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Reducing engine variant bore seems to not affect valve float


Reducing bore reduces valve float because the valve is smaller and thus lighter, and this works on the family stage but change variant bore seems to have no impact on valve float, as though valve float only looks at family maximum bore.


I’ve certainly noticed this, but I assumed it was as intended.

Trim variants are limited to something ~10% reduction of bore, and a slightly lower bore variant isn’t likely going to have an entirely different valve train developed in the real world, only different pistons to suit the difference in machining. Conversely to reduce stroke only requires crank (although often connecting rods are changed too), and the smaller engine will enjoy the full benefits of lower piston speed without the need to engineer any other changes.


Wouldn’t that mean then that the valves wouldn’t ever be as large as possible unless the variant was at its minimum bore?

Or wait, is it that a debored variant has extra room between the valves? That would still be a lighter valve, though.


Your partly right, but the valves aren’t placed onto a flat disk the size of the bore, nor do they move parallel to it. Thus the outer edge of the valves when seated do not need to be within the width of the bore, and any difference in compression ratio can be made up with a thicker\thinner head gasket (common on older cars) or slight changes to the chamber shape between the valve seats, or even to the piston\rods themselves.


Indeed that is intentional, you are not changing the head design by lowering family bore. :slight_smile:


Hmmmm, I’d expect the bigger valves to have some kind of benefit at some point in the RPM range, at the moment all that changes is more drop off at the high end.