Geek theory
By picking a manufacturer like Cosworth, we suddenly have a wealth of engineering resources available: engine simulation software, flow bench data and computational fluid dynamics (CFD) head flow analysis. These guys possess the same tools OEMs use to design an engine from scratch. In theory, We could run engine simulations of how different bore, stroke, rod length, piston, compression ratio and displacement combinations would offer optimum power, torque, flow and engine speeds.
Our original hope was to reverse-engineer the VQ and analyze the engine to see why Nissan made certain choices in its design and what could be optimized without sacrificing durability or its innate character. But while Cosworth has the technology, the labor involved is far beyond the scope of this project-this isn't an IRL or an F1 engine we're talking about here.
As powerful as these analysis and modeling tools are, they're still only guides to be used in conjunction with practical and realworld knowledge. So we've gone for a more conventional method of design and tuning, working around the VQ's basic architecture.
Real-world tuning
Our high-output VQ will be loosely based on Cosworth's concept for a drop-in crate motor, featuring an assortment of parts already released or in testing. The Top Shop motor will take it up another notch.
Since we're using a stock block, our engine design is constrained by some basic physical limitations. The Castrol Syntec Top Shop Challenge makes no restrictions on displacement or flow, so we've started from the bottom end to aximize our total displacement and take advantage of the lack of a displacement modifier for NA engines.
A simple approach to increasing displacement is to punch out a motor as far as its bore spacing will allow and add as much stroke as possible. This works for old rev-limited V8s with cast iron blocks (where piston speeds aren't such a concern, since reciprocation mass ultimately limits revs). But in a VQ with the potential to spin up to 9000rpm, that's not the case. From a basic displacement perspective, the available room allows Cosworth to increase stock stroke by 6mm from 81.4 to 87.4mm and bore (limited by the factory bore spacing) from 95.5 to 96.0mm, increasing total displacement from 3498cc to 3796cc, or 3.8 liters.
Increasing the stroke by 6mm has a noticeable effect on piston speeds-a significant concern for an NA engine designed for highrev power. The original dimensions are over-square (larger bore than stroke) with a bore/stroke ratio of 1.17, while the 3.8-liter dimensions would bring the engine closer to square (equal bore and stroke) with a ratio of 1.10. Although the engine is still over-square, which is typically good for higher revs, the mean piston speed at 8500rpm has increased from 23.1m/s to 24.8m/s; 25 m/s is roughly F1 engine territory.
However, mean piston speed (or the average speed of a piston through one revolution) only helps classify the type of engine. Higher speeds usually mean higher performance. When changing an engine's internal geometry, such as adding stroke and changing rod lengths, what matters more is the piston velocity profile and the amount of sideways thrust added to the piston.
When an engine is stroked, the rod journals get moved further outboard from the crank centerline. The original VQ35DE has a stroke of 81.4mm, meaning the rod journal rotates 40.7mm (or half the stroke) from the crank center axis. This is because an engine's stroke is the total vertical distance the piston travels as the crank rotates 180 degrees between one and six o'clock, which translates exactly to the distance the rod journal has to travel. By adding 6mm of stroke, Cosworth had to increase the journal offset radius by 3mm. The added radius has a side effect, though, since it will shove the piston harder against the cylinder wall as the crank sweeps from one o'clock to five o'clock on the combustion stroke. This can increase wear, drag and may introduce ring flutter and piston wobble at high speeds.