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VALVE TIMING. Intake open before TDC Intake stays open long after BDC Exhaust opens long before BDC Exhaust stays open after TDC Page 194. VALVE OVERLAP. End of Exhaust , Begining of Intake strokes Longer overlap produces more power at high RPM Longer overlap produces poor vacuum at idle
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VALVE TIMING • Intake open before TDC • Intake stays open long after BDC • Exhaust opens long before BDC • Exhaust stays open after TDC • Page 194
VALVE OVERLAP • End of Exhaust , Begining of Intake strokes • Longer overlap produces more power at high RPM • Longer overlap produces poor vacuum at idle • Long overlap, (late exhaust closing) causes “Lope” • Long overlap cause valve to piston clearance problems • Page 196
CHECKING CAM TIMING • Degreeing in the cam • Dial Indicator • Degree Wheel • Use Intake centerline Method
ROCKER ARM RATIO • Standard Ratio is 1.5:1 • 1.6:1 and 1.75:1 are also common ratios • .300” lift at cam would be .450” at valve with 1.5 ratio rocker
MEASURING VALVE LIFT • Must use a “solid” lifter and Dial Indicator • Or special tool • Net Valve Lift - measured at cam • Gross Valve Lift - measured at valve
PERFORMANCE CAMSHAFTS • Camshafts are a compromise • Preformance - Economy • Low RPM Torque - High RPM Horsepower
PERFORMANCE CAMSHAFTS • Coil Bind • Valve Springs • Retainer to Valve Guide Clearance • Rocker arm slot length
LIFT • High Lift Cams increases all valve train wear • Minumum .125” clearance between • valve stem seal and retainer. • Limited by valve to piston clearance
DURATION • Longer duration increases RPM for Max Horsepower • Shorter duration produces more low and mid range Power • SAE at .006” • Performance manufactures (SAE) at .050” • High compression can offset for long duration