# Cam Questions



## IMCauley (Oct 14, 2010)

So I don't think I'm completely knowledgeable around camshafts and lift and heads. So my general question is (so later I don't look like an idiot), how do you decipher what kind of "lift" you want out of your cam, what do the measurements mean (ex. 232/230), and if I get a new cam since ls2 engines are overhead valve should i still replace the heads? :confused
Thanks for any feedback!


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## jetstang (Nov 5, 2008)

You dont' have to replace the heads ever for a cam swap. You may have to change valve springs, the incresed lift may need stronger springs to prevent valve float or coil bind.


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## BearGFR (Aug 25, 2008)

IMCauley said:


> So I don't think I'm completely knowledgeable around camshafts and lift and heads. So my general question is (so later I don't look like an idiot), how do you decipher what kind of "lift" you want out of your cam, what do the measurements mean (ex. 232/230), and if I get a new cam since ls2 engines are overhead valve should i still replace the heads? :confused
> Thanks for any feedback!


Wow, did you ever open the barn door with that question... I'll try to hit the high points without boring you too much. Bottom line, an engine is a little more than a big air pump. The more air you can get into and out of it, the more fuel you can burn, and the more power/torque you can make. The concept is that simple. All the performance mods that people make in an effort to make power are aimed at that one goal: more air in and out.

Bigger displacement = bigger pump = more air.
Forced induction (turbo, blower) = more air.
Headers = more air (due to better flow).
Cam changes = open the valves 'higher' and/or longer = more air.
Nitrous = more oxygen into the cylinders = more air.
Cylinder head changes / port work = more air flow = more air.

You get the idea...

There's a whole lot beneath the surface on all those topics, especially cam shafts, because of the dynamics of air flow due to the valve opening/closing events and the fact that the air flow is not a constant, but stopping and starting every time the valve closes and opens. The air/fuel mixure, having mass, also has inertia so it takes "time" for it to start moving and stop moving - which is why a cam that works well at high rpm will fall on its behind at low rpm, and vice versa. Ok, enough science for now... you asked what the measurements mean.

Duration: expressed in crankshaft degrees (degrees of crankshaft rotation) during which the valve is "open". For single pattern cams (both intake and exhaust are the same) this will be one number. For dual pattern cams, two, usually with the intake specified first. The numbers will usually be in the 200 to 300 range. "Open" can mean different things for different cam manufacturers. Some define .004" cam lift as "open", others use .005, others might use valve lift. This makes it hard to compare cams. To address this, there is a standard measurement of duration that's measured at 0.050" of lift measured at the cam lobe. You'll often see that expressed as something like 236/242 @ 0.050 ---- which would mean in our example 236 crank degrees of intake duration, 242 crank degrees of exhaust duration, measured at 0.050 lobe lift. These are the numbers you should use to compare different cams. Other duration values as just marketing bs.

Lift: Maximum cam lift on each lobe (intake and exhaust). These too can be specified differently, in terms of lift at the cam lobe or lift at the valve. If measured at the valve then you also have to know what ratio rocker arms were used to get the measurement (often 1.5:1) in order to "back into" the actual cam lobe lift. Again, to compare different cams, always use the actual lift at the cam lobe.

Lobe Separation Angle (LSA). This is the angle between the centerline of the intake lobe at peak lift, and the exhaust lobe at peak lift. This measurement is in *camshaft* degrees instead of crankshaft degrees. (The cam turns half as fast as the crank, so one cam degree = two crank degrees). The LSA has a direct effect on the overlap period (period of time when both intake and exhaust valves are open) and that influences cylinder pressure (compression can't begin until both valves are closed) and, along with duration, the rpm range where the cam is efficient. LSA is usually in the vicinity of 106 to 118 degrees.

All of the above measurements are ground into the cam, so the only way to change them is to change the cam.

Another important measurement is the relative positions of the intake lobe centerline (max lift) and cylinder TDC on the compression stroke. This is something you can change depending on how you "install" the cam and the procedure for setting it is called "degreeing the cam". The measurement is expressed in terms of the number of crankshaft degrees before top dead center (BTDC) where the intake lobe centerline/peak lift occurs. This is usually somewhere between 100 and 110 degrees. Installing a cam "straight up" *usually* means that the cam is installed such that both the intake and exhaust lobe centerlines are the same distance away from TDC. If you install the cam such that the intake lobe is "farther away" from TDC than the exhaust lobe (the intake valve closes "earlier") the cam is said to be "advanced". Similarly, if you go the other way such that the intake valve closes later, the cam is said to be "retarded". The amount of advance or retard is measured in degrees. 
Time for an example:
Assume a cam with a 110-degree LSA (remember cam degrees are double crank degrees).
If you install the cam such that the intake lobe centerline is at 110 crank degrees BTDC, and the exhaust valve centerline is at 110 crank degrees ATDC, then the cam is installed "straight up".
If you install the same cam such that the intake lobe centerline is at 112 degrees BTDC, that will put the exhaust at 108 degrees ATDC and the cam is installed "2 degrees advanced".
If you go the other way (intake at 108 and exhaust at 112), the cam is "2 degrees retarded".

