I know what lift is, or at least I think. It is how far the valve goes off the seat, right? I know there is a correlation with this and cylinder heads, i.e. brand X flows Y CFM at Z lift. Now, are there some cases where, say, a head doesn't flow well to .600 lift, just depending on what it is. Now, if you get a cam that doesn't raise the valve that high off the seat, does that flow number matter? Am I making sense or am I missing something? Thanks.

Originally posted by OUChevelleSS:
I know what lift is, or at least I think. It is how far the valve goes off the seat, right? I know there is a correlation with this and cylinder heads, i.e. brand X flows Y CFM at Z lift. Now, are there some cases where, say, a head doesn't flow well to .600 lift, just depending on what it is. Now, if you get a cam that doesn't raise the valve that high off the seat, does that flow number matter? Am I making sense or am I missing something? Thanks. Taylor,
I think I got it & you are correct, opening the valve further than where the head flows is pretty much counterproductive.
You would be better off with a cam of the same duration you are needing that opened the valve to where the head flowed & the upside is it will be a better lobe as it will end up "rounder" over the nose which will drop lifter acceleration (& also lobe wear)which is usually a good thing.
The only place I can think of where opening the valve further is where you are running a shelf cam & want X amount of duration for a longer period of time.
Then opening the valve further would let the cylinder "see" the port for a longer period of time at full flow lift.

Did I confuse you enough??

No you didn't confuse me actually, thankfully you were able to stay on my wavelength somewhat (which I know was hard since mine was crazy), so all makes sense. Thanks.

It works the other way as well, like you were saying. Assuming you buy a head that has real impressive flow numbers at .800 inches of valve lift, but not all that impressive at anything below .600, there is really no reason to consider these heads if you are building a hot street motor with a cam with say .520 inches of valve lift. Basically, you were correct. If your cam and rocker arm combo does not lift the valve off the seat enough to see these port flow numbers that the head is capable of at higher lifts, you will never take advantage of the cylinder heads capability. Normally this will equate to a loss of torque at low revs too, cause if the head is designed to flow huge high lift numbers and run at high rpm, it will normally struggle to run well at low rpm and not be able to perform right witout the proper cam and valve lift to match it. That is why everything in any particular motor combo should be optimize so that everything is meant to perform over the same rpm range. Like Mike was sayin, way too much cam for a given set of heads isnt normally the best idea cause the heads will take away from the camshafts potential cause the cam will wanna run high and make horsepower, but the heads choke the motor off and wont let that happen. Once again, you are on the right track, just remeber to keep everything, not just the cam and heads, matched for a particular power curve to meet your goals/needs.

Wow, I'm glad I pretty much got everything.

Now my next question is, how do rocker ratios fit in and what do they mean? I know SBC is typically 1.5:1 and that it can go 1.6:1, and I think a BBC is typically 1.7:1...and can go 1.8:1? I don't know. Why do ratios matter and how do they come in?

They matter b/c the 1.8 ratio rockers open the vlave higher then the 1.7 rockers.

Lobe lift example of say .300" as a start..

1.8 ratio rocker x .3" = .540" lift whereas,

1.7 ratio rocker x .3" x .510" lift.

Make more valve lift with the same lobe instead of use a higher lobe is all, that and some other stuff pertaining to slight duration changes too..

pdq67

Ah okay well that's very simple too. And so therefore, 'valvetrain geometry' matters as in...the right length pushrod for the right type of rocker? The right cam so it doesn't lift too high with a 1.8 rocker and smash a valve into a piston? Am I still on the right track here?

Originally posted by OUChevelleSS:
Ah okay well that's very simple too. And so therefore, 'valvetrain geometry' matters as in...the right length pushrod for the right type of rocker? The right cam so it doesn't lift too high with a 1.8 rocker and smash a valve into a piston? Am I still on the right track here? Taylor,
I am going to confuse you yet :D
You still be trackin,,,

The geometry always matters but has little to do with rocker ratio except for the fact it will change when changing anything in the valve train which includes the cam, lifter brands, rocker ratios or brands, valve length, head or block milling etc.
With a stud mounted rocker you normally adjust the geometry with pushrod length & in some cases valve length.
With shaft rockers systems you alter the height of the stands for the best pattern at the valve tip & then adjust pushrod length to fit.
You want the pattern as centered on the valve as you can get it with the least amount of sweep across the tip of the valve.
I would much rather have say .040 of sweep slightly off center on the tip than say .060 sweep perfectly centered.

Altering rocker ratio with a solid cam will change what the engine sees.
Using 1.5 to 1.6 as an example,,

Assuming you adjust the valve lash for the difference in ratio you will make the engine see a more aggressive cam.
It will have the exact same seat timing with a few more degrees at .050, a few more at .100 etc.
And of course more lift AT THE VALVE...

You are not changing the actual cam timing just what the engine thinks the cam is.

If you have a cam that adjusts at .020 with a 1.5 rocker & change to a 1.6 rocker you now need to adj the lash at .021 to achieve the same seat timing you had.

old lash / old ratio * new ratio = new lash