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.
Increasing the journal offset radius by 3mm also brings up another issue: now the piston will pop 3mm above the top of the block into the head at TDC. Since the VQ's deck height can't be changed, there are only two options to compensate for this. Either shorten the rod by 3mm, or modify the piston design and push the piston wrist pin position up by 3mm so the piston sits lower than stock. Here's where rod ratios come into play. Rod ratio is basically rod length divided by stroke length. Long rods or short strokes will have a large rod ratio (roughly 2:1), like high-revving engines such as the B16B or SR16DE. Short rods or long stroke will have a low rod ratio (approaching 1.5:1) like Nissan's 2.5-liter QR25. These can't spin fast, but have massive low-end torque. While rod ratio has implications on torque and engine speed, it's more an indication of how much side load the pistons are subjected to.
In our case, if rod length was decreased by 3mm to match the stock deck height, we would end up with a lower rod ratio of 1.61:1. Imagine the right triangle formed by rod, crank and piston position. A larger journal radius (which makes up the horizontal leg of the triangle) combined with a shorter rod (the diagonal leg) will increase the angle the rod forms to the cylinder wall. The larger the angle, the harder the piston shoves sideways into the cylinder wall. The increased rod angle would also require shorter piston skirts that might compound the issue of piston wobble.
In the VQ35DE, using a shorter rod to compensate for the stroke increase would be a double whammy. Instead, Cosworth chose to reduce the wrist pin depth and retain the stock rod length.
This gives a slightly higher rod ratio of 1.65, compared to the stock VQ35DE's 1.77, resulting in a peak piston speed increase of 9m/s (or, for ease of comprehension, 22mph). The only compromise here is that the ring packs also have to be moved up, reducing the space available for each ring land. Loss of ring land is bad, since it doesn't hold up as well under knock. But as we're running 100-octane gas under NA power, the trade-off is minimal. Small ring lands are also cleaner on emissions and could provide a fractional increase in power.
Compression ratio is only limited by the fuel used. And since our engine will deploy off-the-shelf, CNC-ported Cosworth cylinder heads-designed for both turbocharged or NA applications-our 12.1:1 compression ratio will come strictly from the high-compression Cosworth slugs thrown in. This ratio is based on previous experience, since combustion efficiency, chamber design, tuning and octane all interact to restrict how much compression an engine can run.
That takes care of our bottom end.
Next time, as we continue work on our Castrol Syntec Top Shop Challenge engine, we'll address all the less analytical and more black-art stuff requiring more real-world testing, like heads, ports, cams and tuning.