Most cam manufacturers will specify how their various cams should be installed for "best results".

So, why would you advance or retard a cam? To "fine tune" the torque curve and cylinder pressure. Generally, "retarding" the cam will lower cylinder pressure (because the intake valve is closing later) and it will also move the torque peak slightly higher in the rpm range (more horsepower) but it costs you in terms of reducing low rpm torque. Advancing the cam has the opposite effect. 

Commonly, when you install a cam using the factory timing chain/gears with everything in the factory location, you've installed the cam "straight up". I say things like 'usually' and 'commonly' because that's not always the case. Some manufacturers for some of their cams will grind them in such a way that when you use all the factory pieces and marks, the cam is actually advanced or retarded according to the manufacturer's recommendation.

How do you advance or retard a cam? There are several ways, such as using "offset" keys for the crank gear and/or the cam gear that move the gear slightly away from its factory location, or using an aftermarket timing set (cam gear, crank gear, and chain) that have multiple keyways in them that permit installing at the desire offset.

All that's just scratching the surface, but hopefully it's enough to get you started and thinking.

Hope I didn't put anyone to sleep....

Bear


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## ALKYGTO (Mar 29, 2010)

Zzzzzzzzzzzzz.................:lol:


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## IMCauley (Oct 14, 2010)

BearGFR said:


> Wow, did you ever open the barn door with that question... I'll try to hit the high points without boring you too much. Bottom line, an engine is a little more than a big air pump. The more air you can get into and out of it, the more fuel you can burn, and the more power/torque you can make. The concept is that simple. All the performance mods that people make in an effort to make power are aimed at that one goal: more air in and out.
> 
> Bigger displacement = bigger pump = more air.
> Forced induction (turbo, blower) = more air.
> ...


No not at all I was very much awake throughout the whole thing hahaha that's all of the information I need and then some! thanks a lot! 

Would you say it's better to find cams similar to your specifications or find a place that can make them to your specs? I'm sure there is a huge difference money-wise.


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## Poncho Dan (Jun 30, 2009)

You cannot go higher than .600 lift without changing out valve springs and lifters on the LS motor.

Or is it fly-cutting the pistons? I can't remember.

That's a bridge I'll cross when I get to it. :lol:


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## BearGFR (Aug 25, 2008)

IMCauley said:


> Would you say it's better to find cams similar to your specifications or find a place that can make them to your specs? I'm sure there is a huge difference money-wise.


Having a one-off custom grind done just for you can get pricey. The cam I'm running in my 69 "400" is a custom grind, but its not MY custom. It was spec'ed by Jim Lehart of Central Virginia Machine and he sells them to his customers, so the price wasn't bad. With as much stuff as is available for the LS, and more coming out every day, I'd be surprised if there weren't something "off the shelf" that would fit whatever your goals are.




Poncho Dan said:


> You cannot go higher than .600 lift without changing out valve springs and lifters on the LS motor.
> 
> Or is it fly-cutting the pistons? I can't remember.


Could be both springs and pistons. What would drive a spring change would be if the existing springs can't deal with more lift without going into coil bind (the springs compress solid and can't compress any more). That tends to break other valve train parts in very ugly and expensive ways. Likewise, you might have to cut the pistons to keep the valves from "kissing" them due to the higher lift. Having a valve hit a piston is even worse than what can happen with coil bind. Even just lengthening valve duration alone without increasing lift can lead to piston-to-valve clearance problems because the valve is open longer, allowing the piston to get closer before the valve starts to close.

Roller lifters can be reused with a cam change if they're in good condition. Flat tappet lifters never can - because the lifter and the cam lobe wear in together in a complimentary wear pattern. Once installed in an engine and run, the same lifter has to remain on the same cam lobe "forever" or until both are replaced. Changing one or the other by itself, or even just moving a lifter to a different cam lobe, most often results in "wiping off" the corresponding cam lobe in very short order.

Bear


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