Speaking from experience with other cars, lighter flywheels won’t make much noticable difference in acceleration. The flywheel isnt just there to slip the clutch in first, it’s mostly there to dampen out the torque that fluctuates because of the combustion pulses (not to be confused with the crank and driveline torsional vibration/chatter, which is where the actual crank damper and dual mass flywheels come in) . It’s true that heavier flywheels are used by most OEMs to make slipping the clutch and everyday driving easier
Where light flywheels make the biggest difference is in rev-matching. An ultra light aluminum flywheel and 3 or 4-puck clutch makes for beautiful clean downshifts (and fast up shifts) because the engine and transmission under no load can change speed quickly.
Aluminum flywheels are usually a terrible, unnecessary idea in a street car that sees any kind of stop and go in traffic or stoplights/signs. Middle of the road “chromoly” flywheels are a good compromise.
Flywheel weights are a bit deceptive when it comes to comparing between vehicles, but on this car, 11 lb is probably a bit on the light side. A 15-18lb single mass flywheel is probably a decent compromise.
On a Miata (which I’m more familiar with having done a chromoly and driven a car with an aluminum flywheel once), the equivalents are 8lb and ~11-13lb flywheels.
The slightly larger flywheel on the Fiesta means the larger flywheel has a lot more “dead” weight at the core as the vast majority of the “work” is done by the very outermost rim of the flywheel, and the actual inertia of the flywheels is probably similar (8->11, 13->17)
Aluminum flywheels are also a terrible idea for the street because the separate friction surface insert is not well cooled by the rest of the flywheel, so they are much easier to overheat.
TL;DR if you’re not absolutely sure of what you’re doing, and aren’t building a race car that will never see stoplights, hills or traffic, but you still want a lighter flywheel, go with the middle of the road single mass chromoly flywheel (15-18lb) which is still much lighter than stock, and I guarantee you won’t totally hate it.
You also won’t be disappointed in the difference it makes compared with the stock one, but it will still be forgiving enough on the street to drive around town.
