LSA....what does it mean

[strokerjlk]

It's yet another of those terms in which Pope's proscription regarding the effect of dilettante scholarship serves us well.

The percentage of otherwise superficially proficient people who understand the subject after a single read of even the most helpful article is really, really small.
The number that actually have any need to understand it would fit in my bathroom.

Second-guessing your engine builder, head porter or cam manufacturer is pretty much in the same league with telling your heart surgeon that he's installing your replacement mitral valve improperly.
He's not, which is why he isn't listening.

depends on the builder,porter ,cam designers knowledge.
a brief read of this thread ,shows why you have to second guess some in this industry.

[panic]

In my experience, your comment will receive the same attention and respect regardless of the "professional's" level of competence.

"There is a strong tendency among engine builders... toward "false causation", or improper deconstruction, in which a favorable outcome (more power) is attributed to something they predicted, planned, made, installed &c. (rather than accidents, luck, coincidence, acts of God, &c.).
You'd be surprised at how many successful engine builders have only a general idea of what is actually happening inside an engine in terms of thermodynamics and other laws of physics, but continue to build engines (and make money) because they're duplicating a successful prior project.
In my experience, more people are capable of building a strong engine than of understanding why it worked.
The assumption they make is "My car is fast, and faster than yours. Therefore I'm an expert, my analysis of how the engine functions is accurate, and my predictions are reliable".
This happens because of "positive feedback" (or, what would now be called a "closed loop"), in which the prior results are used to determine future designs:
»   if it's faster, you'll use the same method the next time
»   if it's slower or blows up, you won't
This is therefore at least partially self-correcting.
Be wary of those who dismiss calculations or engineering with glib phrases such as "books are OK, but in the real world....": he's telling you that he has figured out how to bypass the laws of physics – but he's wrong."

(re-printed with author's permission)

[aswracing]

The confusion going on in this thread is exactly why I dislike the term "LCA" ... it means different things to different people.

Truth is, many accomplished engine guys use "LCA" to describe the separation angle as opposed to the lobe centerlines. Hell, follow that link that was posted, it goes to a Vizard article ... read his books, for that matter. He's not alone in using LCA that way either.

LSA & duration are just a different way to describe a cam, rather than talking about the 4 timing events. But they're not as complete. For example, advance your cams. What happens to LSA and duration? Nothing. But you've definitely changed your cam timing!

If you know your 4 cam timing events, you can calculate the LSA, the duration, and the lobe centerlines for that matter. For example, if you have these cam specs:

Intake: 30/54
Exhaust 60/30

And you want to know duration, lobe centers, and LSA, it's easily calculated:

• The intake valve opens 30 degrees before the intake stroke and closes 54 degrees after the intake stroke
• that gives 264 degrees of duration (30 degrees before the intake stroke + 180 degrees of intake stroke + 54 degrees after the intake stroke)
• Halfway through the lobe is the center. So if it's 264 degrees wide, the halfway point is 132 degrees past the open point.
• Since the open point occurs 30 degrees BTDC, the intake center is 132 minus 30, or 102 degrees.

• The exhaust valve opens 60 degrees before the exhaust stroke and closes 30 degrees after the exhaust stroke
• that gives 270 degrees of duration (60 before the exhaust stroke + 180 degrees of exhaust stroke + 30 degrees after the exhaust stroke)
• Halfway through the lobe is the center. So if it's 270 degrees wide, the halfway point is 135 degrees past the open point.
• Since the open point occurs 60 degrees BBDC, the center is 135 minus 60, or 75 degrees ABDC.
• But exhaust center is normally expressed in degrees BTDC. So 180 minus 75 gives us a 105 degree exhaust lobe center.

Separation angle is the distance between the exhaust center (105 BTDC) and the intake center (102 ATDC). 105 +102 = 207 degrees. However that's in crankshaft degrees. LSA is normally specified in cam degrees, which is half that much, or 103.5 degrees.

So just knowing the 4 timing events, you can easily calculate LSA, duration, and lobe centers.

[Matt C]

Yeah I noticed Vizard interchanges these two terms in his books too. I tried to contact him to get clarification
but he never got back to me (big suprise, he's probably pretty busy). So yeah, allot of people mistakenly call
LSA "LCA". Two totally and completely different things.