Hardened valve seats are useful in racing engines and diesels, but car makers stopped using them in gasoline-fueled road cars simply because they weren't worth

Hardened valve seats are useful in racing engines and diesels, but car makers stopped using them in gasoline-fueled road cars simply because they weren't worth the cost regardless of whether the fuel was leaded. Reports of excessive valve seat recession after the phaseout of TEL were all anecdotal, and for every mechanic who swears there was a a rash of worn seats, there are others who say there wasn't. The theory that leaded gas prevents micro-weld induced valve abrasion has not been verified in controlled tests. The final report from the EPA's Valve Seat Recession Working Group found no evidence that leaded gas reduces engine wear under any but the most extreme operating conditions: https://archive.epa.gov/international/air/web/pdf/vsr-finaldraft.pdf https://archive.epa.gov/international/air/web/pdf/vsr-finald... "In real world conditions, virtually no evidence of excessive valve wear has been found in vehicle or engine operation in normal everyday use, and several studies that monitored vehicles in actual daily service in countries that eliminated lead found no excessive valve wear."