*FYI: Vizard uses the term LCA (Lobe Centerline Angle) to identify what most of us know as LSA (Lobe Separation Angle). So in the formula below, he's not talking about the intake centerline. He's talking about the overall LSA.
Vizard has a strange way of calculating the cam specs. "Strange" in that the required duration is a derivative of other factors, rather than the seat of the pants guess we usually make when trying to optimize street manners vs. performance. If you guys look at the cam catalog I posted above, he specs different cams not only based on compression ratio but also displacement. Going on the interwebs shows why: He has a formula to calculate the LCA (Lobe Centerline Angle) that uses engine displacement as one of the variables. The optimum LCA for a smallblock chevy can be calculated as:
Vizard claims this can result in torque outputs of as much as 1.4 ft/lbs per cubic inch. So, for a 2nd gen LT1, that works out to:
128-(350/8/1.94*0.91) = 107.5 LCA (LSA). Are you starting to see why people have such a hard time accepting his claims? (LOL)
Next, you need to determine where in the rpm band you want that torque to occur:
10-1. This chart shows a typical spread of overlap for various applications. The middle of each range represents the amount for a typical two-valve engine having 21.6 ci of cylinder displacement for each inch diameter of intake valve. This puts multi-valve engines at the lower overlap of each range and big-displacement under-valved engines at the bigger overlap values of each range.
1) is for street towing
2) is regular street
3) is street performance
4) is street-strip
5) is race
6) is full race
*Note that these numbers are at .006" lift
So assuming we want something mainly for the street, the amount of overlap is 50-75 degrees, so pick 60 degrees overlap. I got lazy and just used the Wallace Racing cam overlap calculator to figure out the required duration. The answer is a 276 degree lobe. At 0.050", that means the 218/218 cam on the catalog page at 108 LSA. I note that Vizard specs it at 4 degrees advance. this is identical to what most cams are ground to.
This also explains why so many modern cams have split durations. Since most modern cams don't run that tight of an LSA, in order to get the required amount of overlap, they have to run more duration on the exhaust.
Now, how to test that? I went back to Lunati's website and clicked on their 2nd gen LT1/LT4 page. I found the cam that uses the 276 intake lobe. It happens to be the first cam on the page. Its specs are listed as 218/228 at 112 LSA at 0.050". Hey, whaddaya know, Lunati has their own version of the LT4 HOT cam. But check it out. The specs at 0.006" are 276/286 at 112 LSA. Going back to the Wallace Racing calculator, that works out to 57 degrees of overlap. Bingo. Now we know why they run more duration on the exhaust on so many cams. It's so they can get the correct amount of overlap.
So, according to Vizard, if you feel like the LT4 HOT cam is the cam for you, by going with Vizard's 218/218 @108 LSA version of the cam, it can net you as much as 20 ft/lbs over the regular HOT cam while actually running LESS duration.
Camshaft Overlap Calculator - Wallace Racing
Chevrolet Small Block LT1 and LT4 (1987 - Up) - Chevrolet/GM - Camshafts & Kits - Products
It apparently works on LS engines as well:
There are plenty of guys out there running the LT4 HOT cam. I'd LOVE to see how this cam stacks up against it, assuming the other mods are the same.