How important is lift vs. duration in making power?
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From: Bring it........ b*tch
How important is lift vs. duration in making power? This question is in regard to going from say a G5X2 112 sized cam to a 242/250/109 0.578/0.578. The X2 has approx .600 lift both intake and exhaust but doesn't have the duration. The larger duration cam is lacking in the lift department a little. I have stage II LS6 heads and with some research both of these cams will theoretically fit without having to flycut (which I don't really want to do - yes it's tight as hell I'm aware.)
I'm looking for a max effort here, not a daily driver. In anyone's opinion, would it be better to sacrifice some lift for the extra duration in a max effort application? I know my heads flow well up to .600, so I'm wondering if I'd be short changing myself with a cam that doesn't take advantage of that. Anyone's opinion is appreciated.
TIA.
I'm looking for a max effort here, not a daily driver. In anyone's opinion, would it be better to sacrifice some lift for the extra duration in a max effort application? I know my heads flow well up to .600, so I'm wondering if I'd be short changing myself with a cam that doesn't take advantage of that. Anyone's opinion is appreciated.
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From: Bring it........ b*tch
Originally Posted by Dom
Duration is more important.
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From: Bring it........ b*tch
Originally Posted by Cstraub
Lift is the ability of camshaft to get to a power range, also the quickness of the engine to get there. Duration is the ability to maintain power in the power band.
Chris
Chris
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From: Foley, Alabama-southern Alabama
The duration determines at what rpm the motor makes its power,that little bit in lift probably wouldn"t make that much difference in the hp.Look at the ASA cam with its .525 lift. 
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Launching!
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That all depends on how the heads flow, the port velocity, the desired RPM range, and probably tons of other stuff.
Think about it like this: Ideally the intake valve would close at BDC, and at BDC cylinder pressure would equal pressure in the intake runner. But in the real world the valve and port only flow so much, and at BDC cylinder pressure is less than runner pressure. So the valve must remain open longer, until the pressure in the runner is equal to the pressure in the cylinder... Problem is, by then the piston is already moving up in the bore, so your loosing volumetric effeciency. The faster you can open the intake valve and the more you can get the intake port to flow (by using more lift), the sooner you can close it (and thus get better VE and DCR).
Think about it like this: Ideally the intake valve would close at BDC, and at BDC cylinder pressure would equal pressure in the intake runner. But in the real world the valve and port only flow so much, and at BDC cylinder pressure is less than runner pressure. So the valve must remain open longer, until the pressure in the runner is equal to the pressure in the cylinder... Problem is, by then the piston is already moving up in the bore, so your loosing volumetric effeciency. The faster you can open the intake valve and the more you can get the intake port to flow (by using more lift), the sooner you can close it (and thus get better VE and DCR).
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Brett Foote Thread Starter
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From: Bring it........ b*tch
Originally Posted by FASTONE
The duration determines at what rpm the motor makes its power,that little bit in lift probably wouldn"t make that much difference in the hp.Look at the ASA cam with its .525 lift.
The main purpose I posted this thread was to see if there is a linear trade off between duration and lift. The last thing I want is a vehicle that has much worse driving manners yet doesn't produce any more power. I must say though that most of the stock cube LS1's making big power have very large duration cams in them from what I've noticed.
The larger cam I listed BTW is Mikey's (RMS) "Dominator" cam that made 500+RWHP on a stock bottom end, but he used 1.8 ratio rockers to achieve a lift of .612 on that car.
I think the breathing is really area under the curve
(lift*duration) but with one critical thing - the air
column inertia and resonance effects. A cam with
infinite lift and zero duration would not move much
air, the train would never get rolling. Of course a
stumpy little train is not too interesting either.
But duration "builds out" what lift you have and
as a good part of narrow-duration cams is chewed
up by the opening and closing ramps, incremental
duration increase I think pays off more than 1:1.
(lift*duration) but with one critical thing - the air
column inertia and resonance effects. A cam with
infinite lift and zero duration would not move much
air, the train would never get rolling. Of course a
stumpy little train is not too interesting either.
But duration "builds out" what lift you have and
as a good part of narrow-duration cams is chewed
up by the opening and closing ramps, incremental
duration increase I think pays off more than 1:1.
FWIW, If I remember correctly, Mikey wanted more lift to take advantage of the flow characteristics of his LS6 heads, but was limited to the .578 lift by the lobes he chose, which is why he went with 1.8 rockers...
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From: Bring it........ b*tch
Originally Posted by LawmanSS
FWIW, If I remember correctly, Mikey wanted more lift to take advantage of the flow characteristics of his LS6 heads, but was limited to the .578 lift by the lobes he chose, which is why he went with 1.8 rockers...
Any other opinions are welcomed.
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From: Bring it........ b*tch
Originally Posted by FASTONE
Think about this:how much time would the valve spend from the .578 to the .600 lift????NOT VERY MUCH!! 
Thats only .022 difference anyway!
Thats only .022 difference anyway!
Their is a direct relation of course with duration at tappet lift on the lobe. You can add duration to make up for less lift and in certain racing applications this should be done. This gives a very narrow power band. An engine in an 1800lbs altered that can launch at high rpm, hook, and not hurt parts is prime for this way of camming.
