Valve recession with unleaded fuel (UL94)

My thought is that high utilization UND service included running all their engines from fresh overhaul, and a lot of those freshly overhauled engines were run for lots of hours after overhaul on unleaded only, enough to unearth the issue they’ve apparently now found. Also they were measured periodically after their overhaul cycle, in a structured program looking for valve recession specifically based on previous car experience.

Same thing with valve recession on freshly overhauled engines happened on cars and motorcycles in the late 70s/early 80s after unleaded fuel was first introduced, then used widely and inspected after long use. It wasn’t found until unleaded fuel was used exclusively for lots of hours on engines that had never been run on leaded fuel. Then rebuilders and manufacturers went in all kinds of wacky technical directions for a while before the issue was put to bed by the late-80s or so. It’ll be interesting to see if this is a repeat performance or not.

Hardened valve seats are not the answer for this issue based on industry experience. For example in about 1980 BMW installed hardened valve seats in their motorcycle engines as a preemptive measure against unleaded fuel then becoming the norm in the US. This ended up being a disaster, because hardened materials have worse thermal conductivity and made the exhaust valves run hot, that worsened the problem and valve recession was extreme. By 1985 they had moved to another alloy for both valves and seats (a matched pair) that solved the problem of their own making. Pre-1980 or so engines never had a major problem then or now regardless. So it’s not so simple but the valve and seat materials that generally work with unleaded fuel are now established.

Vic, I’d suggest reading easily found articles like this or this or this to understand that high thermal conductivity of valve seat materials in combination with strength/hardness is a key requirement in cooling the valves of an engine with hot (e.g. air cooled) cylinder heads, and preventing the valves from recessing. Less thermally stressed (lower power density and/or liquid cooled and/or multivalve) engines can more often get away with lower conductivity valve seats as long as they’re hard enough.

Strength/hardness and thermal conductivity are two things that are typically hard to find simultaneously in one material – alloys that one typically don’t have the other. Beryllium copper is an example of an optimum valve seat material in this regard. However it is also costly and toxic – nothing is perfect