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Discussion Starter · #1 ·
Its an ammount of time .
The degrees in which a crankshaft turns from when a valve is closed to when its open. 050" on a given engine is just that and cannot be measured by a time frame.
Why didn't they call it "crankshaft degrees @ .050" of lift "instead of "duration @ .050 " "
Lol. Silly but .....
 

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Discussion Starter · #3 ·
Yes its how LONG somthing happens.
That's an ammount of time . The definition of duration is an ammount of time.
How long as in how much time. Not how much distance or number of camshaft degrees
 

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As I read it years ago, duration paper timing numbers were all over the place with cams and cam makers, I mean really, GM had a BBC cam that had over 370°advertised, lol really, I don't think so. So in the 70's every aftermarket cam manufacturer started testing and degreeing in cams at .050 instead of the unreliable .006 by the 80's everyone found that @ .050 cam duration numbers were much more accurate and reliable and that became the std. of the industry. There are also other things I noticed with duration numbers the intrigued me, like when a advertised duration number was like only 30-35° away from the .050 effective number making it a cam with explosive power out of the corners in circle track racing. I soon learned that it would work well in drag racing (for me anyway) and I do look at both the advertised and effective duration numbers very close when picking my camshafts.
 

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Discussion Starter · #7 ·
Always assumed it was 'duration in crankshaft degrees' or simply duration for short.
It is "duration in camshaft degrees and duration for short . But again duration is a period of time . A measurement of time .
What it isn't is a dimension or degrees of anything.
Silly semantics on the one hand but the word duration really is being applied the wrong way
In this instance.
Duration is simply a period of time .
How long in time .
 

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Discussion Starter · #9 ·
As I read it years ago, duration paper timing numbers were all over the place with cams and cam makers, I mean really, GM had a BBC cam that had over 370°advertised, lol really, I don't think so. So in the 70's every aftermarket cam manufacturer started testing and degreeing in cams at .050 instead of the unreliable .006 by the 80's everyone found that @ .050 cam duration numbers were much more accurate and reliable and that became the std. of the industry. There are also other things I noticed with duration numbers the intrigued me, like when a advertised duration number was like only 30-35° away from the .050 effective number making it a cam with explosive power out of the corners in circle track racing. I soon learned that it would work well in drag racing (for me anyway) and I do look at both the advertised and effective duration numbers very close when picking my camshafts.
All true but duration is still a measurement of time . The ammount of time it takes for a valve to reach .050 is an infinite ammount of different periods of time .
 

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All true but duration is still a measurement of time . The ammount of time it takes for a valve to reach .050 is an infinite ammount of different periods of time .
If you really want to get picky, one could degree in every lobe to see if the cam is actually ground right.
 

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Manufacturers often list 2 different duration values:
  • Advertised Duration is the degrees of crankshaft rotation that the lifter is raised more than a predetermined amount. This predetermined amount varies between manufacturers.
  • Duration at 0.050" is the degrees of crankshaft rotation between when the lifter is raised 0.050" and when it is 0.050" from its resting position. This is standard among all manufacturers. You should use this value to compare camshafts.
How is it measured?
Most Cam Cards will list duration. However, if you want to find the duration for the camshaft in your engine, you can calculate it.
  1. Using a Dial Indicator and Degree Wheel, find the opening and closing points of the valves at 0.050 in. of lift.
    1. If the intake valve opens AFTER TDC, use a negative value.
    2. If the exhaust valve closes BEFORE TDC, use a negative value.
  2. Add the numbers together.
  3. Add 180° to find duration.
For example:
  • Intake: Opens at 7° BTDC, closes at 39° ABDC.
  • Exhaust: Opens at 51° BBDC, closes at 3° ATDC
Intake Duration = 7° + 39° + 180° = 226°
Exhaust Duration = 51° + 3° + 180° = 234°

How does it affect performance?
At high rpm, longer intake duration fills the cylinder. It also allows more exhaust to escape. This creates more power. (It is why a camshaft's RPM Range is based on its duration.)
However, at low rpm, the open valves reduce the pumping pressure of the piston. This results in lower cylinder pressure and less low-end power.
Long duration camshafts also create more Valve Overlap. At high rpm, this helps promote the Scavenging Effect. But at low rpm, it also contributes to Exhaust Reversion.

I hope this helps.
 

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Discussion Starter · #15 ·
The duration(ammount of time)that the valve is open on a 230 @ .050 cam is less than the the duration (ammount of time ) than if a 270 @ .050 cam is used.
It takes a longer period of time for the 270 cam to open the valve to .050 lift than does the 230 .
As relative to one another the word duration is being used correctly there but to isolate it and say its a "230 "duration " camshaft " is actually incorrect isolated like that .
Semantics
.
 

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Okay. I get it.
Now, how about Timing. Many of you refer to "advancing" spark timing as "adding timing" which is totally incorrect

I can "add timing" two ways...retard or advance, but can only "advance" timing one way.

Capeesh ?
 

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I know all about all of that .
The point is a number of degrees is not a frame of time .
Duration is a time frame
This semantics as you said. It is the number of degrees crank travel for the duration (time) of the valve event.
 
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