake manifold vacuum. Interesting stuff! The water for the EGR cooler can be tapped to cool the turbocharger’s center section.
You can see the large EGR transfer tube on the right side of the engine. The thin wall plastic intake manifold is going to limit the amount of boost we can run without it failing. High boost would require a fabricated intake manifold most likely. We think that the stock manifold is probably OK for 10 psi or less. We’ll see!
This is the large EGR control valve. It is bigger than some small engines’ throttle bodies! We are going to get rid of this system for our off-road turbocharged use, as it will probably wreak havoc with performance tuning.