The 4-5 HP & torque seen by Ford Racing is pretty typical for UDPs.
Most of the power gain is via underdrive. The weight loss is less important because it is has small diameter (lower moment of inertia). 6 lbs lost here is less effective at freeing up power than a flywheel that is 3X the diameter. Also note that power gain will be more significant in lower gears, as benifits diminish with greater rotational inertia. There is an equation (where they discussed this in SCC about a year ago). A #10 lighter flywheel (on a zetec Focus) gained about 22 HP is 1st, about 9 HP in second, about 5 in 3rd, only 2 or so in forth. Sice we dyno in forth..that is all you will SEE when you dyno. But you will benifit from bigger power in low gears, percieved as faster reving. This is why flywheels are nice but a 6+ drop from a UDP would only see a small fraction of that gain..probably only noticed in first gear.
Its a nice mod, unless it screws up your crank harmonics..
