Tetraethyl lead as an additive raises the octane, which means you can run higher compression before knocking occurs, and it also deposited a small amount of metallic lead on the exhaust valve seat after the burn. The small deposits were enough to stop the very hot exhaust valve trying to fuse itself to the head, and then tearing that fused area out when the valve next opened.I don't think lead had anything to do with compression, that would be octane. Lead was used to lubricate the valves, mostly the exhaust IIRC. John
That weld/tear was what created valve seat erosion when the lead was removed from fuel. Modern castings / different metallurgy solves the problem without using additives.