How duration is measured and what's required to calculate duration
07-01-2004, 09:11 AM
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How duration is measured and what's required to calculate duration
Duration
If you look at a cam lobe carefully, you'll notice that the lift is created gradually using a slope. The amount of time (in degrees) that lift is generated is called the duration of the lobe. Here's where we throw you a mild curve. Camshafts operate at half engine speed. This is easy to see because the gear that turns the camshaft is twice the diameter of the crank gear that drives it. That means that the cam spins at half engine speed. Because of this, camshaft duration is always expressed in crankshaft degrees. This makes it easy when it comes time to degree the cam to ensure it is positioned accurately in the engine.
So let's take a typical cam and look at how duration is expressed. The point at which lobe lift first begins is often difficult to identify since the profile is very gradual at this point. A long time ago, the Society of Automotive Engineers (SAE) decided that 0.006 inch of valve lift was a good place to start, but not all cam manufacturers adhered to that standard. They chose instead to use different heights of >> tappet lift, usually between 0.004 and 0.006 inch. Using 0.004 inch as an example, once lobe lift achieves 0.004 inch, you start recording the number of crankshaft degrees it takes for the lobe to run all the way through max lift and back to 0.004-inch lobe lift on the closing side. Let's say this is 270 degrees. This is the advertised duration of the lobe because this is the number that most cam manufacturers use when referring to their camshaft duration numbers in advertising.
The problem with advertising numbers was that not everyone used the same lobe-lift figure to determine duration. This led to significant confusion when it came time to compare numbers. Legend has it that Harvey Crane suggested that all the cam manufacturers use 0.050 inch of tappet lift as a common lobe-lift point that all cam manufacturers would use so that we could compare the cams. This is the number that most people use when referring to duration specs since it uses a common data point. For example, a Crane PowerMax 278 flat-tappet hydraulic has an advertised duration of 278 degrees on the intake and 290 degrees on the exhaust side. The duration at 0.050-inch tappet lift is 222 degrees on the intake lobe and 234 degrees on the exhaust. Because this camshaft uses different intake and exhaust duration figures, it is referred to as a dual-pattern cam. If the intake and exhaust durations are the same, then it would be a single-pattern cam.
We should also go through some information about opening and closing points as well. Each cam company calls out valve opening and closing points differently. For example, a Comp Cams timing card >> will indicate opening and closing points at 0.006 inch of tappet lift. Crane delivers the opening and closing points at both 0.004 inch and 0.050 inch of tappet lift. They both use some abbreviations that you should know. The intake valve opens at a given number of degrees Before Top Dead Center (BTDC) and closes the intake After Bottom Dead Center (ABDC). The exhaust valve generally opens Before Bottom Dead Center (BBDC) and closes After Top Dead Center (ATDC).
There is a simple way to determine duration. Let's say you have a cam in an engine and you're not sure of its duration. Set up a degree wheel on the engine and determine the cam's opening and closing points. Let's say that the intake lobe opens at 4 degrees BTDC and closes at 44 degrees ABDC, both measured at 0.050 inch of tappet lift. Add the opening and closing points together along with 180 degrees and you will have the cam's duration of 228 degrees at 0.050-inch tappet lift (4 + 44 + 180 = 228 degrees). This also works for the exhaust side and you can determine advertised duration the same way. Cool, huh?
Now since Audi uses 1mm for their tappet (lifter) lift all of their opening/closing degrees are based on that number. Same way that if you buy a Schrick or any other european camshaft their published rate of 1mm lift will also be on it's spec sheet. So all camshafts including the opening/closing rates printed in the Bentley manual for the AAH/AFC and ALL other Audis ever made are listed at their rate of lift. Figuring duration from those number is really easy. The europeans have standardized to specs using 1mm lift including Volvo, Saab, Mercedes, Porsche, BMW, Jag and a host of others. Americans do one thing, Europeans do another. Long as we know the number used it's easy to do the math.
Now lets take the idea that "all cams must open/close at the same point" thats been thrown out here. It is true for "single pattern" camshafts. And admittedly there are some single pattern cams available out there as our esteemed colleague has pointed out. But you probably wouldn't want one as it it's less than ideal for any dual use and/or even in a serious race effort. It's also never the "greenest/cleanest" cam you can use and it's inefficient. If valves sizes were identiical it might make more sense but its not the best you can do for your engines breatheability dynamics in ANY situtation. The better path is whats called a "dual pattern" cam that has a distinctly different cam profile for intake and exhaust. All single cam Audis use a dual pattern cam and much the same thing is accomplished by twin cams. These cams are maximized for best efficiencies on both the intake and exhaust sides rather than a "one size suits both" approach. These cams intake/exhaust lobes will NEVER open/close at the same/identical degree of rotation... EVER!!
