Can a heavy piston rob power?

[TonyL]

I only ask as I'm sure I read JB's tuning of the Airsporter RB2 and he said the piston was too heavy, I was chronoing the Fenman last night and it is running at 10ftp with the drop in Diana gas ram, it was also doing the same with the spring kit I have, but that didn't have any preload slip washers fitted, I had a 5mm thick disk welded to the front of the Fenman piston so I could fit a Diana 35 seal, I first thought it might be too heavy but have just thought that it's been short stroked because of the longer seal and the added disk, so tat might be why it is lower power with spring fitted.

I think I read somewhere that Theoben rammer pistons are hefty old lumps because they have thick walls as they have a lot of pressure in them, so most probably heavier than a standard springer piston, and because of the extra weight they need extra high pressure to propell them.

... which as you say is possibly more likely to be down to effectively short stroking it ?

Yep, taking this into account, in this case I think the lower output is, most likely, down to losing some of that stroke. Sounding like a big, heavy piston to begin with also, so is going to require some extra ooommpphh to get it working properly? Upping spring force should help in terms of, hopefully, finding that power and quickening up the cycle. A lighter piston with the standard stroke should help, also. But if you are not wanting / able to source another piston, I'd be looking to up the spring (obviously keeping in mind the firing cycle and usability).