Cam Lobe question
11-14-2002, 01:28 AM
#1
TECH Senior Member
Thread Starter
Cam Lobe question ok.. what does XE-R mean? I know it means fast ramp speed.. but what does it mean?
I've been looking at head and cam comparisons but don't remember enough calculus to compute the area under the curve (all that matters)
with a cam, if you take the center line, go up to max lift, and make a triangle, seems the less area filled in the better?
(faster ramps?)
regardless... how do you compare XE-R to TR ramp speeds? There has to be a way of calculating the area under the curve/triangle... I'm not looking for opinions here.. i'm looking for hard facts aka math
thanks
Mike
guess what i mean is, draw a right triangle, then a line from the top 45* angle to the bottom... If you draw the same as the hypontonuse (SP?) it's a long distance, but if you draw it at a less than 45* angle it will reach the top (max lift) faster...
/=
//=
//=
// =
// =
// =
// =
// =
// =
// =
//______=
/=
//=
/ /=
/ / =
/ / =
/ / =
/ / =
/ / =
/ / =
/ / =
/ /____=
EDIT: what i see now looks cool... but what you see looks like crap... multiple spacfes aren't acceptd
looking at these two , the 2nd looks more aggressive since the angle at the top is less (ignore the fact that it looks like ****... this is ascii geometry) (there are errors in diagrams, but you cans ee the difference in the inner line....
There must be an equation for ramp speed ...
If we can dtermine a standard for measuring everything then we can fairly compare cam lobe speed, head flow/velcity, etc..
<small>[ November 14, 2002, 01:54 AM: Message edited by: horist ]</small>
I've been looking at head and cam comparisons but don't remember enough calculus to compute the area under the curve (all that matters)
with a cam, if you take the center line, go up to max lift, and make a triangle, seems the less area filled in the better?
(faster ramps?)
regardless... how do you compare XE-R to TR ramp speeds? There has to be a way of calculating the area under the curve/triangle... I'm not looking for opinions here.. i'm looking for hard facts aka math
thanks
Mike
guess what i mean is, draw a right triangle, then a line from the top 45* angle to the bottom... If you draw the same as the hypontonuse (SP?) it's a long distance, but if you draw it at a less than 45* angle it will reach the top (max lift) faster...
/=
//=
//=
// =
// =
// =
// =
// =
// =
// =
//______=
/=
//=
/ /=
/ / =
/ / =
/ / =
/ / =
/ / =
/ / =
/ / =
/ /____=
EDIT: what i see now looks cool... but what you see looks like crap... multiple spacfes aren't acceptd
looking at these two , the 2nd looks more aggressive since the angle at the top is less (ignore the fact that it looks like ****... this is ascii geometry) (there are errors in diagrams, but you cans ee the difference in the inner line....
There must be an equation for ramp speed ...
If we can dtermine a standard for measuring everything then we can fairly compare cam lobe speed, head flow/velcity, etc..
<small>[ November 14, 2002, 01:54 AM: Message edited by: horist ]</small>
11-14-2002, 04:53 AM
#2
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Re: Cam Lobe question If you want to calculate the area under a dyno curve, you need to do a numerical integration. You can use Simpsons 3/8 rule for something like this. Just take data points every 100 rpm's and do the numerical integration. It's fairly easy to do this by hand. Let me know if you need more help on that.
As for the ramp differences....I dunno <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />
As for the ramp differences....I dunno <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />
11-14-2002, 07:07 PM
#3
TECH Fanatic
Re: Cam Lobe question </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by horist:
<strong>
I've been looking at head and cam comparisons but don't remember enough calculus to compute the area under the curve (all that matters)
with a cam, if you take the center line, go up to max lift, and make a triangle, seems the less area filled in the better?
(faster ramps?)
regardless... how do you compare XE-R to TR ramp speeds? There has to be a way of calculating the area under the curve/triangle... I'm not looking for opinions here.. i'm looking for hard facts aka math
thanks
Mike
looking at these two , the 2nd looks more aggressive since the angle at the top is less (ignore the fact that it looks like ****... this is ascii geometry) (there are errors in diagrams, but you cans ee the difference in the inner line....
There must be an equation for ramp speed ...
If we can dtermine a standard for measuring everything then we can fairly compare cam lobe speed, head flow/velcity, etc..</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Mike,
Don't get too lost in the math without a good basic understanding of what valve timing a particular engine needs.
You are on the right track with 'area under the lift curve', but more, not less is generally better.
The units for this area are (x-axis) Duration[in degrees] X (y-axis) Lift [in inches], often expressed as "degree-inches". If you really wanted to know the exact values, the easiest way I know of is to use a computer engine analysis program called 'Engine Analyzer Pro'. But how that area is achieved may be even more important. This gets long, but it's not very math heavy.
Probably the limiting physical factor in the valve train is the slope of the cam flank (see SStroker Ace's post). This is limited by the diameter of the foot of a flat lifter, and by somewhat similar geometry of a roller lifter. There is a practical physical limit to how much the lifter can rise per degree of camshaft rotation. This is expressed in "Inches per degree" and is called "cam velocity".
Here's the non-intuitive part: Cam Velocity is not related to engine rpm. It is a mechanical relationship between the cam flank and the lifter. The higher the cam velocity, the more stress placed on the valve train at every rpm.
Now to get more area under the lift curve, you can increase either or both the lift and duration. Because duration plays such a big part in what the overall characteristics of the engine performance will be, a designer may want to limit duration to say 225 degrees (@ .050 cam lift). If you checked a number of hydraulic lifter cams with approximately that duration you might find lift at the valve (with 1.5 rocker ratios) from .450 inches to .567 inces. The area under those lift curves would be very different, as would the engine performance. The .450 is a flat lifter design, and the .567 is a "very aggressive" hydraulic roller design.
