A few things we would like to know. What engine and is freshly rebuilt with high compression? Is battery CCA 650 or higher? And do you shut off all other unnecessary power uses when cranking? If jumping relay to start causes same problem you're eliminating all the car wiring, and just have cable to gnd. on the engine and cable to the starter or starter as the problem. I hope you cleaned terminals to battery very good, both Gnd & positive. Not too many things left after this. Also how old is battery?
