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Camshaft tech.

T

thefarside

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Just and FYI but Chevy High Performance mag has a great article about camshafts, May 2004 issue. They start out with a overview of terms and basic cam math and go into examples on a small 305 to a wild 383 stage III. Damn good reading! It helped me avoid a lot of needless posts here on the forums. /forums/images/graemlins/thumb.gif
 
I'm not sure I agree with them 100%. I am no master engine builder or expert on the subject but in response to:

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On the other hand, simply changing to a 1.6:1 ratio would deliver an additional 0.031 inch lift to move the valve a total of 0.496 inch from its seat. Of course, with this additional lift comes additional duration as well, which brings us to our next subject.

Valve Duration

The next most popular camshaft measurement is the duration of time a camshaft lobe keeps the valve off its seat. The purpose of a valve is to regulate the flow of air. In order to measure a camshaft's duration, you must use a degree wheel in conjunction with a predetermined lift point. The most common duration references are advertised duration measured at 0.050-inch lift. While the advertised numbers are supposed to be measured at 0.020-inch lift, some cam grinders measure farther down the lobe to make their camshaft look more radical than it really is. Because of this confusion, most engine builders use the 0.050-inch duration figure, as will we throughout the rest of this story.

Duration is probably the biggest factor in determining an engine's overall character. When the length of the duration is increased, the engine's maximum top-end potential will increase as well. This is due to the intake and exhaust valves being held open longer to move more air and fuel through the cylinder. While this long-duration technique is great for making upper-rpm power, there is also a negative effect that must be considered. When the valves stay open longer, it requires them to leave the seat sooner and close back down on it later, which causes an overlap condition. This allows the combusted cylinder pressures inside the cylinders to fall at low rpm, which in turn creates a loss of low-speed torque. >> As for upper-rpm cylinder-pressure bleeding, there isn't enough time for a considerable amount of pressure to disperse before the next engine cycle can begin.

Sooner or later, the question of ultimate valve timing manipulation will enter the picture and issues of piston-to-valve clearance will become a problem. While most enthusiasts believe that total valve lift is primarily responsible for piston-to-valve clearance, it actually plays only a small part. Pistons contact the valves due to extensive amounts of duration. Camshafts that typically measure less than 0.550-inch lift and 220 degrees of duration (at 0.050-inch lift) are usually safe with flat-top pistons, but even then they should be checked for acceptable clearance.


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A cam lobe baseline is 0, any portion of the bump/lobe is >0, and that lobe only exists for X degrees of camshaft rotation. That never changes (except for wear).

That being said if the lobe exists from 25 degrees to 75 degrees, at any point before or after the valve lift is 0. 0 times 1.5 or 0 times 1.6 is still zero. The only thing changing the when rocker arm ratio changes is lift, NOT duration.......unless of course I missed something in physics class.

Am I right or did I miss something?
 
I fully agree with you and wonder why they say duration will change. It is impossible for it to change unless the cam is worn or modified.
 
i guess maybe theyre thinkin if you open it further it is gonna take longer to shut therefore making it seem as though more duration?
 
The valve opens farther but the lob is rotating at a given speed therefore the valve travels a greater distance in the same period of time. Doesn't faster valve operation help in making power. /forums/images/graemlins/dunno.gif sounds right.
 
The duration at a valve lift of 0.05 will change when switching from 1.5 to 1.6 rockers, The 1.6 rockers will make the cam appear to have more duration. The duration from seat to seat will not change.
 
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The duration at a valve lift of 0.05 will change when switching from 1.5 to 1.6 rockers, The 1.6 rockers will make the cam appear to have more duration. The lift from seat to seat will not change.

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I agree with what you are saying, but your wording could allow for confusion. I agree valve lift will change, and therefore flow and cylinder pressures will change (ie. the reason for changing ratios without having to swap a cam), but the word duration is misused in the first portion of your sentence.

In reality, it may Appear to the engine to have the effects of longer duration cam (from a flow standpoint) by using a higher ratio rocker arm, but the actual duration Never changes.


I'm going by the following definition:
Main Entry: du·ra·tion
Function: noun
1 : continuance in time
2 : the time during which something exists or lasts

It is when the valve is open vs. closed. Not how far the valve is open or how much flow is allowed.

