Today I replaced the closely wound copper (?) wire regenerator in my Ross type stirling engine with a much looser one (lower fill factor, more flow area) made of knitted stainless wire (actually one of those kettle lime-scale traps). With the inner sleeve and regenerator fitted into the hot cap I performed a flow test (alright, I blew down it!) which showed that the looser mesh did indeed give significantly lower resistance to air flow.
Once started and thoroughly warmed through (this takes some time, I think due to the substantial lump of iron on the top end of the inner sleeve) the engine ran up to a higher speed than before, no doubt due to lower pumping losses, it did not however feel significantly more torquey.
After a good long run I took the hot end apart and was rewarded with a nice range of oxide colours on the regenerator mesh indicating the temperature gradient through the engine. If I could remember the colour/temperature relationship from school metalwork (and wasn't colour blind) I might even be able to estimate the temperatures involved.
Next material I have to hand is pan scourer made of coiled up stainless strip.
Photo of regenerator and sleeve after run at :-(last pic in 'all hot air' album)