I have heard of some people swapping out their original pistons for aluminum ones. Why? Is it meant to speed up the car? Are there other modifications people are doing during these engine restorations?
Usually done when you have to go oversize. Aluminum is what is available in oversize. Other common modifications are Stipe higher lift cam, Scat crank, and advanced cam gear.
Aluminum pistons lighten the reciprocating mass on the rather spindly crankshaft.
"Aluminum pistons lighten the reciprocating mass on the rather spindly crankshaft."
And that lighter reciprocating mass will reduce the wear on rod and main bearings.
Some folks believe that heavier pistons contribute to a smoother-running engine by increasing flywheel effect, but when one considers the start, stop, reverse-direction nature of pistons, that doesn't make sense.
Lighter pistons impose less stress and wear on connecting rods, bearings and the crankshaft (and anything you can do to reduce the amount of stress absorbed by the Model T's fragile crankshaft would be a good thing). -Because it takes less energy to start, stop and reverse direction of a lighter weight object than a heavier weight object, efficiency is increased and with that, to some degree, performance.
The ideal weight for reciprocating parts is zero.
Oops... Steve and Mike were quicker on the draw than me!
Used when you have to go oversized? Would that be because the cylinders are wider than normal from all the years of use? Do aluminum pistons increase speed at all?
Jason -- Cylinders wear into an oval shape and tapered from top to bottom. When that occurs, the cylinders need to be bored (oversize) to be round again, and non-tapered. When you have your cylinders bored, you will need larger pistons. New pistons are made of aluminum, because they're better (for the reasons mentioned above). Since those are what everyone wants, those are what are made today. I don't know of anyone producing cast iron Model T pistons these days, but there may be someone doing it.
In theory (and maybe in reality; I'll bow to the engineers on this one), an engine should be able to run a bit faster with a smaller reciprocating mass.
You'd probably have to have some sophisticated measuring equipment to see any HP gain from the lighter pistons. I seriously doubt you would notice it from the driver's seat. The real gain is that you are not as likely to receive a not-so-free membership to the Two Piece Crankshaft Club.