I'm not a cam expert, but I have run a few combos that I can talk about. What I know is;
Compression cannot begin until the intake is closed.
The sooner you close it, the sooner compression can start, and all things being equal, the end Cranking Pressure will be higher.
The LSA has NOTHING to do with making pressure.
The ICA in a given engine, is what determines the pressure.
The LSA is what ballparks how soon you can close the intake, and not screw up the PowerStroke.
But, if you have a HIGH SCR, and do not need to close the intake so soon, then you can lengthen the intake stroke for more power. Eventually the longer intake stroke will end up going deep into the exhaust stroke on overlap, which will cause your engine to run rough, which will reduce your power at idle and points beyond, which will require a higher idle speed to overcome, and a higher stall to find power. And so, your power peak is driven ever higher up in the rpm band; requiring more attention to the top end parts to prevent the engine from destroying itself.
There is NOTHING inherently wrong with a 292/292/108 cam. If you build the engine around it, and, IMO, run an automatic with a higher than stock stall.
If you have a manual trans with street gears of 3.55s or less, Good Luck trynta drive slowly, with a very late closing intake, cuz at typical street timing, the engine is NOT gonna like idling at less than ~700, which with 3.55s and a 2.66Low gear, is about 6mph.
Whereas with a hi-stalled auto, you can thump along at say 2mph. and besides that, how long is that 292 rocker gear gonna last at under 700 rpm?
So you know, the 292 is fine for the correctly built engine and powertrain.
Whereas, in those days, 3.55s were my hi-limit. So I pulled it and sold it to a budding young racer, who reported that he loved it.
The point is that, the LSA is a tool to be used to match a particular cam, to a particular engine, and/or application, and/or to a lesser extent, the combo.
IMO;
Since this is sorta backwards for most of us, it is cheaper and easier, to choose a suitable cam, and then build the engine and combo to maximize it.
But if your engine is already built, and is an oddity, then perhaps the cam will also have to be odd.
But if your engine is still in the planning stages, think about this; the compression cycle only has to be just long enough to get the pressure up to where you want it to be. If your Scr is gonna be 8/1 then yur gonna need a lot of compression degrees to get to optimum pressure. But if yur ar 12/1 Scr, you can easily overshoot the pressure that pumpgas can support. So in both cases, the ICA has to be adjusted to make it work...... optimally without detonation. and if the Ica is optimized, it may require the LSA to be changed as well.
So then
IMO,
For a NA-streeter, it's hard to take an LSA number, and say that this is what's best, cuz invariably it will be difficult or expensive to build an engine to that so-called "best" number. I mean inside the accepted norms of say 106 to 112.
Furthermore, there is a plethora of cams in the 108>110 window, such that there should not be a reason to go outside it.
I'd be willing to bet that just about every performance street cam will have or be able to be adjusted to, an Ica of 58 to 68 degrees. Why is that? Because 58* works reasonably well in lo-compression engine, and 68* works reasonably well in a hi-compression street engine.
Furthermore, that small window, leads to a similar sized window on the power stroke from ~104 to maybe 112*, which then sorta sets the exhaust degrees. and again, the LSA adjusts the overlap for more power over the nose, or more torque/fuel-economy.
That leads to the bulk of the differences being in the overlap.
As an aside;
If you know the exact Scr of your engine, and are willing to commit to a certain grade of fuel, and are not gonna run an octane booster, then, it is possible to calculate an ICA, within a few degrees, to optimize your cylinder pressure for the chosen fuel.
From there, you can bias the Power stroke, from power to economy. and
from there, you can usually chose an amount of overlap that you are willing to tolerate in the rpm zone that your engine is gonna spend the bulk of it's time.
Once those three have been established whatever is left over from the "four cycles plus overlap", gets divided between intake and exhaust. As long as this is done reasonably accurately with regards to choosing the overlap, you'll always get a pretty good cam.
Once all those numbers are settled, the LSA will come out of it as a matter of math. If it falls in the window of 106 to 112, the cam will be good. If it doesn't, it's probably because you got greedy on the overlap.
Here's an example
Suppose you have determined that with your Scr, you want an Ica of 64* leaving the compression at 116*, and
you want some economy, so you choose a power stroke of 111*, and
You're thinking that overlap of 53* is about right. Okay so
116 +111 +53 = 280.
The total number of degrees for all this to happen is 720 (two revolutions) plus the 53 overlap is 773. Subtracting the 280, we get 493 for intake plus exhaust. Lets say we install that at ~split-overlap, so 26* goes to intake and 27* to exhaust.
Ok then 360(one revolution), plus 26*, and less 116 to compression leaves 270 for intake duration. Exhaust thus has to be 360+27, less 111=276.
The Lsa thus, by the math is (270+276)/2 less the overlap, and the result divided by 2 equals ...... 110, which is a good number, so there you have it; the perfect cam for the three parameters that you choose, namely compression, power, and overlap.
Had you chosen any other number or numbers, this would spit out a different cam. Now all you gotta do is find one. lol. In this case, the picken's are easy. But it doesn't always work out that way. I used numbers for which I already knew the results, cuz that is how I came to choosing my cam.
Lets do another one for which I have no idea how it will turn out.
lets say you have a low-compression engine and 58* gets you decent cylinder pressure, which is 180 less 58= 122* of compression. and,
you don't care much about fuel economy, so say 105* of Power, and you get real greedy on the overlap, say 70*.
OK then the total amount of time available measured in degrees is 720+70=790 degrees.
From that we subtract (122+105+70)= 297 leaving 493 for intake plus exhaust. To put that in at split overlap, I have to give half of 70 to intake and half to exhaust. Thus intake will be (360+35) less 122= 273 intake duration, and
Exhaust will be (360+35) less 105=290, and
the Lsa will be, (273+290)/2 less 70, and lastly divided by 2= 105.75 .. Wow, that just missed it into the window (106 to 112), But look, the exhaust duration is out of sync with the intake. Typically, you will not find a cam with more than 8* split, nor is there any need to go more........ So in this case, the result is a poor cam choice. Your options are more intake duration, less exhaust duration, and/or less overlap If you opt for changing the cam-timing, you will change your pressure .
FYI
What I have discovered is a relationship between exhaust duration added to power duration, that likes to be around 225>230 for a decent pressure streeter; To the highside for a low compression engine and can be towards the low side if a hi-Scr design.
Once you go below 220, fuel-economy rapidly disappears.
Once you go below 210, you are approaching race-car territory
No matter what, on a streeter, you want the Power cycle to have at least, IMO, 104*, cuz any less than this is wasting gas right out the tailpipe, and there is nothing you can do about it. I'm currently at 106, and my Barracuda was retired from DD at 111.
Lets say your pockets are deeper than mine, and you're ok with 104* Power. And you have plenty of Scr, so yur looking at maybe 113* of compression. Added together that make 217, and, since this is below 220, I can guarantee that this combo will be hard on gas.
I don't care what cam you pick, if the .008 advertised Compression plus Power are less than 220, she will be hard on gas, UNLESS,
you are building a specific-purpose engine that allows more Power duration than is typical.
At the ever-rising cost of fuel today, I recommend the Power duration not be less than ~108 degrees, and make sure the ignition Timing starts soon enough for the load and rpm.
This means that the Compression degrees will be short, and so the only cure for that is a higher SCR.
Happy HotRodding
PS, I know this post got long, and some of the old guys will roll their eyes, and azzhole comments will invariably follow. But this post is not for them, it's for you. I hope you get out of it what you need.
