[Aaron]

Ok, got to doing some reading on flow numbers and valve lift. Kind of confused me.

Question is:

In looking at cam selection and flow numbers.

What are the factors to consider?

Lets leave duration and compression totally out.

What tells you when to stop adding lift to cam? If a head flows say 268 at .550 and you only gain a little more cfm at .600, .650, and .700.

 

[RB69SS396Conv]

The valve spends only a TINY fraction of its time at or near full lift. (unless of course you're using one of those solid rollers for moderate RPMs ONLY, with almost "square" lobes)

 

[Aaron]

The valve spends only a TINY fraction of its time at or near full lift. (unless of course you're using one of those solid rollers for moderate RPMs ONLY, with almost "square" lobes)

Not sure I follow exactly what your talking about.

I think what I need to re ask is this.

If you have a set of heads pretty much having max flow at say .600 is there any reason to run a cam over .600 lift?

I mean if the heads "maxed out" at .600 lift and at .650, .750. it only gains 5 to 10 cfm....what is the point to run a cam with say .650 lift...you aint moving any air.

 

[Chad Speier]

Not sure I follow exactly what your talking about.

I think what I need to re ask is this.

If you have a set of heads pretty much having max flow at say .600 is there any reason to run a cam over .600 lift?

I mean if the heads "maxed out" at .600 lift and at .650, .750. it only gains 5 to 10 cfm....what is the point to run a cam with say .650 lift...you aint moving any air.

Few reasons. One, if that is at 28' on a flow bench, it's far sooner on a running engine. So the first thing is fixing the heads. .700 lift of positive flow only needs a .700 lift is a myth.

The reason you would run more lift than positive flow is because you would have more crank degrees of that positive flow. All your doing at that point is stretching molecules with nothing in return.

 

[Inquisitive Gearhead]

Aaron and others that like this kind of stuff,:thumbsup:

The real issue is called "area under the curve" when one looks at the intake curve and where the intake valve closes (which has a considerable influence on dynamic compression ratio). In order to get the maximum effect of the flow curve (valve lift vs flow numbers) then the valve events must support getting all the flow possible in order to capture the flow capacity. You might consider that the valve motion is on the flanks of the cam twice while the maximum lift issue is simply a way (and place) to change directions.

Having said all the drivel above, best bet is to work with a cam company that understands the issues of flow data and more often than not they will suggest a best selection of a bumpstick to compliment your application. IF they won't talk flow numbers or can't do so, choose another cam company to assist you in spending your hard earned money.

In a book I wrote, Engine Airflow, chapter 11 deals with camshaft selection based on flow numbers. Not a scholarly work, but might be worth reading anyway.:boring:

Respectful Regards to all that like this kind of stuff,
IG

 

[RB69SS396Conv]

Imagine that you've got a cam lobe with the usual "sine-wave wrapped around an axle" kind of shape. (it's not that, actually, but the image is helpful)

Now instead of that, imagine a dentil molding, like you'd find in expensive crown molding or something, that looks like square teeth, wrapped around an axle.

That's the difference between a "mild" lobe, and an "aggressive" one.

OK now imagine the fraction of the time, that on a typical "street"-ish cam with the former style of lobe, that the valve will be spending anywhere near full lift. Pretty small, eh? Most of the time it will be lifting off the seat or gently dropping back down.

Now imagine how, if you have 2 cams that are IDENTICAL IN EVERY WAY except that one has max lift of say .500" and one has max lift of say .750", how LITTLE of the time the actual lift that's in effect, will be above whatever that "maxed out" point is. MOST OF THE TIME, the valve is going to be .100", .200", .300", etc. off the seat.

OK, so it's pretty easy to see that the cam that has .750" of peak lift will have .150" when the other has .100", .300" when the other has .200", .450" when the other has .300", and so on. Eh?

Does it REALLY matter that the "peak" lift doesn't flow very much more, when ALL THAT TIME ON THE FLANKS OF THE LOBE will have so much more flow? In fact, if you took that .750" cam and "squared off" the tip of the lobes, such that it got up to say .600" just as quick as it did before but didn't go all the way to .750", it would flow pretty close to the same as it would at .750"; right?

