Valve train mods

·         Noblet

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Hi,
I heard that the valves on 2200 engines are now repelled with TWO coaxial springs.
What about that? purpose, and technical info?
Thanks
Louis Noblet
France

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·         Doug Smith

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Hi Louis,
The story of the dual valve springs, courtesy of Alex, is:
- Double valve springs add redundancy to the system, if one spring should happen to fail the valve will still be contained and the engine will continue to run (with reduced power). To prove this, an engine test run was conducted with the outer spring (i.e. the larger spring contributing the most seating pressure to the valve), broken (with an angle grinder) in two places and installed on exhaust valve along with an ordinary intact inner spring on #4 cylinder of a 4 cylinder engine. Engine test run showed that 2700rpm could be developed on the ground with a wooden prop (which would normally develop just under 3000rpm under normal circumstances).
- The springs are wound in the opposite direction to prevent spring binding (this is common practice in multi spring arrangements)
- With two different springs of different spring rate the valve train system is not inclined to resonant at the natural frequency of one or the other spring, this is mostly relevant because it makes the valve less inclined to bounce on the seat when closing.
- The double valve spring system generates more closed seating pressure (about 20% more) which improves valve sealing. This is particularly important for the exhaust valve in prevent high temperature combustion gases escaping during the power stroke and overheating the valve which can lead to failure.
- During design it was decided to use a lightly thin gauge wire for the large outer spring. This was to prevent the spring binding in the event the lifter pumped up solid and would not bleed down, damage would not result on the spring.

Best Regards,
Doug.

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·         Noblet

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Thanks Doug.
Could you pse tell us if Jab intends to provide kits to overhaul/upgrade old engines?
Regards
Louis Noblet

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·         Doug Smith

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Kits will become available, yes: anyone interested should get in touch with Jabiru spares directly (or their local dealer).
Regards,
Doug.

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