So in closing remember that all cam openings/closings have been given to us with their lifter pre-loads calculated in. Therefore it's easy to do the math when figuring their durations. American cams have almost all started using .050" as their standard and the europeans use 1mm. But any measure of duration taken from the Bentley manual already has the 1mm feature calculated in. It's printed on the cam spec page in every Bentley for EVERY Audi cam as the measurement used. And even the aftermarket cams that come with a spec sheet that will contain the same info. So as I said earlier what we're chasing here is the duration. And duration is based on an "event", ie; opening and/or closing. And the event, if based on cam grinders specs is followed will always be xxx degrees of duration based on 1mm pre-lift with our Audis. And the only time an apples-to-oranges comparison could conceivably happen is if you try to use Crane, Crower or some other american manufacturers number of lift for your Audi cam. But that only happens in pgoyer's world.
If you look at a cam lobe carefully, you'll notice that the lift is created gradually using a slope. The amount of time (in degrees) that lift is generated is called the duration of the lobe. Here's where we throw you a mild curve. Camshafts operate at half engine speed. This is easy to see because the gear that turns the camshaft is twice the diameter of the crank gear that drives it. That means that the cam spins at half engine speed. Because of this, camshaft duration is always expressed in crankshaft degrees. This makes it easy when it comes time to degree the cam to ensure it is positioned accurately in the engine.
So let's take a typical cam and look at how duration is expressed. The point at which lobe lift first begins is often difficult to identify since the profile is very gradual at this point. A long time ago, the Society of Automotive Engineers (SAE) decided that 0.006 inch of valve lift was a good place to start, but not all cam manufacturers adhered to that standard. They chose instead to use different heights of >> tappet lift, usually between 0.004 and 0.006 inch. Using 0.004 inch as an example, once lobe lift achieves 0.004 inch, you start recording the number of crankshaft degrees it takes for the lobe to run all the way through max lift and back to 0.004-inch lobe lift on the closing side. Let's say this is 270 degrees. This is the advertised duration of the lobe because this is the number that most cam manufacturers use when referring to their camshaft duration numbers in advertising.
The problem with advertising numbers was that not everyone used the same lobe-lift figure to determine duration. This led to significant confusion when it came time to compare numbers. Legend has it that Harvey Crane suggested that all the cam manufacturers use 0.050 inch of tappet lift as a common lobe-lift point that all cam manufacturers would use so that we could compare the cams. This is the number that most people use when referring to duration specs since it uses a common data point. For example, a Crane PowerMax 278 flat-tappet hydraulic has an advertised duration of 278 degrees on the intake and 290 degrees on the exhaust side. The duration at 0.050-inch tappet lift is 222 degrees on the intake lobe and 234 degrees on the exhaust. Because this camshaft uses different intake and exhaust duration figures, it is referred to as a dual-pattern cam. If the intake and exhaust durations are the same, then it would be a single-pattern cam.
We should also go through some information about opening and closing points as well. Each cam company calls out valve opening and closing points differently. For example, a Comp Cams timing card >> will indicate opening and closing points at 0.006 inch of tappet lift. Crane delivers the opening and closing points at both 0.004 inch and 0.050 inch of tappet lift. They both use some abbreviations that you should know. The intake valve opens at a given number of degrees Before Top Dead Center (BTDC) and closes the intake After Bottom Dead Center (ABDC). The exhaust valve generally opens Before Bottom Dead Center (BBDC) and closes After Top Dead Center (ATDC).
There is a simple way to determine duration. Let's say you have a cam in an engine and you're not sure of its duration. Set up a degree wheel on the engine and determine the cam's opening and closing points. Let's say that the intake lobe opens at 4 degrees BTDC and closes at 44 degrees ABDC, both measured at 0.050 inch of tappet lift. Add the opening and closing points together along with 180 degrees and you will have the cam's duration of 228 degrees at 0.050-inch tappet lift (4 + 44 + 180 = 228 degrees). This also works for the exhaust side and you can determine advertised duration the same way. Cool, huh?
