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EFI Tuning Fundamentals: Valve Timing Events

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Valve Timing Events

02.35

00:00 - In the last module I talked about the points in the engine cycle when the intake and exhaust valves open and close.
00:06 While I said that the intake valve opens at TDC and closes at BDC.
00:11 In reality this isn't quite the case.
00:14 A more realistic description of the valve opening and closing events are shown by this graph.
00:19 You can see here the full engine cycle of 720 degrees displayed with the position in crankshaft degrees displayed on the x-axis.
00:27 While valve lift is displayed on the y-axis.
00:30 The zero degree point represents TDC on the compression stroke.
00:35 And as we move to the right the first 180 degrees represents the power stroke.
00:40 We can see that before the piston reaches BDC the exhaust valve has already begun to open.
00:46 This point is often abbreviated in cam specifications sheets to EVO which stands for exhaust valve opens.
00:54 As we move past BDC we move into the exhaust stroke and we can see the exhaust valve lift climbs, peaks and then begins to close again.
01:03 We can see though that the exhaust valve actually remains open past TDC slightly into the intake stroke.
01:11 EVC on the graph stands for exhaust valve closes.
01:15 While we've been discussing the exhaust valve, you can see the intake valve has begun opening before we reach TDC.
01:22 So we have a brief period either side of TDC when both the intake and exhaust valves are open together.
01:28 This is known as the overlap period.
01:31 IVO on the graph stands for intake valve opens.
01:35 As we move though the intake stroke, the intake valve lift increases, peaks and then drops again.
01:41 And we can see the point labelled IVC or intake valve closes occuring after BDC as we move into the compression stroke.
01:50 This course isn't going to cover cam design or how to optimise your cam timing specifically, however it is worth understanding when the valve timing events are likely to occur in a real engine.
02:01 And the important point to take away from this section is that the valves don't open and close exactly at TDC and BDC.
02:09 Lastly, while we will be touching on continuously variable cam control in the engine management section table section later in the course, if you are working with engines that utilise variable valve timing, I highly recommend checking out our dedicated Variable Cam Control Tuning course once you've completed this fundamentals course.