Performance engine preparation Valve events What are they?
Performance engine preparation Valve events What are they? How are they measured? Which cam lobe leads the other? Performance engine preparation Valve events IVC (Intake valve closing) Determines effective compression ratio
Piston direction @ IVC ? Early High cylinder pressure, detonation Late Low cylinder pressure, intake reversion Occurs after BDC to take advantage of inertia Performance engine preparation Valve events EVO (Exhaust valve opening) End of power stroke Piston direction @ EVO ?
Early Shortened power stroke, lower output Late Incomplete evacuation, more effort required to remove exhaust gas (pumping loss) Blow down is between EVO & BDC Performance engine preparation Valve events IVO (Intake valve opening) End of exhaust stroke & beginning of overlap
Piston direction @ IVO ? Early Exhaust gas dilutes intake Late Reduced intake stroke & efficiency Ideal Cylinder & manifold pressures are equal Performance engine preparation Valve events EVC (Exhaust valve closing) End of overlap period Piston direction @ EVC ?
Early Pressure rise at end of exhaust stroke Resists piston travel Reduces intake flow Late Shortened intake stroke Exhaust gas dilutes intake Manifold vacuum drops Performance engine preparation Camshaft terms Duration Valve open period
Lift Cam lift or valve lift Overlap End of exhaust stroke, both valves are open Base circle Time valve is closed Ramp To take up valve train slack before opening To reduce shock on valve closing Flanks Determine opening & closing rates Limited by lifter diameter Nose Transition from opening to closing Lobe centerline Centerline of the lobe in relation to crank angle
Lobe centers Angle between intake & exhaust lobes Performance engine preparation Camshaft terms Lobe taper For lifter rotation Symmetrical cam lobes Same opening & closing contour of lobes Asymmetrical cam lobes Different contours ground on opening & closing flanks Dual pattern grinds Intake & exhaust lobes ground to
different duration and/or lift Performance engine preparation Cam selection considerations Intake Be realistic about RPM range & redline Measure or calculate static compression ratio Choose IVC for 7:1 effective comp ratio Duration based on centerline Lift based on flow data or valve diameter
Performance engine preparation Cam selection considerations Exhaust Does exhaust flow 75% (80% for turbo) of intake? Add duration for low percentages Reduce duration for high percentages Good port efficiency is preferred to long duration More duration = more overlap = poor vacuum Exhaust valve open as far as possible by BDC
Performance engine preparation Cam selection considerations Lobe centers Narrow lobe centers . . . Increase overlap Improve VE at high speeds Increase intake reversion (lope) Reduce vacuum
Performance engine preparation Cam selection considerations (forced induction) For selecting duration, include CR and boost Increase in compression = boost pressure atmospheric pres Final compression ratio = static CR (increase + 1) Performance engine preparation Cam selection considerations (forced induction) Exhaust flow should be higher than NA engine
80% compared to 75% with NA Increased exhaust duration required to deal with higher VE Lobe centers may be spread to reduce overlap
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