Don't make me get out a belt and take you 'cross
my knee!!!!!!
> >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.
How are you abusing the term "duration"? Are you measuring
the number of degrees on a cam lobe where the diameter
of the cam lobe is .050" inches greater than your
"base diameter"? This is not the cam duration!!!!
Cam timing is not measured by putting a dial indicator
on the lobes of the cam. As a mechanical designer/engineer,
you may think in those terms, but they aren't advertised
that way and the end product (valve lift) sure don't work
that way.
> 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.
Ok you Pro-E, AutoCAD using freak. Follow me:
We have a cam with a lobe that starts "lifting" at zero
degrees (let's start at zero for the sake of the experiment).
Let's say it takes 10 degrees with 1.5 rockers to get the
valve .050" off the seat. On the other side, 200 degrees
later, the valve goes below .050 inches of lift.
So we go up at 10 degrees and back down at 200 degrees.
If we increase the rocker ratio from 1.5 to 1.6 or even
1.7, the valve will get off its lazy seat a few degrees sooner
(perhaps at 5 degrees) and will stay open longer (perhaps
205 degrees total) before the valve gets closer than .050"
to the seat.
By changing the rocker ratio, we have gone from
having our valve open at 10 degrees and close at
200 degress to having our valve open at 5 degrees
and close at 210 degrees. We have gained 10
more degrees of CAMSHAFT SPINNING TIME
that the valve will be higher than .050 from its seat.
Yeah, I know we haven't changed the shape of the lobe.
The increased ratio is what did the magic.
Our "Little Shop of Combustion" is open earlier
in the morning and closes later at night to suit
your gas-burning needs.
> Also...duration figures are measured in degrees of *crankshaft* rotation.
> Lobe separation angle is specified in degrees of *camshaft* rotation.
It's always good to keep those straight.
> 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:
>
[SNIP]
That Comp Cams article is pretty good. Did you notice
the part where it mentions measuring lift AT THE VALVE?
Ha ha ha!!!
I'll learn ya pretty soon, ya feisty young-un.
-Scott
"No, Homer, very few cartoons are broadcast live. It's a
terrible strain on the illustrator's wrist."
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