Now since Audi uses 1mm for their tappet (lifter) lift all of their opening/closing degrees are based on that number. Same way that if you buy a Schrick or any other european camshaft their published rate of 1mm lift will also be on it's spec sheet. So all camshafts including the opening/closing rates printed in the Bentley manual for the AAH/AFC and ALL other Audis ever made are listed at their rate of lift. Figuring duration from those number is really easy. The europeans have standardized to specs using 1mm lift including Volvo, Saab, Mercedes, Porsche, BMW, Jag and a host of others. Americans do one thing, Europeans do another. Long as we know the number used it's easy to do the math.
Now lets take the idea that "all cams must open/close at the same point" thats been thrown out here. It is true for "single pattern" camshafts. And admittedly there are some single pattern cams available out there as our esteemed colleague has pointed out. But you probably wouldn't want one as it it's less than ideal for any dual use and/or even in a serious race effort. It's also never the "greenest/cleanest" cam you can use and it's inefficient. If valves sizes were identiical it might make more sense but its not the best you can do for your engines breatheability dynamics in ANY situtation. The better path is whats called a "dual pattern" cam that has a distinctly different cam profile for intake and exhaust. All single cam Audis use a dual pattern cam and much the same thing is accomplished by twin cams. These cams are maximized for best efficiencies on both the intake and exhaust sides rather than a "one size suits both" approach. These cams intake/exhaust lobes will NEVER open/close at the same/identical degree of rotation... EVER!!
So in closing remember that all cam openings/closings have been given to us with their lifter pre-loads calculated in. Therefore it's easy to do the math when figuring their durations. American cams have almost all started using .050" as their standard and the europeans use 1mm. But any measure of duration taken from the Bentley manual already has the 1mm feature calculated in. It's printed on the cam spec page in every Bentley for EVERY Audi cam as the measurement used. And even the aftermarket cams that come with a spec sheet that will contain the same info. So as I said earlier what we're chasing here is the duration. And duration is based on an "event", ie; opening and/or closing. And the event, if based on cam grinders specs is followed will always be xxx degrees of duration based on 1mm pre-lift with our Audis. And the only time an apples-to-oranges comparison could conceivably happen is if you try to use Crane, Crower or some other american manufacturers number of lift for your Audi cam. But that only happens in pgoyer's world.
07-01-2004, 09:42 AM
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Re: How duration is measured and what's required to calculate duration
Mance that was excellent, and I am not kidding. The thing that bothers me though is that the Eurospec cams are measured from 0.1mm, not 1mm and so the duration values are not directly comparable. You have said that yourself in your disertation.
I will be measuring my cams as you described so we can do the comparison in a meaningful way.
I will be measuring my cams as you described so we can do the comparison in a meaningful way.
07-01-2004, 10:07 AM
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it's impossible to get there with old heads or more accurately cost-prohibitive...
you would need to have the heads set up and built by a head shop to achieve those numbers on your stock heads. Over the years/miles valve seats get hammered in and valve seaing rings do also. This raises the stems higher than they were and puts more pre-lift back into the lifter/lobe ratios. But your duration remains largely unaffected, ie: it will reduce some but probably only in the sub-1-2 degree range. Unless starting with new valves, lifter, springs, seats those numbers are going to be off from advertised no matter aht you do. Those numbers only work in a head set-up and macbined to "make" those values work. But in any used engine thay cannot and will not work with just throwing cams and lifters in then buttoning up and calling it a day.
Only way you can determine "exactly" what you have for duration is to do the cams and then do the degree wheel checking. But on used heads you have to do a TDC for EVERY cyinder due to the aforementioned valve head/seat wear and increased stem height.
And even this it's much to do about nothing as on a street cr with more than 10k miles it cannot be within .5mm of every valves original installed height.
Also you will get more HP with higher duration. I don't think anyone here is arguing with that. It's just that HP it will be "peak" horsepower and will come later in the RPM's. In other words you will be faster than someone in a drag race with side-stepping the clutch. But you will lose any/all 2nd gear 1000 RPM rolling starts to the 60mph redline. And duration is responsible for that affect. And 18HP cannot overcome even a half carlength lead in that race. It would take closer to 60HP to do that. Beating someone is relatively easy if properly prepared. Overcoming someone who has pulled on you is next to impossible in any race shorter than top speed. It takes HUGE gobs of HP to overcome any early defecit... far more than is usually attainable with simple cams no matter how aggressive.
