12/24/2023 0 Comments Valve timing diagram![]() To calculate the duration of any intake valve timing event, add 180° to the intake opening and closing time. Most of the air flow into the cylinder should therefore occur somewhere between 45° and 135° ATDC. Crankpin angle is critical because the air entering the cylinder doesn’t achieve maximum velocity until the crankpin approaches 45° after top dead center (ATDC). Let’s look at how air actually flows through a naturally aspirated engine and why the proper camshaft can aid in cylinder fill for a chosen RPM range. ![]() This is based on time in milliseconds and is critical to producing power. ![]() New parts don’t mate well with worn surfaces. Also never use new lifters with a worn cam or vise versa. If the lobes don’t measure up, don’t reuse the cam. At high RPM inertia of the air charge continues to fill the cylinder ABDC as the piston begins to ascend in the bore. Extending the duration also increases the responsiveness of the air/fuel mixture to piston location. This benefits breathing by placing the valve at a higher more useful lift as the induction stroke begins. By simply increasing duration (milliseconds), time is added to help fill the cylinder efficiently.Īs duration is added, the opening and closing point of the valve are extended. All engines will eventually “tap out” as mechanical movement becomes too fast for airflow to fill the cylinder. The valve is open for the same amount of degrees, but time opened diminishes as RPM is increased. As RPM increases a corresponding reduction in valve open time occurs. Cylinders hold an equal volume of air regardless of RPM.Įxample: an engine at 650 horsepower uses approximately the same amount of air charge as it does at idle. To make use of the added duration an increase in RPM is also necessary. Increased duration allows more of the air charge to fill the cylinder extremely critical at high RPM. For purposes of this article we will use IVC as the most critical.ĭuration of the intake lobe design is critical for building power. Considerable debate does exist as to which event (IVO or IVC) is the fundamental aspect of power generation. Even small changes in engine design can have serious effects. Intake events tend to be less forgiving to change than exhaust events. You will notice that the intake valve events are placed in the first two positions of importance. On some engines, you have to count the chain links between sprockets to get the proper alignment. Always refer to the OEM timing procedure so you know how the timing marks are supposed to be aligned. Exhaust valve opening (EVO) Overhead cam timing can be tricky on some engines. The valve timing events occur in this order of importance (well, this is debatable, somewhat).Ĥ.
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