> All modern engines are fuel injected, so the intake valve only ever sees filtered air. How modern are we talking? There are plenty of fuel injected engines

> All modern engines are fuel injected, so the intake valve only ever sees filtered air. How modern are we talking? There are plenty of fuel injected engines in production/use today that inject fuel into the manifold [1], right before the valve. Those valves definitely make contact with fuel-laden air, not to mention EGR[2] recycling exhaust gasses into the intake manifold for emissions/efficiency improvements. What you're talking about is direct injection, which is becoming more and more common, but is not "the norm" if you take the cross section of total vehicles on the road today [1] - https://en.wikipedia.org/wiki/Manifold_injection https://en.wikipedia.org/wiki/Manifold_injection [2] - https://en.wikipedia.org/wiki/Exhaust_gas_recirculation https://en.wikipedia.org/wiki/Exhaust_gas_recirculation