>What is "duration"? It is the number of degrees of camshaft rotation
>that the valves are at .050 or higher. A higher ratio rocker gets
you up to .050 quicker and keeps you there longer.
Actually the duration *is not* affected. Think of a section view thru
a cam lobe. As the lifter travels the base diameter of the lobe the valve
sees no lift. The ratio of the rocker during this period is
inconsequential. At a certain point you reach the "lift" portion of the
lobe. Since the rocker ratio isn't coming into play until that initial
point of lift, and at the end of lift the lifter returns to the base
diameter rendering the ratio inconsequential again, the duration of the
lobe is unaffected.
The rate of lift and lift itself will be affected but not the duration. If
you want to move the powerband you need to change the duration. If you're
just lookin' for more power in the range you're already in you'll want to
go with increased ratio rockers to simply increase lift.
Also...duration figures are measured in degrees of *crankshaft* rotation.
Lobe separation angle is specified in degrees of *camshaft* rotation.
This stuff is all real crazy to follow. I found a great cam tutorial on
Comp Cams webpage some time ago at http://www.camhelp.com/valvtim1.html
that really cleared things up for me. There's a great graphic of dur. vs.
lift on there. Crower also has some great info on their page as well at
http://www.crower.com .
Here's a little of the info I've come across:
Camshaft Tech:
Varying Lobe Separation Angle
+--------------------------------------------------------------------------
------+
| Tighten | Widen
|
|---------------------------------------|----------------------------------
------|
| Moves Torque peak to lower | RPM Moves Torque peak to lower
RPM |
| Increases Maximum | Torque Increases Maximum Torque
|
| Narrows Powerband | Narrows Powerband
|
| Builds higher cylinder pressure | Builds higher cylinder pressure
|
| Increases chance of knock | Increases chance of knock
|
| Increases cranking compression | Increases cranking compression
|
| Increases effective compression | Increases effective compression
|
| Idle vacuum reduced | Idle vacuum reduced
|
| Idle quality reduced | Idle quality reduced
|
| Open valve-overlap increases | Open valve-overlap increases
|
| Closed valve-overlap increases | Closed valve-overlap increases
|
| Natural EGR effect increases | Natural EGR effect increases
|
| Decreases Piston-10-valve clearance | Decreases Piston-10-valve
clearance |
+--------------------------------------------------------------------------
------+
Lobe separation Angle
+----------------------------------------+
| Above 114 degrees = Extremely wide |
| 114 - 112 degrees = Wide |
| 112 - 110 degrees = Moderately wide |
| 110 - 108 degrees = Moderate |
| 108 - 106 degrees = Moderately tight |
| 106 - 104 degrees = Tight |
| Below 104 degrees = Extremely tight |
+----------------------------------------+
Advancing/Retarding Cam Timing
+--------------------------------------------------------------------------
---------+
| Advancing | Retarding
|
|-----------------------------------------|--------------------------------
---------|
| Begins intake event sooner | Delays intake event sooner
|
| Open intake valve sooner | Keeps intake valve open later
|
| Builds more low-end torque | Builds more high-end torque
|
| Decrease piston-intake valve clearance | Increase piston-intake valve
clearance |
| Increase piston-exhaust valve clearance | Decrease piston-exhaust valve
clearance |
+--------------------------------------------------------------------------
---------+
Sorry the post is so long.
*** 97 RPM Dakota - 14.98 @ 91.76 *** http://surf.to/real-per4mance ***
sean meldrum
Email @ Home: spm@c3net.net
Email @ Work: sean_meldrum@ittauto.com
Truck mods: http://www.geocities.com/MotorCity Downs/7875/signature.txt
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