I can see 80-85% eff at the pump as possable because of the lower operating pressure but 90% does seem a bit much. I know I had to figure my valve volume a couple times because of mistakes...The first time I forgot to subtract the poppets volume from the valve volume...
Those are barrel pressures as the pellet travels, right?.... If we compare a 0.13333 CI valve at 1500 psi with a 0.200 CI valve at 1000 psi, they both start with the same amount of air inside, and in theory it took the same number of pumps to put it there.... (Actually, since pump efficiency drops with pressure, it might take an extra pump to get to 1500 with the small valve, but you get the idea).... However, if you look at the pressure as the pellet moves down the barrel, the smaller valve at higher pressure gives the pellet a harder initial "kick", so the velocity would be greater.... I ran the numbers, assuming a 14.3 gr. pellet and 60% internal efficiency, and the .200 CI valve at 1000 psi produces 11 FPE, while the 0.13333 CI valve at 1500 psi produces 13.3 FPE.... This is why stuffing a valve works IF the larger valve isn't being filled to the pressure capacity of the pump.... Once you start working at a fixed pressure, however, the larger valve will produce higher velocities (but require more pumps).... as per the graph at the beginning of this thread.... If you want higher efficiency (more velocity per pump), use a smaller valve.... If you want more pure FPE, use a larger valve, but be prepared to pump more.... Even then, going past a valve volume of 1/2 the barrel volume isn't really worth it, as shown above....Bob
That still seems unreasonably high for FPE, as I was only getting 9.3 FPE with 10 pumps and a 14.5" barrel (then add about 1 FPE for an 18").... and that with stock valve volume (~0.09 CI).... If we assume 80% pumping efficiency, and 10 pumps moving 18 cc per pump, then you are putting ( 10 x 18 x 0.8 ) = 144 cc of air into ~ 1.48 cc, so call it 100 bar (1450 psi).... With an 18" barrel, to reach 12.6 FPE, you would need 73% internal efficiency, which is very high.... I was only achieving about 60% internal efficiency, and that with very careful flow and porting work.... The larger you make the valve above stock, the less likely you are to achieve over 600 fps on 10 pumps....Bob