There's been a lot of discussion about cam specs here, lot's of it pretty technical. Some of it I understand, at least the basics, of lift, overlap, measured height. More duration seems to come with more lift, more overlap causes rough idle with lower vacuum. Lobe centers, which I assume is the center-line difference of intake and exhaust, fully open, confuses me a bit. I've degreed in cams, but aside from following the instructions ( it seems they were normally retarded a couple degrees) I don't really understand what that means or accomplishes. Why not just grind it to go in straight up. Looking at Mummert's site, he has a 280-I-07, and a 280-I-11. The difference appears to be the lobe centers at 107" and 111" respectively. On a street V-8, of say 300 CID, what's the difference driving from, say 1500 or 2000 to 4000 or 5000 rpm? Assuming the heads, manifolds etc. are the same, and compatible to the cam, why does it cause the difference? And finally, how do (real general here) bore stroke/ over square/ under square affect this? I don't like to seem too dumb, but I'm pretty sure I'm not the only one, and there's so many combination out there, there must be some differences of opinion even among cam designers. How about a little remedial education for some of us.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

As a general rule, camshafts are installed 2-4° advanced and not straight up.  This is for a couple of reasons with the first being to compensate for timing chain wear.  As a chain stretches from use, the cam retards itself so advancing the cam initially helps to delay this.  The second reason is chain elasticity where the chain stretches as the rpms go up and comes back in size as the rpms come back down.  This can be seen with a timing light once all the ignition timing is in and where you can actually see the timing start retarding while watching the timing marks with a light and varying the rpms.  That’s the camshaft actually retarding and in turn just dragging the ignition system along with it.

 

Regarding lobe centerline angles on a given camshaft, as they get narrower the overlap increases which in turn reduces the vacuum signal.  That’s simply due to both the intake and exhaust valves being open together for a longer period of time.  Narrower lobe centerlines typically make the torque values more peakish with a narrower operating range while wider lobe centerlines have broader power bands with reduced peak values.  Making the lobe centerline angle smaller also permits the intake valve to close earlier which allows the cranking compression values to also increase.  This gives an immediate increase in low end torque.  Because both valves are open simultaneously, this is where the choppiness or lope in an idle comes from and the more the centerline angle is reduced, then the more the lope or chop in the idle characteristics.

 

Here are some past threads related to camshafts and their installation.  Doing a search using different keywords will bring up a variety of discussions on the topic.

http://www.y-blocksforever.com/forums/Topic38920-3-1.aspx

http://www.y-blocksforever.com/forums/Topic42770-3-1.aspx

http://www.y-blocksforever.com/forums/Topic61758-3-2.aspx

http://www.y-blocksforever.com/forums/Topic61625-3-1.aspx

http://www.y-blocksforever.com/forums/Topic55379-3-6.aspx

http://www.y-blocksforever.com/forums/Topic46641-3-1.aspx

http://www.y-blocksforever.com/forums/Topic57116-3-1.aspx

Lorena, Texas (South of Waco)