What is the best manifold insulator spacer to go on a VZ alloytech. 12mm or 25mm, what would the difference be?

The 12mm manifold insulator gives around a 10% increase in low down torque, while the 25mm item gives around a 13% increase over a broader range. In summary, *if you spend most of your time in the mid to upper RPM range then I would be leaning towards the 12mm manifold insulator. *if you spend most of your time in the low to mid RPM range then I would be leaning towards the 25mm manifold insulator. In saying the above though, you would be sincerely happy with either manifold insulator upgrade.