cams

[Squad]

Question: How does overlap, lobe separation affect the performance of a cam shaft. There are so many cams out there with different lifts- durations, I mostly pay attention to MP cams is there really that much difference between cam manufacturers.
Also MP B/RB cams have the same duration/lift for intake exhaust and most other companies don't. Why?

[340king]

Hey squad good question. I'll try to answer withwhat little I know about cams. I know it is little because I have read Gary Vizards article in SS&DI June 1994 five times and it still makes my head hurt each time.

Lobe seperation and overlap are joined at the hip. A change in one influences the other.

I'll start with overlap. When you look at four stroke engine design, overlap is hard to grasp. The easiest way I can explain its purpose is with an analogy. Think of the gasses as they exit the engine during the exhaust stroke. They are expanding and under pressure. As the exh. valve opens the pressure is reduced over time causing the exhaust gasses to exit slower. Then the piston comes up and pushes them out. This works great until the piston gets near TDC. Then the exhaust acts like pop in a straw. With the intake closed(finger tip over the end of the straw), the exhaust cannot come out. When you open the intake(straw) the exhaust(pop) runs out. That is what overlap does for your engine.

Some overlap is good. Proper sizing can be critical for good volumetric efficiency. As this exhaust is leaving, it pulls the intake gasses into the chamber by creating a slight vacuum. This is often refered to as scavenging. Pulse tuning, exhaust sizing and reversion all play into this.

Lobe seperation determines where in the rpm range your peak scavenging occurs. Tighter lobe seperations, ie. 104-106 boost low end torque by closing the intake valve sooner, and increase midrange by producing more overlap. This induces scavenging at middle rpm.

Conversely, wider lobe seperations cause the intake to close later and hurt peak torque, but the drop in overlap make them more efficient in the lower rpms. Wider seperations also pull harder up top due to pulse effects and overlap timing/duration.

The reason for split duration cams is to increase the amount of scavenging for a given intake duration without causing intake reversion. Reversion can occur when the intake is opened too soon and the exhaust pressure is too high. By splitting the duration, cam designers increased the overlap without subjecting the intake to higher exhaust pressures.

This is a very brief explanation and I recommend that you seek out the SS&DI article as it is much more in-depth, correct and thorough.

[This message has been edited by 340king (edited December 09, 1999).]