More theoretical tech questions.

Why is a cams intake center line usually around 102 to 112?

I'm asking the theory behind this, not how to degree a cam.

From what I have noticed, this sets the maximum valve lift when the piston is about 72-73% down the cylinder bore (varies by rod ratio.)

Usually changing duration, just changes the open/close points equally on each side, but centerline remains fairly constant

I always figured that the engineers found thats the best spot for max lift to make everything work good to fill the cyl for max power. If you set it at 90 degres after TDC then it would be at max lift halfway down the cyl. I will say I see so many car guys that just dont understand what lobe seperation angle is when related to installed centerline. Many think they are the same. But like I said I guess the engineers who design engines for the manufactors and race cars found thats the best spot in the 100 to 115 degree area to best fill the cylinders and for max power. Ron

Centerline on cams vary got lots of reasons,
As well as what lash you use on valve adj,
I look at anything above 108 c/l to be that of a motor used with power adders
Those on lower spectrum to be that of torque motor used to move some what of heavy cars or underpower lol

Reality is all a tuning device for getting max power , torque out of your combo

David Vizzards' book How to Make Horsepower, 2nd edition, goes into great detail about centerlines and where to install a cam, as well as every other aspect of cam design. Basically the lobe separation angle is about how well a motor breathes at overlap. A 318 with great heads on it, with large valves can start to fill a cylinder much better than a stroked 440 with old iron heads and small valves for the cubes they feed. The 318 will want a smaller amount of overlap, so a wide LSA is better. The stroker 440 would want a very large overlap, so the LSA would be much narrower to produce that overlap needed. There is a limited time that the exhaust pulse produces a partial vacuum at the exhaust valve, and having both valves open at the same time the right amount of time and the right amount of lift for the rpm range you want to run is critical. That overlap period puts a huge draw on the incoming air and fuel, much greater than most would think.
The length of overlap needed grows at a rate about the same as the total duration does for any given rpm range you want to run, so a certain engine combo will want about the same LSA regardless of rpm range. Notice that split pattern cams with more exhaust duration have a wider lsa. BUT, the overlap for that motor and head combo will be close to the same as a single pattern cam, and the only opening and closing event that really changes for the cam is the exhaust opening event. The higher the compression ratio, the quicker the cylinder pressure drops, and then the exhaust valve can open earlier to help the motor breath better. My 528 race motor with 15/1 compression can open the valve very early, and the cam is 283 @.050 intake, 296 @ .050 exhaust, but the lsa has been spread to 114 to keep that overlap in about the same range of a single pattern cam. Put this cam in a low compression engine, and the exaust valve would open WAY before the work was done, because of the low compression keeps cylinder pressure high later in the power stroke, providing more work closer to BDC.

If you set it at 90 degres after TDC then it would be at max lift halfway down the cyl

90 degrees isn't ever halfway down the cylinder.

I was thinking on how in a pure sinusoidal wave, the root mean square is 70.7% of the maximum value, and it seems centerlines are around 72-73% of the stroke length (piston position) and if they are somehow related? taking into account the reciprocating part and rod lengths?

90 degrees after TDC the piston would be halfway down the cyl since the readings are off the crank. 180 after TDC it would would be at the bottom. Ron

Repeating it doesn't make it true.
90 degrees isn't ever halfway down the cylinder.

Damn I am an NA guy and all my junk is well over 108...Apparently been doing it wrong..IMO the reason you see all the narrow LSA's is you are looking at old school Mopar grinds(my guess)and that is an old way of thinking

Should that be applied to your post as well?

Poly is right and the detractors haven't thought it through. Picture a crankshaft pointing to 3 o'clock. Now put a connecting rod on it and center the cylinder over the crank centerline. There is no way that the piston will ever be at halfway down when the crank is at 3 o'clock. The connecting rod makes that an impossibility.

There is a mechanism that could produce this, it's called a Scotch Yoke. I know of no automobile engines that have ever been built using a Scotch Yoke instead of a simple connecting rod.

R.

LSA has absolutely NOTHING to do with how agressive (or how soft) a lobe is. LSA should be based on head flow at overlap. The better the head, the wider the LSA can be. That is where Comp has failed. They decided 110 is the de facto LSA. Not a good way to decide LSA.

Exactly

You know what I mean. If the piston is 90 degrees from TDC its halfway through its travel of its stroke. At 90 degrees from TDC the piston should have gone the distance of the crank throw. At 180 it would have done a full stroke and be at the bottom.
Dont you agree if the pistons travels 180 degrees it has traveled through its stroke from TDC to BDC ? Ron

doesn't work that way, punch in some numbers and see for yourself.

http://dansmc.com/mc_software2.htm

At 90-degrees crank rotation the crank pin is 1/2 around, but the piston rod is 1/2 way down and 1/2 way over so the piston is more than 1/2 way down the bore at 90-degrees.

Anyhow, I was talking about intake lobe centerline, not lobe separation angle. LSA is usually nearly the same number resulting in an equal split on overlap.

I think we need to visualize the fulcrum effect, the Trig, if you will of the connecting rod and piston motion relative to the stroke, 90 degrees after TDC does not put the piston 1/2 way down the bore....not with a connecting rod greater than 0 in length anyway, lol

But it is interesting to note that when the ROD is 90 degrees to the Crank throw this is the point of maximum piston VELOCITY in the bore. Think of your leg pushing on a bike pedal.

The intake centerline is wherever you put it. It can have a 114 LSA and you can install it at 102. It would be 12 degrees advanced. But you could still do it.

This is a true statement.

Actually (also) the better the port, the wider the lobe centerline can be. A good port and combustion chamber with little to no valve shrouding doesn't have to rely on "fifth cycle" scavenging (overlap) to keep the cylinder charged and the VE up, also if the port moves air and fuel efficiently as a charged mass, you can delay the Intake centerline (and more importantly the Intake closing event) which can effectively RAM more air and fuel into the hole before the valve closes then the natural "effective displacement" ie < than the static displacement which gets smaller for every degree past Bottom Dead Center.