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All Springer/NP/PCP Air Gun Discussion General > Show Off Your Air Guns With Mods (SHOW and TELL)

An experiment in low-power springer tuning: Diana 240 .177 soft tune

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Jim-in-UK:
Before his untimely passing in 2015, a friend and I conducted a good deal of research into transfer ports, him largely from the physics standpoint (him being a Professor of engineering), me from experimental engineering.

Before the adoption of synthetic piston seals, most airguns from German manufacturers sold in the UK had 4mm diameter transfer ports, and they all had leather piston seals bearing lubricant that supplied a ready source of fuel for dieseling, which they relied on to make their muzzle energy. When they switched from leather to synthetic piston seals, the dieseling stopped, and they had to reduce TP diameter to nearer 3mm. So why?

Dieseling creates a pressure/temperature spike that gets the pellet moving earlier in the stroke (about 74% into the stroke with a typically 400psi start pressure 1980s pellet), and the 4mm port delayed the point at which sonic choking occurred in the barrel, which would otherwise occur while the piston still had some way to travel. Without dieseling, the pellet would not start until nearer 86% into the compression stroke, so sonic choking had to be advanced to closer coincide with the piston coming to the end of the stroke, which compensated for the loss of diesel energy with improved energy efficiency.

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