Excerpt from this online tech tip, from the book CHEVY BIG-BLOCKS: HOW TO BUILD MAX PERFORMANCE ON A BUDGET. by David Vizard (geared towards BBC but he applies to many engines)
https://www.chevydiy.com/chevy-big-bloc ... in-events/
"The Importance of Overlap
The events that occur in the exhaust/intake overlap period are of utmost importance. They take place before the piston really gets underway down the bore on the intake stroke. From many hours studying pressure traces of a variety of engines, one thing becomes clear, and it is the very point that makes the overlap the most important valve event, rather than the often-supposed intake closing point. This is an important point you should never let slip from your mind when it comes to valve events for maximum output...
...It should now be clear that the most important factor toward both big torque numbers and top-end power is the way you deal with over-lap. The “overlap triangle” (Figure 9.2) must be positioned at about TDC so it experiences the optimal benefits of the very strong exhaust extraction pulse that a performance exhaust system can deliver. Anything short of optimal is, in effect, like compromising an otherwise free induction event. In addition, this is critical in the case of an under-valved big-block; you need to get the intake valve underway well in advance of the piston demand. This produces a higher lift earlier on in the piston phase of the induction cycle and thus partially compensates for an intake valve that is too small for the job."
Text for the cam image:
Fig. 9.3. The most important cam parameters are the intake (1) and exhaust (2) centerlines, the LCA (6) and its relation to cam advance (A) and retard (R). Remember, cam advance/retard (7) assumes the crank is held still during cam adjustment and that is why the advance and retard appears the wrong way around. The intake duration (3) and the exhaust duration (4) positions on the cam are controlled by the LCA (6), and this positioning, along with the duration, dictates what the overlap (5) is. For a Chevy big-block, the lobe lift of the intake is a very important factor. Because the cam is physically bigger (1.950-inch journal diameter) than a small-block (1.872-inch journal diameter), the lobe can, for a given duration, have a higher lift. However, be aware that many cam companies use the same profiles for a small-block. If the profile is designed especially for a big-block and fully utilizes the extra diameter, it can result in a profile with as much as 0.012-inch greater lift.
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