Lobe Center

[wilf leek]

Great - a chance here to ask a damm fool English question!!

I understand the definition and meaning of lobe separation (used to be called overlap here in the UK).
What I don't know is the "limits" of the actual measurement, e.g. is 108 degrees "very" small, or 112 degrees "large"?
Anyone care to give a list of typical applications and the "normal" lobe sep for them, you know, from Drag car to Tow car??

[Excaliber]

360 degress in a circle. Less degress, smaller circle, more valve overlap (as I too am used to calling it). 180 degrees would be two lobes directly opposite each other, no over lap would be possible at all!

108 would open 4 degrees of rotation sooner than 112. At low rpm and 108 degrees the exhaust would tend to blow back up into the just opening intake valve port. "Flow" would be erratic and disturbed (like me). A rough idle would result.

As rpm increases' the "inertia" of the exhausting and entering gas flows would tend to keep them smooth and flowing in a constant direction. The exhausting gas velocity would actually help "pull in" the fresh intake charge, thus effieciency would be dramatically improved. NO overlap means NO help with one flow complementing the other.

Header\tube\muffler length would play a key role in maintaining this effect. Good cam profile but with a bad header setup, won't be as "effecient".

I'm "guessing" a motor built for torque, low rpm, will have little or no over lap, but what "number" is that? Gas flows at low rpm would not be fast enough to create the "inertial" effect. High rpm with very fast "gas flows" would be able to take advantage of a LOT of over lap. But if your cam "comes in" at 5000 rpm and the intake manifold works best at 4800 rpm and the headers like 6000,,,,you see the problem?

Ernie

[A Snake]

Two cams can have identical lift and duration but vary dramatically on lobe separation.

108 degrees is racing stuff. 112 grocery getter or RV.

Al

[petek]

Neat stuff on the net (including some graphs and pictures):

http://www.newcovenant.com/speedcraf...camshaft/3.htm

and

http://www.muzzys.com/articles/lobe_centers.html

[craigayl]

Venom S. In my 514, I recently changed form the .647 solid roller cam to the Comp Cam .585 Hydraulic flat tappet. After 5000 miles I had 6 broken valve springs with the roller. I changed to the CompCam for more reliability and for something that would be easier on the valve springs. I don't really know how they compare for power, but the .585 certainly sounds and feels just as healthy.