Valve Overlap
07-19-2002 | 12:43 AM
#1
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Valve Overlap
Ok, i need an explanation on this.
When aggresive N/A cams create overlap (intake and exhaust valves open at the same time) how is this beneficial? I would think that having both valves open at the same time would cause problems.
When aggresive N/A cams create overlap (intake and exhaust valves open at the same time) how is this beneficial? I would think that having both valves open at the same time would cause problems.
07-19-2002 | 01:46 AM
#3
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From: Rancho Palos Verdes, CA
Vehicle: 2008 Toyota Prius 2006 Suzuki SV650S
In a nutshell.
The wave form engery (sound and motion), plus the kinetic energy (particles) of the exhaust leaving the cylinder, can, if designed right, create a lower than atsmopheric pressure in the combustion chamber. This aids in sucking the fresh air/fuel charge into the cylinder.
When done right, the more valve overlap is due to the intake valve opening earlier to take advantage of this lower than atsopheric pressure, and the exhaust valve staying open as late as possible to KEEP the pressure low.
The *overlaps* are timed for certian RPM. What works great at 6000 rpm, will give you a lousy idle. What works great at say 2000 rpm, probably won't affect your idle, but would hurt your top end. The length of overlap, and the timing of the overlap, should, in theory, be timed to have maximum advantage at your torque peak rpm. However, the profile of the cam, and the overlap can shift the torque peak around some.
It's a rather complicated process to go into without bringing up lots of technobabble and formulas. Red's better at explaining this stuff, he'll probably be able to give you a good explaination without the technobable and math.
The wave form engery (sound and motion), plus the kinetic energy (particles) of the exhaust leaving the cylinder, can, if designed right, create a lower than atsmopheric pressure in the combustion chamber. This aids in sucking the fresh air/fuel charge into the cylinder.
When done right, the more valve overlap is due to the intake valve opening earlier to take advantage of this lower than atsopheric pressure, and the exhaust valve staying open as late as possible to KEEP the pressure low.
The *overlaps* are timed for certian RPM. What works great at 6000 rpm, will give you a lousy idle. What works great at say 2000 rpm, probably won't affect your idle, but would hurt your top end. The length of overlap, and the timing of the overlap, should, in theory, be timed to have maximum advantage at your torque peak rpm. However, the profile of the cam, and the overlap can shift the torque peak around some.
It's a rather complicated process to go into without bringing up lots of technobabble and formulas. Red's better at explaining this stuff, he'll probably be able to give you a good explaination without the technobable and math.
07-19-2002 | 03:23 AM
#4
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The process that Random is referring to is called "scavenging." It makes use of the inertia of the exhaust gasses leaving the combustion chamber to actually draw more intake charge into the cylinder. This is the same kind of theory that goes into header design, and why small engines need some backpressure to maintain low end torque. The key is smooth flow.
At a certain RPM, the frequency of the exhaust discharges is such that for a small period of time, the exhaust system will actually have vacuum in it. Changing the exhaust system will change the harmonics, which is why cams and a header become something of a matched set, and both together will gain more power than the sum of either one alone.
At a certain RPM, the frequency of the exhaust discharges is such that for a small period of time, the exhaust system will actually have vacuum in it. Changing the exhaust system will change the harmonics, which is why cams and a header become something of a matched set, and both together will gain more power than the sum of either one alone.