Lift or duration, which of these is the most important? Let see if I got this right, lift is just what it implies how far it lifts the valves, and duration is how long it is open for right?

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87 Z28 305 TPI, Auto, 2.73, Gutted air boxes, K & N fillters, SLP Airfoil, Dyno Max 3" cat back, Accel 8.8 mm spirals, Accel cap and rotor, Accel coil, rapid fires, and 160 Stat.
Best 15.414 @ 87.47.

 

I'm no cam expert but I would thik the LSA or lobe seperation angle would be top because it determined how much valve overlap there is which makes for a smooth idle or a real loppy idle.
It usually expressed in degrees from 108° to 114°

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86 Camaro Sport
383 Speed-O-Motive Crate Engine, Trick Flow Heads, 58mm Accel TB, 3.73 Auburn Pro, SLP Cold Air Induction and Headers, Dynomax Cat-back, Serpentine belt setup, Dual IROC Fans, Jamex Springs.
http://www.sethirdgen.org/octride.htm

[This message has been edited by John Millican (edited January 01, 2001).]

 

New,

John makes a good point. Lobe separation, combined with lobe duration, determines the overlap (amount of rotation that the intake valve is opened before the exhaust valve closes). Too much overlap can result in low vacuum, unsteady idle, and higher emissions.

The most important factor for air flow is the "area under the curve", or how much valve opening for what period of time. There are two ways to increase the amount of air flow - lift and duration. Either open the valve farther, or hold it open for a longer duration. Or, both. The trick is to use both to their fullest advantage for the type of car the engine is used in and type of driving intended.



Some cams are ground with a very high lift but relatively short duration, like some factory performance cams and the "moderate" street/torque grinds offered by aftermarketers. These cams are said to have a high ramp, because they open and close the valves very quickly. This allows good flow and torque output at lower RPMs, but also limits the top RPM because of the potential for the valves to float off their seats and not follow the cam profile precisely at higher RPMs.

Some cams have moderate or high lifts over very long durations. These produce more power at higher RPMs, but lack idle quality and low-end power (torque). The valves open and close very slowly, allowing high top RPMs of the engine, but also have a lot of overlap, preventing good flow at low RPMs. Overlaps of more than 40° start to create a rough idle and reduce power in the low RPM ranges.

A good street cam profile is a compromise of both of these principles. Enough lift and duration to get good flows, but at short enough duration to allow good lobe separation and decent idle/emissions. The decreased overlap also improves low-end torque output.

Roller cams can be an advantage, since the relatively small contact area of the roller to the cam lobe allows a little better control over the duration while allowing higher lifts.

High lift cams also require different valve springs, since most stock valve springs are not designed to survive lifts above 0.470" without binding/breaking.

It's also important to match the cam profile to the engine displacement. It's far too common to over-cam a small displacement engine and not have decent power levels until 4,000 RPM or higher. Unfortunately, this doesn't work well with many injection systems, since the fuel delivery starts to fall off at around 4,500 RPM. It's also important to decide the weight and intended use of the vehicle. A heavy car or truck that needs a lot of low-end torque to get rolling will have a slightly diffrerent ideal profile than a lighter car with the same engine. A car used for circle track racing that seldom runs below 5,000 RPM will have a different profile than a car used for ¼ mile jaunts.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
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[This message has been edited by Vader (edited January 01, 2001).]

 

I would Like to jump into this Discussion With a Question of My own pertaining to Cam Selection

Ive got the Lg4, as you probrably already know. Now I would like to Swap the cam out, but Im not entirely Sure What I should Look For. This will be In Conjunction With a performer RPM intake and I already have a Pretty healthy Carb. I was considering the Performer RPM cam But I have dirveability Concerns, and Also Head Flow Concerns. There Is not Much point Having a cam that Out Flows my heads, And I dont Want to Dive into the Head World ($$$) just yet, If ever On this Motor. ( it is a Lg4, after all ). The performer has a gross lift of .488 or so, I Dont recall the LSA or Duration Off Hand, But this is a pretty Healthy Lift.

I will need to Pass Smog with this Cam in the Motor, Altho I will put all the other Emissions stuff Back on. As usual I would like the Best of both Worlds with Low end Torque and high end Power, But Realistically I would prefer To Make Power that Peaks out at about 5500, as opposed to Low end grunt. ( altho 55 isnt all that High )

The Only Driveability Concern I Have Is Gas Milage and Emissions, Im Young I can Live with the Unpleasent ( ha ) Nature of all the rest.

This Will be With 3.42's out back eventually, and m not Opposed to having to Go to a higher stall, and Will pick that to match the cam as opposed to the other way around. The Car is In Full Plush trim, So its fairly Heavy.
Its My Daily Driver, But Will see Some Definate Track time.

Thakns For your Time , And Sorry To Butt In New87 but this topic is beaten to death and I didnt want to Clutter the Board with another Cam Topic

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60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver

Livin' the Stereotype

 

Hey Bort, in the interest of staying smog legal, you'll probably want to stick with a cam that has a lobe separation of 112 degrees or higher. Too much overlap causes unburnt fuel to be dumped into the exhaust system. I would also try to keep the duration at or below 280 degrees in advertised figures(220 or lower @.050 lift). Since you plan on sticking with the stock heads for now, I'd try to stay at .450 lift or lower since the stock heads wouldn't be able to flow enough to match any higher lift. Also, your stock valve springs should be okay with that amount of lift. Hope this helps narrow it down some for you.



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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."