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Discussion Starter · #1 ·
I know that this could generate a host of opinions, but I'm trying to rationalize how it works, in theory. I'm also aware of the standard, "it depends on the combo" answer, but I'd like to try and keep that at a minimum. I'm thinking of a broad-based, "in-theory" discussion, as much as possible. At some point, yes, every combo is different and requires specific analysis, but that's not really useful.

The part that is really confusing to me is how LIFT can make a significant difference, assuming the value is, "all the way open." Clearly, if you go with a cam that has .450 lift, when your heads peak flow is at .600 lift, the head(s) will never flow their peak because you aren't opening the valve all the way. It almost seems like you didn't have the correct camshaft installed in the first place; the lift in relation to the valve diameter probably wasn't correct.

But going with a cam that has a, say, .700 lift on the same heads seems totally pointless. Once the valve is "open," opening it further doesn't seem like it would generate more air flow at all.

DURATION, on the other hand, seems pretty brainless in terms of understanding its impact. The longer you can keep the valve near the lift where the head flows the most, the more air you can move.

The strange part about all of this is that I continue to read and hear about how LIFT has a significant impact on an engine's performance.

For instance:

"Lift is how far your valves open at a certain duration. To understand pick 2 equal duration cams but different lifts.
Cam A opens the valve for a period X time and a distance Y (certain lift)
Cam B opens the valve for a period X time and a distance Y+Z (higher lift)
so mathematically speaking, cam B will make more power because more air was allowed in, therefore you could add more fuel and make more power."

HUH?!?!?

Unless the valve wasn't all the way open, that seems totally illogical. Atmospheric pressure is atmospheric pressure--it doesn't increase because the value opened more.

What I'm missing??? :hmm:
 

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You must remember too that with a higher lift cam the "ramp" of the lobe is now significantly different, meaning that the valves will be open further compared to a shorter lift cam at any given point (your last equation-valves begin to open sooner and stay open longer).
Opening the valve further should increase air flow, think of a water tap, the more you can open it the more is allowed to flow.
Overlap is also very important to consider, especially forced induction.
 

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Discussion Starter · #3 ·
You must remember too that with a higher lift cam the "ramp" of the lobe is now significantly different, meaning that the valves will be open further compared to a shorter lift cam at any given point (your last equation-valves begin to open sooner and stay open longer).
Agree. But that's just duration, correct?


Opening the valve further should increase air flow, think of a water tap, the more you can open it the more is allowed to flow.
Here's where I get confused:

Since we are only talking LIFT here, duration is held as a constant, yes? X degree at 0.050 with a shorter lift cam vs. the same X degrees at 0.050 with a higher lift cam.

(I 100% agree that once DURATION is added into the equation, & increased, more air should flow, in theory.)

As such, the ramp has to be faster to get the lift higher, but it also has to close faster as well, by definition. Well, assuming the valve is, "all the way open" with the same duration/shorter lift cam, it shouldn't matter seems to me that the higher lift cam goes past, "all the way open;" atmospheric pressure is atmospheric pressure and should fill the cylinder just the same.

DURATION seems to me to be the entire game, assuming, of course, that the lift is enough for the head(s) to simply flow properly.

I guess I'm missing something...

Overlap is also very important to consider, especially forced induction.
Totally agree. NA vs. FI need to consider overlap and they likely will be significantly different.
 

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James,

I believe the term is "effective area." This is a product of lift and net valve opening area. The amount of flow possible is a function of the effective area, valve opening ramp, and duration. Given that the valve seat is smaller than the manifold runner, the max flow could only approach, but never equal what the heads could flow with no valve present at all. So lift does matter because it increases the effective area for any given duration.
 

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Discussion Starter · #6 ·
James,

I believe the term is "effective area." This is a product of lift and net valve opening area. The amount of flow possible is a function of the effective area, valve opening ramp, and duration. Given that the valve seat is smaller than the manifold runner, the max flow could only approach, but never equal what the heads could flow with no valve present at all. So lift does matter because it increases the effective area for any given duration.
Yeah, I understand the math part of the equation, but I'm still getting tripped up with how that translates into more air getting into the cylinder & I think it's here that I'd like someone to help explain that to me.

Duration at 0.050 is constant, regardless of lift. As long as the valve is opening far enough so that the head isn't choking off the intake charge, I really struggle with how more air is allowed in with a higher lift, but same duration.