Only way you can determine "exactly" what you have for duration is to do the cams and then do the degree wheel checking. But on used heads you have to do a TDC for EVERY cyinder due to the aforementioned valve head/seat wear and increased stem height.
And even this it's much to do about nothing as on a street cr with more than 10k miles it cannot be within .5mm of every valves original installed height.
Also you will get more HP with higher duration. I don't think anyone here is arguing with that. It's just that HP it will be "peak" horsepower and will come later in the RPM's. In other words you will be faster than someone in a drag race with side-stepping the clutch. But you will lose any/all 2nd gear 1000 RPM rolling starts to the 60mph redline. And duration is responsible for that affect. And 18HP cannot overcome even a half carlength lead in that race. It would take closer to 60HP to do that. Beating someone is relatively easy if properly prepared. Overcoming someone who has pulled on you is next to impossible in any race shorter than top speed. It takes HUGE gobs of HP to overcome any early defecit... far more than is usually attainable with simple cams no matter how aggressive.
07-01-2004, 10:31 AM
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Re: it's impossible to get there with old heads or more accurately cost-prohibitive...
OK, that was also very helpful. Now, my engine has only 17k on it today. But, that is beside the point. I intend on measuring both the stock cam and the Eurospec in the same head. So the valve seat issue should be a wash. What I will get is a relative value, not an absolute one. Since we know the absolute value for each cam (be it the Audi or the Euro) we can then tell from our relative numbers, where they both stand.
Also, it may be very true about losing 0-60 time. That's why I am doing the dyno. I want to see where the cams are making power and where they are giving it up. If I don't like what I see, I'll be switching back, or better yet, I may try the AAH cams.
Anyway, like most things, there isn't any such thing as an "exact" science.
Thanks for the help. You really do know alot about cams. I think we had a misunderstanding, and if it was on my part, I'm sorry.
But listen, when I spoke with CFI, they too knew alot about cams. They said that this cam would make power in the mid range, not upper. That should help, not hurt my 0-60.
We will see
Also, it may be very true about losing 0-60 time. That's why I am doing the dyno. I want to see where the cams are making power and where they are giving it up. If I don't like what I see, I'll be switching back, or better yet, I may try the AAH cams.
Anyway, like most things, there isn't any such thing as an "exact" science.
Thanks for the help. You really do know alot about cams. I think we had a misunderstanding, and if it was on my part, I'm sorry.
But listen, when I spoke with CFI, they too knew alot about cams. They said that this cam would make power in the mid range, not upper. That should help, not hurt my 0-60.
We will see
07-01-2004, 11:19 AM
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About measuring duration with 1mm, .1mm, or .050"....
I hope I don't get flamed here, but pgoyer is right about one thing: measured duration will be different if you start measuring at .1mm or 1mm or .050". I'll draw a pic to describe:
<img src="http://www.justboring.com/audi/images/cam-duration.gif" />
Correct me if I'm wrong, but the duration would just be *similar* to the circumference (as in: in most cases, the bigger the circumference, the more duration [excluding larger peaks]), measured from where the edge became more than 1mm away from the beginning radius.
Obviously, the circumference is <b>not</b> the same as the duration. Well, I can't think of a quick way to describe it... oh well
[enter flames]
<img src="http://www.justboring.com/audi/images/cam-duration.gif" />
Correct me if I'm wrong, but the duration would just be *similar* to the circumference (as in: in most cases, the bigger the circumference, the more duration [excluding larger peaks]), measured from where the edge became more than 1mm away from the beginning radius.
Obviously, the circumference is <b>not</b> the same as the duration. Well, I can't think of a quick way to describe it... oh well
[enter flames]
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07-01-2004, 12:29 PM
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I think it was misunderstood to mean that something with...
measuring the duration. Measuring duration with 1mm will give a different number than measuring it with .050". They can't give the same number, as the cam would contact the measuring device at different times and angles depending on what height you used to begin measurement.
This is the afflicting post:<ul><li><a href="https://forums.audiworld.com/12v/msgs/30290.phtml">https://forums.audiworld.com/12v/msgs/30290.phtml</a</li></ul>
This is the afflicting post:<ul><li><a href="https://forums.audiworld.com/12v/msgs/30290.phtml">https://forums.audiworld.com/12v/msgs/30290.phtml</a</li></ul>