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[Steve]

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[Steve]

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[Steve]

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[scarceller]

Lobe separation angle is likely the most important number to get correct. A lot of cam design tools calculate this number first! A LSA of 100 means that the intake valve reaches max lift at 100deg ATDC. Why is LSA important? Because it dictates the peak torque potential for the cyl. A few cam designers describe it this way, the larger the diameter of the bore the more LSA you need. Look at he factory cam specs (see Bruce Anderson's book) and you'll notice how the smaller bores had smaller LSAs. Here's a formula called the 128 rule written by David Vizard for estimating LSA, it's very interesting formula and get you a LSA to high degree of accuracy.

LSA = 128 - (CID/# of Cylinders/In Valve dia. in inches x 0.91)

Just for fun let's run the numbers for a 98mm 3.4L bore

111.73 = 128 – (207cid/6/1.93*0.91)
Using David’s 128 rule we calculate best LCA for the 3.4L to be LSA=112

David has built 1000s of motors, although not 911 motors he has done a lot of hemi engine work. That formula is simply a guideline to sanity checking a cams LSA for a given bore size.

His procedure for picking a cam has these basic steps:
1 - Select LSA
2 - Select duration based on RPM range
3 - Select lift based on flow bench numbers
4 - Select static compression based off intake valve close event, you need to pay close attention here as compression can NOT start till intake valve closes. You then figure out what the dynamic compression will be from the close event to the top of stroke. Ignoring this step will have a cam improperly matched to static compression!
5 - Let overlap fall as it may. David seems to not worry so much about overlap as it's a function of getting the above steps correct first. Others may disagree with him on this but he simply points out that overlap is a means to an end in getting LCA and duration as desired. I found his theory here very interesting.

Here's the 128 rule calculator with a few more conditions considered, this calculator takes into account inlet valve size as it's also important:
http://www.wallaceracing.com/calc-lca-vizard.php

David explains his 128 rule and theory in detail here:
http://www.speedtalk.com/forum/viewtopi ... 35#p590898