In our last story of our saga of turbocharging the MR20DD, we looked at the engine’s external features to see how they would be would be suitable for building a turbo system. Despite some odd features, like the siamese exhaust ports, we didn’t see anything that would make the engine totally unsuitable for turbocharging. With that, we proceeded to disassemble the engine to see how the engine internals looked.
First, we took off the oil pan where we found this assembly with balance shafts. We are not fans of balance shafts, as they spin at 2X the engine speed. They whip up the oil, heating and aerating it. They suck up about 5-10 hp and really don’t make an engine that much smoother. When building an engine we normally remove them as they do not have anything to do with attenuating the engine’s torsional or reciprocating imbalance, they just counteract a 4-cylinder engine’s up-and-down shaking moment. In the case of the MR20DD the balance shafts drive the oil pump, so we are not going to be able to easily remove them. Perhaps in later development we can, but not for now. On a positive note, the oil pump is large and seems like it will have good volume capability.
Turning our attention to the engine’s bottom end we were disappointed to find that the main caps were tiny and unsupported. This isn’t going to hold a ton of power and our dreams of pumping an easy 500 hp out of this engine ended right here. Remember that this engine isn’t designed for this and a stiffening bottom-end girdle can rob as much as 10 hp by adding windage losses. Reducing mileage eating friction at the intended target power was probably a compelling reason to not add excessive reinforcement.
On a positive note, the Turbocharged MR16DDT engine found in the Juke has a very similar bottom end and people have modded them to make almost 400 whp for street-driven cars. However, we cannot see this MR20DD lasting very long under hard use in its current form at a higher power level.
For right now, this bottom-end arrangement will have to do. If the budget was higher and if we had significantly more time available, we would align bore the block and drill the main caps for larger ARP studs. We would also come up with some sort of main cap reinforcement. Possibly a girdle tying all the caps together with the oil pan rails. For now, we will just keep our power targets and RPM limits lower.
Here is a look at the entire bottom end. The main cap bolts are pretty small and we will replace them with brand-new OEM bolts when reassembling the engine. The crank is partially counterweighted. For performance use we prefer full counterweighted cranks, but some performance potential was probably sacrificed to keep the reciprocating mass low.