Problem is, cam lobes don't work that way. They are designed using a series of curves, where there's one curve on the ramp, another as the rate of rise gets steepest, another over the top, and so on; might even be different on the rising and the falling sides. (usually are, in modern "state of the art" ones) In order to minimize noise and stress on parts and whatnot, a good cam design arranges for all derivatives of all those curves to be equal at the point where they change over. As you will recall from your calculus classes, the first derivative is the rate of change in the valve's location; the 2nd derivative of its location is its acceleration; the 3rd derivative is "jerk", the rate of change in acceleration; the 4th, 5th, & 6th derivatives are called "snap crackle & pop" and are a little harder to relate any physical phenomenon to, but are still just as real. Given that mathematical constraint, it's REAL TOUGH to design a cam that snaps the valve open as fast as that .750" example up there, but stops short of .750" of full lift. You get the lift, even if you don't "need" it, just so that the parts are moved in a manner consistent with long life and all that.

Which is why it's fallacious (and "wrong" too) to pick a cam solely on the basis of the lift of "maxed out" head flow.

 

[Inquisitive Gearhead]

I had to smile when snap, crackle, and pop terms were mentioned. Good memories that track back to Harvey Crane, et al.

IF anyone out there is really interested in the DESIGN phase of camshafts (few really want the headaches the addtitional knowledge brings). You might take a look at the following link:
http://www.jamisonequipment.com/itemdetails.cfm?Product=12391&type=3
as it is an offer of a book written by the late Don Hubbard on camshafts and design techniques and other issues. The book is a bit pricey for the average guy or gal, but a very good read if you are truly in need of such data.:yes:

Regards to All that like this kind of stuff,
IG

 

[Aaron]

Good reading here. Lots of learning.

 

[540Hotrod]

Another way to think about it in simple terms is like shift points.

If you make peak power at 6000 rpm...then at every point below it you're making less (flowing less). Now if you can get the power to *hang on* after peak power and not take a dive (port stalling or going turbulent)...then extending the shift point past peak power keeps you in the "fat " part of the powerband longer.

You can do this with less lift and a *square lobe* that has a ton of duration (area) at the upper part of the lobe.

Or you can do it with more lift that takes it past peak flow and stays in that area much longer overall than if you had immediately started closing it at peak flow.

I can assure you that the larger lift lobe will generally be easier on valvetrain parts than the first version since there is more lobe to work with to slow down the lifter at peak lift, change directions and hang there and then accelerate to close..and then slow it down before it hits the seat.


JIM

 

[pancherj]

Good discussion. Something I have thought about...do we even care what it flows at max lift and 28" of depression? From the few pressure traces I have seen posted, the depression is lower than 28" at max lift and much higher than 28" at really low lifts.

 

[steelcomp]

Sometimes more lift is just a function of the lifter continuing on a reasonable path as opposed to the "table top" lobe Jim mentioned. Some classes have lift restrictions that don't allow the extra lift, in which case the lobe can get real agressive, like a table top. POP the valve open, hold it there, then drop it closed. Look at the valve path on a servo actuated valve. There is no "curve". The curve comes from having to translate rotating motion to linear motion and the limitations of physics.

 

[540Hotrod]

Man...we're getting near "lofting" discussions now!

JIM

 

[Inquisitive Gearhead]

There are many good technical papers available to provide reliable test data about engine airflow issues.

One SAE paper that might interest some is: Measuring Absolute-Cylinder Pressure and Pressure Drop Across Intake Valves of Firing Engine, Paulius Puzinauskas, Joseph C. Eves, Nohr Tilman, 1984, SAE.:thumbsup:

The 28"H2O test pressure numbers are somewhat of a quasi standard that is widely accepted in the automotive afermarket industry. If that number does not please you, select another and compare data accordingly by use of the square root of the pressure ratio.:boring:

However, be very wary of telling some of the folks on this and other forums that you might read something of the technical ilk lest you be referenced as too interseted in being "book smart" or some such nonsense.

Regards to All that like this kind of stuff,:yes:
IG