I understand a lot about engines and am always wanting to learn more so I was wondering you guys could explain the duration and lift and those numbers related to a cam and why spring rates would affect that and lifter ratio and push rod length and how it all comes together. I am looking at doing a cam in my 92 vette and dont know where to start, or which springs or lifters and what not to use. And I dont want to read a random forum and see that someone said that a certain cam worked for him. I have seen a lot about the hot cam but thats fro the LT4 and some things need to be changed. I want to keep my daily driavabitly but want to feel the power more and i love the sound of a cam too. I dont plan on forced induction. I also have gears on the way(went with 3.54) I dont plan on buying all parts at once, just collect them till i can do it all at once. I already have headers and rear mufflers are gone and an intake.
Thanks!

 

Duration is the number of degrees of crankshaft rotation that the valve is off the seat. Typically it is measured at .050 lift.
Lift refers to maximum amount the valve is "lifted" off its seat at the cam lobe’s highest point.
Spring rates don't effect the cam specs. Its the other way around. A cam that moves the valve further off the seat will require stiffer springs for several reasons. None of which are as important as knowing that certain cams require stiffer springs.
Pushrod length and the rest all is related to the geometric relationship between the rocker arms, valve stems and pushrods.
I'm never heard of "lifter ratio", I think you mean rocker arm ratio. This is a multiple applied to cam specs by the design of the rocker arms. A 1.5 ratio arm will move the valve less than a 1.6 ratio arm and so on ...

 

 

1.6 moves the valve more than the 1.5, which is why some folks think its better.
I'm sure someone else will weigh in on the direct effects of changing to the 1.6

 

valve springs return the lifters and control the the lifters following the cam. When the cam lobes/ramp/ are of greater angle and or higher, stiffer springs may be needed. The downside to that is faster cam lobe wear and on stock heads, the rocked arm stud may pull out of the head. There is a different spring design offered called Beehive springs. Instead of using double springs they get more resistant the higher they get squeezed.

 

Let me put it a slightly different way - the valve springs job in life is to keep the rocker arm in contact with the push rod- the pushrod in contact with the lifter, and most importantly to keep the lifter in contact with the cam lobe. As engine speeds rise - the cam accelerates the lifter more "aggressively" - and if you don't have enough spring tension - the lifter can actually lose contact with the cam - this is called valve float. The reason its called valve float is that the valve closing "ramp" on the cam is ground to control the valve closing motion, and to "gently" let the valve back down on the seat. If the springs aren't stiff enough - the valve tends to be dropped back on the seat - which is bad for the valve and bad for the seat insert. If the engine has hydraulic lifters, and the valves float- the lifters try to "pump up" to take up he excess play - and this means the valves simply don't close for a period of time - also not good for a variety of reasons.

In state-of-the-art racing engine design / building - phenomenal amounts of time and money are spent in this area. If you google "spintron" - you'll see people (read that as camshaft producers) have gone to crazy lengths to put together a test rig to see how well a valve spring is able to control the valve motion of prototype camshafts

The simple answer might seem like installing monster springs will solve all the problems - but as the springs get stiffer - the pressure on the lifter - to camshaft contact surface increase and both cam and lifter wear go up. Most street cars have wide open valve lift spring loads of 350 or so lbs. In some racing engines - the loads are closer to triple that - but the camshafts / lifters typically don't last very long....

So - back to reality - stock valve springs are typically fine with stock cams, and stock RPM limits. Go to an aftermarket cam - where the cam grinder may try to push the valves open a bit faster to achieve better performance, and it would be prudent to install the springs the cam producer recommends, and to verify that the installed height is correct (shims can be used to get the installed height correct if needed). Slightly stiffer than stock springs are also just fine - with stock cams and sometimes even preferable - as the aftermarket spring makers typically use better steel for their springs than the OEM's do.

Hope this helps...