Early generations of the VQ also used small bearings, which have less surface area to reduce friction and mass. The drawback is more load per bearing area. When coupled with a bearing material intolerant of high temperatures, that results in a lot of spun bearings in hard-driven cars. This is why we're seeing larger and larger bearing surfaces on updated versions of Nissan engines, as well as oil temperature sensors wired into the ECU. Add the fact that most new engines feature piston oil squirters, which transfer more heat from the pistons into the oil, and there's a snowball effect of hotter oil, bearing material transfer, increased oil clearances and reduced oil pressure.
Over-revving the stock VQ is also an issue. Newer engines use light pistons with small ring lands and low ring tension to decrease friction and reciprocating mass, and to clean up emissions. Under stock conditions, this shouldn't be a problem, but throw in boost, rich fuel mixtures and raised rev limits, and there's the potential for accelerated cylinder wear and eventual blow-by. The stock rev limiter was designed specifically so that piston speeds do not exceed 4100ft/second, based on the stock bore and stroke. Missed shifts or raised rev limiters could push the stock, low-tension, thin-strip rings beyond the margin of safety and either gouge the cylinder liner wall or cause ring flutter.
We saw the end result of both these issues in our VQ. The engine consumed and degraded oil at an excessive rate and it was the lack of proper lubrication that ultimately led to main and rod bearing failure. But, much to Nissan's credit, the engine steamed along like this for a good 30,000 miles. As long as we kept adding oil, the bearings kept going, although the blow-by and power loss kept getting worse.
Much of the burnt oil could be seen exiting the exhaust under boost or while the car was cold, so we had a suspicion that the stock rings and bore had taken a beating from being driven hard before reaching proper temperatures. And since the car was supercharged, boost only increased the blow-by effect-as well as the oil degradation-from fuel mixing with the oil that wasn't scraped away by the pistons or within the crankcase itself.
The Bottom End
Rarely do we have the opportunity to tear apart and completely rebuild an engine in our project cars, since the terms 'rebuilding engines' and 'cheap speed' never usually occur in the same sentence. It's a risky affair with a lot of potential pitfalls and expense if not done right. But in this case, building a high-performance bottom end able to withstand 500 wheel-hp is absolutely crucial.
The stock open-deck engine was designed to be light and strong enough only for the original 287bhp. Everything was designed from the ground up to save fuel, reduce weight and satisfy emissions regulations. In order to contain over twice the power, we would need much stronger internal components. Several options were open to us, but we decided to maintain the stock bore and stroke so we didn't have to deal with the limitations of increased piston speeds or gear ratios that didn't work for our power range.
We addressed compression ratio and piston strength with 8.5:1 compression forged aluminum pistons and rings from JE. The piston has been designed for forced induction, so it uses thicker rings (under more tension), as well as larger ring lands. The larger ring land (the area between the top of the piston and the first ring) adds significant resistance to damage from detonation. The stock piston uses a small ring land in order to reduce hydrocarbon emissions, as well as gain a couple of hp.