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Overlap is built into the grind of the cam; it's the separation between the intake and exhaust lobe of any given cylinder. The timing of the cam can't be out by much or you would get some piston to valve contact. The more overlap you have the shorter the time is that both intake and exhaust valves are closed on the compression stroke which is where you build cylinder pressure. With 8:1 pistons and relieved valve pockets you could easily be at 7.5:1 or less. What cams are you using?
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