I must admit that I don't quite understand what you're saying. It
must have been taught in one of those math courses I flunked.
My understanding of the cam / lifter geometry is that if the lobe is
designed to never run off the edge of the lifter, then that's the
condition that will always exist, whether or not we float the valves.
Is this correct?I guess my question would be -- with the cam we're
using, what is the diameter needed to avoid running off the edge --
which gets at how close to the edge of the lifter will the lifter
contact the cam, under any circumstance?
I had my head work done several months ago and forgot to mention to
you guys a problem that I had. It may not have anything at all to do
with this discussion, or it may. All my valve stems were peened over
a little bit on the ends. The machine shop expert's first opinion was
that I had floated the valves a lot. When I told him that I kept the
revs to levels that I wouldn't expect that (6000), we theorized that
because of the diameter of the stud holes in the rocker stands,
perhaps the centerline of the shaft had moved back far enough to let
the rocker tips run off the edge of the stems.
That puzzled me too, because there was evidence of something fishy
going on all the way across the ends of the stems. I purchased new valves.
Later when he was starting assembly he determined that the springs
had lost some of their tension so we put new valve springs in. I
don't know how old the springs were. Our conclusion ended up being
that the weaker springs had allowed valve float.
I have them set the seat pressure between 90 and 100 on all these
heads. That's really quite conservative compared with what the V8's run.
We also decided that the radius on the valve tips looked too shallow
(too large a radius), so he refaced that tips to have a sharper
radius to reduce the possibility of them running off the edge.
I'm also considering replacing my old rocker stands with new ones.
The new ones are much nicer. I've had the old ones milled and shimmed
numerous times over the years - they are ten years old.
I have a hard time thinking that any of this is related to camshaft
design, since as I understand it, the acceleration and velocity we're
using is not significantly higher than used on the stock camshaft --
do I understand this correctly?
Anyway, all this may or may not have some relation to what we're
seeing in the valve train..
At 09:19 AM 1/6/2008, Larry Young wrote:
>I should have mentioned the relationship between velocity and lifter
>size. You'll notice from the graph that the maximum velocity is 0.006
>in/deg. Multiply this number by 360/Pi and you will get the radius of
>the lifter needed. 0.688 in this case. You need add a safety factor for
>chamfer, slop, etc. A good design should use the entire lifter to get
>the maximum performance.
>
>Larry Young wrote:
> >> Here is a graph constructed from Cam Doctor data on a stock TR4 cam -
> >> http://home.swbell.net/cartrip/TR_StockB.gif.
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