We are still on the same subject here, as in Chapter 9. We are going into more detail on valve-event timing and how the spec of the rest of the engine affects optimal events.
This Tech Tip is From the Full Book, DAVID VIZARD’S HOW TO BUILD HORSEPOWER. For a comprehensive guide on this entire subject you can visit this link:
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The topics we need to cover, in order of importance, are: overlap, lobe centerline angle, duration, and valve lift.
What I want to do here is to help you avoid buying a generic cam. That’s one where the cam catalog tells you it’s a good cam in a Chevy small-block with a displacement between 265 and 440 cubes. If you had cause to believe that one cam could be optimal for such a wide range of displacements, then you are going to find this chapter a real eye-opener.
Determining how much overlap is required for the job at hand sets the scene, so to speak, for the rest of the cam spec selection. Just how much should be used depends on what the engine’s primary function is. If it is truly a street engine, then idle quality, low-speed torque, and cruise fuel consumption are almost certainly high priorities. On the other hand, if it is a race motor, outright performance takes precedence over everything else. At the street end of the scale, small overlaps are called for, while the race end needs large amounts of overlap. This means the first factor we need to take into account is the primary function of the engine involved.
For a production engine intended to power a daily driver, overlap values of 10 to 35 degrees are common. Such amounts of overlap favor idle, mileage, and emission considerations. If we consider typical-performance street cams for an engine that’s most likely equipped with an efficient intake and exhaust, we find that overlap values that still satisfy true street requirements can typically fall between 30 and 55 degrees. At the other end of the scale, all-out race cams can have anywhere between 85 and 115 degrees of overlap.
So far, all I have done is give you a vague clue as to what your engine might specifically need for overlap. That’s no better than you could have obtained from many other sources. Obviously, I need to do better