Apologies if this comes across as me being difficult, as that is nowhere near my intention. I'm definitely just trying to understand/learn. ;)


Heres a link to some cam basics, and maybe will help straighten out what you are trying to figure out. I see what your saying but this is easier than me trying to explain what I was trying to lay out.

http://www.chevyhiperformance.com/techarticles/49158_camshaft_engine_basics/index.html
Thanks for the cool article. As stated above, however, I think I need to understand this from an air-flow perspective, for that's how I view an engine--nothing but an air-pump.

That said, does anyone think that LIFT is more important than DURATION? I've read some articles from cam experts that have argued in favor of LIFT, which makes me shake my head in confusion...haha
 

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James,

Try this thought experiment: your job is to fill a bucket with water from a faucet. The catch is you can only have the faucet open for up to 2 seconds at a time (duration). For any given 2 second period, you are allowed to open the faucet as wide as you want (lift).

The faucet is analogous to the valve of course, and the water pipe is the head/manifold. If your water pressure was low and you could get max flow at a quarter-turn, opening the valve further would be of no benefit. However, if the pressure were high enough such that there was still back-pressure even with the valve wide-open, the further you open the valve for any given period, the faster your bucket would fill.

Does that help?
 

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Great analogy..

Although we are trying to keep the discussion simple. LSA (lob seperation angle) plays a large part in this as well.
 

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Discussion Starter · #9 ·
James,

Try this thought experiment: your job is to fill a bucket with water from a faucet. The catch is you can only have the faucet open for up to 2 seconds at a time (duration). For any given 2 second period, you are allowed to open the faucet as wide as you want (lift).

The faucet is analogous to the valve of course, and the water pipe is the head/manifold. If your water pressure was low and you could get max flow at a quarter-turn, opening the valve further would be of no benefit. However, if the pressure were high enough such that there was still back-pressure even with the valve wide-open, the further you open the valve for any given period, the faster your bucket would fill.

Does that help?
That's a great example and I often use a similar example to explain to people how a supercharger/turbocharger works (putting the intake manifold under pressure). :cool:

Let me expand the example above to illustrate my confusion.

Clearly, if the valve on the faucet is only opening part way, that's a different issue & what I was referring to above, with the following quote:

"Clearly, if you go with a cam that has .450 lift, when your heads peak flow is at .600 lift, the head(s) will never flow their peak because you aren't opening the valve all the way. It almost seems like you didn't have the correct camshaft installed in the first place; the lift in relation to the valve diameter probably wasn't correct."

This is what you are talking about, yes? I agree, that would be an issue.

But once the valve is, say, 90 degrees open, opening it, say, 110 degrees won't make a lick of difference...at least, to me it shouldn't. At which point, duration is the entire game.

I guess they must just come up with configurations where the, "valve doesn't open all the way?"

That seems wholly lame & begs the question...why install a head and cam combination that doesn't have the cam open the valve all the way in the 1st place? Hmmm... :hmm:

Outside of a situation where a manufacturer/installer didn't, "open the valve all the way," I still maintain/believe that DURATION is the name of the game. :)
 

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The other things to consider might be NVH, wear factors, and, I'm not kidding, mileage. To increase the effective area for a given duration, you may want/have to ramp the valves open more quickly to the greater lift value. You may need heavier valve springs to prevent float, which impacts valvetrain noise, wear, and possibly even mileage to a minute degree (or have to resort to more expensive lightweight valvegear). Maybe it's my imagination, but the higher lift LS9 cam seems to have added an "edge" to my exhaust note.
 

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Luna, I think I understand where you're coming from, and maybe this perspective will help:

Intake valves are not typically opened to maximum possible lift for optimum flow at max power.

I think what you're thinking is that all cams are [or should be] ground to lift valves to their maximum effective height for maximum flow such that any more lift is not effective. On passenger cars this is rarely true because maximum lift is only effective at high RPM when you're into the power band. As GMX322V S/C alluded to, there are other considerations, such as NVH, emissions, drivability, etc. At low RPM, a cam with high lift contributes to a "lumpy" idle. Part of the reasoning behind this is that at low RPM the intake charge needs more turbulent flow in order to have a complete and thorough combustion, bearing in mind that flame propagation through the combustion chamber is a signficant source of noise and vibration.

As you note, past a certain point lifting the valve any further won't make a difference in a naturally aspirated engine (I seem to remember reading that mathematically speaking, a lift equal to the radius of the valve opening is the point at which no higher lift will precipitate higher flow). However, since production automotive engineers also need to factor in customer satisfaction with regards to NVH, smoothness, reliability, emissions, etc., when not operating at max power, valve lift (as well as duration and overlap) is one of the factors not is not optimized from the factory for all-out horsepower.
 
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