My s38 turned over easily with a standard flooded lead acid Group24 battery. Its mounted in the trunk as well and wired with 0 gauge wire to the starter. The engine block is also grounded to the subframe via 0 gauge. The difference in compression ratio is negligible in my opinion. Not sure what to offer at this point, but I suspect low amperage whether its wire gauge, ground issue or other. If it were mine, I would check each cyl. compression just for grins then if all good, attach a new battery with big gauge jumper cables to the starter and engine block and see what happens.