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Message: <blockquote data-quote="96blak54" data-source="post: 1529167" data-attributes="member: 14727">[ATTACH=full]28608[/ATTACH]So i made the decision to run only one angle, 45 degree, on the exhaust valve. No 3 angle seat and no back cut on the valve. Listening to a few engine building/ racing guys, their claim is low valve lift restriction improves high rpm housepower due to the prevention of exhaust reversion at the end of overlap. And supposedly the exhaust flow evacuation at any lift over .050", the valve seat size becomes null to the extreme exhaust pressures long as the exhaust port is adequate and ideal. Keep in mind im speaking in terms of naturally aspiration.Exhaust evacuation in the chain of cycle for any engine has a direct effect to horsepower and is one area completely misunderstood. One would think to be best the exhaust exiting the chamber quick as possible. We call this exhaust pressure relief "blow down" and ideally at high rpms we want this exhaust pressure to keep working for us pushing down on the slowing piston as it reachs the bottom of the stroke. The cam timing also affects this greatly and needs to be adjusted for the engine build. By restricting exhaust evacuation at the valve cracking off the seat, this slightly improves that restriction and adds a few more degree of pressure on the slowing piston reaching the bottom of the bore. We want pressure on the piston, but also in a state of relief as the piston begins to travel back up the bore....this is when the long tube headers take over pulling a negative pressure on the piston, clearing the cylinder of exhaust.My bad, started rambling on.Long story short, as the exhaust valve shuts at overlap end, the restriction of flow prevents exhaust getting pulled back into the chamber due to negative cylinder pressures while the piston begins down the bore.Ive learned exhaust reversion is revealed by simply looking at the intake ports after hard runs. If the intake ports have a dusty black/gray soot on the port walls, this is a clear sign of reversion.</blockquote>