Duration and lift are ground into a cam profile. The lift can be multiplied by using a ratio'd rocker arm but duration cannot be changed. Does using a higher ratio rocker arm have similar effects of using a longer duration cam....yes. I just wanted to get the wording straight as not to confuse myself (which is easily done /forums/images/graemlins/blush.gif) or someone else. /forums/images/graemlins/peace.gif
 
The advertised duration of a cam is listed at 0.05 in of valve lift using a 1.5 ratio rocker for a SBC. Change that to a 1.6 ratio rocker and the lift at that same point changes to 0.053333 inches of valve lift. To obtain 0.05 in of valve lift using a 1.6 rocker the camshart would have to be in the possition where the 1.5 ratio rocker would give 0.0469 inches of valve lift. Therefore steepnes of the ramp will determine the added duration at 0.05 in valve lift.
 
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The advertised duration of a cam is listed at 0.05 in of valve lift using a 1.5 ratio rocker for a SBC. Change that to a 1.6 ratio rocker and the lift at that same point changes to 0.053333 inches of valve lift. To obtain 0.05 in of valve lift using a 1.6 rocker the camshart would have to be in the possition where the 1.5 ratio rocker would give 0.0469 inches of valve lift. Therefore steepnes of the ramp will determine the added duration at 0.05 in valve lift.

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***
At work so some thoughts are repeats, but I had to keep going back and forth.
***

Close but not quite. If you wrote steepness of the ramp will determine the added amount of available flow due to a higher ratio rocker, I would agree with you 100%

Lobe height at any given point (regardless of steepness) is mutiplied by the rocker arm ratio to give lift at the valve.

Zero times any given ratio equals Zero. The cam (regardless of rocker ratio) will always be at Zero for the same amount of time (do you agree with this)?

Since a steeper ramp will cause the valve to lift quicker, that quickness will be multiplied with a higher ratio rocker (compared to a cam with gradual ramp), that I will agree to. I will also agree the valve will lift higher with a higher ratio rocker. Both of these factors will act as if the cam had a longer duration compared to using a lower ratio rocker, but will NOT increase the duration.


************************

I guess I’m a little confused, either that or we are agreeing in different ways.

I know a lifter that is attached to a cam lobe where the lobe is .05” above centerline will cause the lifter to raise .05” (I know it’s actually a little different as the lobe runs against the flat bottom of a typical lifter causing a ratio other than 1:1, but for this example lifter lift will equal cam lobe height above centerline). A valve attached to a 1.5 ratio rocker will be at .075” (.05 x 1.5) of lift, whereas a valve attached to a 1.6 (.05 x 1.6) ratio rocker will be at .08” of lift. This will allow more air to flow in the given amount of time, but NOT for a longer period of time.

The lifter follows the lobe/ramp of the cam. Unless you change the cam or shape of the lifter bottom, the lifter will move exactly the same amount at the same time regardless of rocker arm ratio. When the lifter is as zero height (baseline of cam) the valve is at zero lift. The valve at the end of a 1.6 ratio rocker will be at zero lift for the same exact amount of time as a valve at the end of a 1.5 ratio rocker which causes duration to be exactly the same.

After the zero mark, the valve at the end of the 1.6 ratio rocker will be at .1 times greater lift than the one at the end of a 1.5 ratio rocker, so you will be able to flow more air, but NOT for a greater amount of time.

Do we agree on that?

If we do, duration is exactly the same nomatter what ratio you want to use on a rocker arm.

I also agree changing the ratio of the rocker arm will give similar effects as if you chose to run a cam with different duration (ie. Where most people get confused), but does NOT change duration.

Am I making sense or not (a lot of times I don’t explain myself well).
 
Im thinking that maybe you should add that duration is measured at the cam, not at the valve, therefore the measurement at .050" lobe lift will be the same no matter what rocker is used. /forums/images/graemlins/thinking.gif
 
I guess I should have put it this way:

If you have a cam where the lobe starts at 3 degrees ATDC and ends at 27 degrees ATDC. The lobe would open and close the valve in 24 degrees of cam rotation. How long does it take for the valve to open and close with a 1.5 ratio and with a 1.6 ratio? Both take 24 degrees of cam rotation (i.e. same exact duration). Which has the ability to flow more.....the 1.6 of course.
It acts as if you gave the came a longer duration without actually increasing it.

I'm hoping this is making sense, or is it way off base?

One of the reasons I am pressing this issue is that I am in the process of rebuilding a SBC and want to understand as much as can for components selection.
 
Here are a few things I pulled from the Crane web site.

What is Camshaft Duration and why is it important?

Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. Duration (at .050" lifter rise) is the deciding factor to what the engine's basic RPM range will be. Lower duration cams produce the power in the lower RPM range. Larger duration cams operate at higher RPM, but you will lose bottom end power to gain top end power as the duration is increased. (For each ten degree change in the duration at .050", the power band moves up or down in RPM range by approximately 500 RPM.) <font color="red"> </font>

What is the difference in Advertised Duration and Duration at .050" Lifter Rise (Tappet Lift)?

In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same. There are two key components for measuring duration-- the degrees of crankshaft rotation and at what point of lifter rise the measurements were taken. Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050" lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts. <font color="red"> </font>

How does an increase in Rocker Arm Ratio improve the engine's performance?

The lobe lift of the cam is increased by the ratio of the rocker arm to produce the final amount of valve lift. A cam with a .320" lobe lift using a 1.50:1 ratio rocker arm will have a .480" valve lift (.320" x 1.50 = .480"). If you install rocker arms with an increased ratio of 1.60:1, with the same cam, the lift would increase to .512" (.320" x 1.60 = .512"). The engine reacts to the movement of the valve. It doesn't know how the increased lift was generated. It responds the same way it would as if a slightly larger lift cam had been installed. In fact, since the speed of the valve is increased with the higher rocker arm ratio, the engine thinks it has also gained 2° to 4° of camshaft duration.

The end result is an easy and quick way to improve the performance of the existing cam without having to install a new one. See the Buyers Guide section for availability of increased ratio rocker arms. Remember, whenever you increase the valve lift, with either a bigger cam or larger rocker arm ratio, you must check for valve spring coil bind and for other mechanical interference. Please review the previous sections concerning these matters. <font color="red"> </font>

Hope this helps.
 
[ QUOTE ]
Here are a few things I pulled from the Crane web site.

What is Camshaft Duration and why is it important?

Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. Duration (at .050" lifter rise) is the deciding factor to what the engine's basic RPM range will be. Lower duration cams produce the power in the lower RPM range. Larger duration cams operate at higher RPM, but you will lose bottom end power to gain top end power as the duration is increased. (For each ten degree change in the duration at .050", the power band moves up or down in RPM range by approximately 500 RPM.) <font color="red"> </font>

What is the difference in Advertised Duration and Duration at .050" Lifter Rise (Tappet Lift)?

In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same. There are two key components for measuring duration-- the degrees of crankshaft rotation and at what point of lifter rise the measurements were taken. Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050" lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts. <font color="red"> </font>

How does an increase in Rocker Arm Ratio improve the engine's performance?

The lobe lift of the cam is increased by the ratio of the rocker arm to produce the final amount of valve lift. A cam with a .320" lobe lift using a 1.50:1 ratio rocker arm will have a .480" valve lift (.320" x 1.50 = .480"). If you install rocker arms with an increased ratio of 1.60:1, with the same cam, the lift would increase to .512" (.320" x 1.60 = .512"). The engine reacts to the movement of the valve. It doesn't know how the increased lift was generated. It responds the same way it would as if a slightly larger lift cam had been installed. In fact, since the speed of the valve is increased with the higher rocker arm ratio, the engine <font color="red">thinks</font> it has also gained 2° to 4° of camshaft duration.

The end result is an easy and quick way to improve the performance of the existing cam without having to install a new one. See the Buyers Guide section for availability of increased ratio rocker arms. Remember, whenever you increase the valve lift, with either a bigger cam or larger rocker arm ratio, you must check for valve spring coil bind and for other mechanical interference. Please review the previous sections concerning these matters. <font color="red"> </font>

Hope this helps.

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You said it [ QUOTE ]
In fact, since the speed of the valve is increased with the higher rocker arm ratio, the engine <font color="red">thinks</font> it has also gained 2° to 4° of camshaft duration.

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Right from the words of the cam manufacturer, there is no increase in duration, it just makes the engine think so.
 
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Obviously it does not make tha camshaft bigger!!

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Yes, we already agreed on that.

The article stated that it increased lift at the valve (which I agree with) as well as increase duration at the valve (which from the way I read, you agreed with). All I tried pointing out is that is incorrect. It may seem as though the duration increases along with the lift, but it does not (it just seems that way).

I am not trying to argue with you or put anyone down, just trying to make a clarification as not to confuse anybody else (including myself /forums/images/graemlins/blush.gif)

/forums/images/graemlins/cool.gif
 
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