The "aggressiveness" of the cam is the slope of the flank that you ASCII-sketched. The .567 lift has a much steeper slope; it has more "cam velocity"
If we divided the lift by the duration, we'd get inches per degree, the same units as cam velocity, which might be a simplified measure of "aggressiviness". In this example, .450/225=.002 and .567/225 = .00252 or 26% more. Now this isn't really "cam velocity" but it is an easy way to compare cams.
As for area under the curve, for comparisons use the formula for area of a triangle, 1/2 base times height. The base, in our case is 225 and height is .567 so an approximation of area is 225/2 x .567 = 63.79 with the aggressive hydraulic roller. To get the same area with say an Xtreme Energy flat hydraulic (less aggressive)would take 246 degrees and .516 lift. That 20 degrees more duration and 9% less lift would give very different engine performance characteristics, even thought the area under the curve would be about the same.
If you really like math, be advised that cam designers work not only with lift, velocity, acceleration, and jerk(3rd derivative) but also with the 4th, 5th and 6th derivatives. I get lost visualizing anything after "jerk", the rate of change of acceleration. How about you?
My $.02
<strong>
I've been looking at head and cam comparisons but don't remember enough calculus to compute the area under the curve (all that matters)
with a cam, if you take the center line, go up to max lift, and make a triangle, seems the less area filled in the better?
(faster ramps?)
regardless... how do you compare XE-R to TR ramp speeds? There has to be a way of calculating the area under the curve/triangle... I'm not looking for opinions here.. i'm looking for hard facts aka math
thanks
Mike
looking at these two , the 2nd looks more aggressive since the angle at the top is less (ignore the fact that it looks like ****... this is ascii geometry) (there are errors in diagrams, but you cans ee the difference in the inner line....
There must be an equation for ramp speed ...
If we can dtermine a standard for measuring everything then we can fairly compare cam lobe speed, head flow/velcity, etc..</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Mike,
Don't get too lost in the math without a good basic understanding of what valve timing a particular engine needs.
You are on the right track with 'area under the lift curve', but more, not less is generally better.
The units for this area are (x-axis) Duration[in degrees] X (y-axis) Lift [in inches], often expressed as "degree-inches". If you really wanted to know the exact values, the easiest way I know of is to use a computer engine analysis program called 'Engine Analyzer Pro'. But how that area is achieved may be even more important. This gets long, but it's not very math heavy.
Probably the limiting physical factor in the valve train is the slope of the cam flank (see SStroker Ace's post). This is limited by the diameter of the foot of a flat lifter, and by somewhat similar geometry of a roller lifter. There is a practical physical limit to how much the lifter can rise per degree of camshaft rotation. This is expressed in "Inches per degree" and is called "cam velocity".
Here's the non-intuitive part: Cam Velocity is not related to engine rpm. It is a mechanical relationship between the cam flank and the lifter. The higher the cam velocity, the more stress placed on the valve train at every rpm.
Now to get more area under the lift curve, you can increase either or both the lift and duration. Because duration plays such a big part in what the overall characteristics of the engine performance will be, a designer may want to limit duration to say 225 degrees (@ .050 cam lift). If you checked a number of hydraulic lifter cams with approximately that duration you might find lift at the valve (with 1.5 rocker ratios) from .450 inches to .567 inces. The area under those lift curves would be very different, as would the engine performance. The .450 is a flat lifter design, and the .567 is a "very aggressive" hydraulic roller design.
The "aggressiveness" of the cam is the slope of the flank that you ASCII-sketched. The .567 lift has a much steeper slope; it has more "cam velocity"
If we divided the lift by the duration, we'd get inches per degree, the same units as cam velocity, which might be a simplified measure of "aggressiviness". In this example, .450/225=.002 and .567/225 = .00252 or 26% more. Now this isn't really "cam velocity" but it is an easy way to compare cams.
As for area under the curve, for comparisons use the formula for area of a triangle, 1/2 base times height. The base, in our case is 225 and height is .567 so an approximation of area is 225/2 x .567 = 63.79 with the aggressive hydraulic roller. To get the same area with say an Xtreme Energy flat hydraulic (less aggressive)would take 246 degrees and .516 lift. That 20 degrees more duration and 9% less lift would give very different engine performance characteristics, even thought the area under the curve would be about the same.
If you really like math, be advised that cam designers work not only with lift, velocity, acceleration, and jerk(3rd derivative) but also with the 4th, 5th and 6th derivatives. I get lost visualizing anything after "jerk", the rate of change of acceleration. How about you?
My $.02
11-15-2002, 12:42 AM
#4
Re: Cam Lobe question The ramp is the area on the Lobe where it transitions from a closed valve postion to an open one.
The flank is the area between that and the top of the lobe.
Actually you want as much area as you can get on the lobe. The total lift area is the important part. Actually cams should be quoted in a total inch degrees, but they are not. I like to have the most area in the least amount of duration I can use, that usually means high lifts.
Bret
<small>[ November 14, 2002, 08:27 PM: Message edited by: SStrokerAce ]</small>
The flank is the area between that and the top of the lobe.
Actually you want as much area as you can get on the lobe. The total lift area is the important part. Actually cams should be quoted in a total inch degrees, but they are not. I like to have the most area in the least amount of duration I can use, that usually means high lifts.
Bret
<small>[ November 14, 2002, 08:27 PM: Message edited by: SStrokerAce ]</small>
