you assume your air pump (engine) is around 65% efficient with those calculation ...the higher the compression and the better the air flow to closer you get to 80% because you will never reach 100% but 80 is pretty dang good
I believe the 340's had 10:1 compression if I remember right. But people buy the manifolds and throw them on there engines and are able to get the them to run good. So I dont necessarily believe that the higher compression is what allowed it.