Join Date: Feb 2004
Location: Victoria. Australia
Re: Solid cam confusion
I found this on the subject,
Overlap and LCA (LSA)
There is more to cam selection than lift and duration contrary to popular belief. The most critical aspect that is almost always overlooked is valve overlap. Another important aspect is intake valve closing point; which is directly related to duration but can also be manipulated a slight amount. These two factors will effect the engine's performance more than a person might think.
Overlap is the amount of time that both the intake and exhaust valves are open when the piston is around TDC. If there is too much overlap, vacuum is adversely affected, reversion will occur, a lean and damaging combustion process will occur, idle quality will suffer, pore low and midrange torque will result, reduced peak engine output and poor gas mileage will occur as well. If too little valve overlap is ground into the cam for a particular build, inadequate cylinder filling, reduced cylinder scavenging/filling and inadequate exhaust gas evacuation will occur; which will all hinder performance.
Valve overlap is directly affected by Duration, Lift and Lobe Center Angle. A cam for 350 CID with a somewhat wide LCA of 114 but long duration and high lift could very well have more overlap than a cam with a tighter 110 LCA that has a short duration and low lift.
The following are overlap's broken into groups that seem to be routinely followed that I find handy to reference. The shorter side of each group works for smaller CID while the longer side of each group is for larger CID.
10*-35*: trucks and RV's that require low-end torque and where gas mileage is a concern.
30*-55*: cars and trucks where idle quality, low-rpm performance, good street manners and strong off-idle throttle response are desired.
50*-75*: high performance cars where performance is the primary goal with reasonable street manners.
70*-95*: roundy round, open road race and strip engines that consistently operate at high rpm's
90*-115*: all-out drag-race engines.
If I'm building street/strip engine and the cam card shows 80* overlap, then I know I should be looking at another grind.
As mentioned earlier, valve overlap is directly affected by duration, lift and LCA. This is where cam selection starts to get difficult. When looking at grinds, all four have to be scrutinized. If the cam is ground on a 110 LCA and the overlap is 30, valve lift will be more during the overlap phase with .600" lift than 500" lift. Although the duration of overlap remained the same, the amount of overlap is increased. When lobe duration is added and the LCA remains the same, the duration of overlap and also the amount of overlap is increased. Keeping this in mind, a grind with an overlap of 30 and lift of .500" might be just right, but increasing lift to .600" might increase the amount of overlap to a point where it is excessive for the build and targeted performance will be lost. All of these considerations can become quite confusing very fast.
LCA is often referred as a reference to valve overlap. While a cam with a LCA of 106 will have more overlap than the same cam with a wider LCA of 114, the amount of overlap will vary greatly with differing lobe duration and lifts. For this reason, I don't rely on LCA alone. I look at the valve events and calculate the overlap if it is not provided with the cam specs.
Performance grinds will usually be ground on a LCA between 106 and 114. When I'm looking at grinds, I like to start out with a 110 LCA and fudge this number in either direction depending on the overlap and duration. If I know that the build will require a duration of 234 @ .050" to target peak power at 6000 rpm, I look at different grinds at this duration. I then compare their respective LCA's and overlaps to find a grind with the most appealing combination. I don't believe that there is a "perfect" LCA for any particular engine and the amount of overlap should be the primary concern.
The following are LCA's that are commonly associated with high performance engines in regards to carbureted SBC CID. These are only guidelines and should not be a primary bases for cam selection. They get me "in the ballpark" only and should be used with discretion.