On Wed, 17 Sep 2003, Don Malling wrote:
> I read this on page 109 of Roger Williams new book.
>
> ===========================================
> Premature wear in the guides is usually the result of valve gear being
> at the wrong height for the valve lift that will be generated by your
> cam shaft. Triumph got the height of the valve gear correct in the first
> place for the cam shafts it was designing into its engines, however if
> you fit a cam shaft that increases the anticipated valve lift, then it
> follows that the valve gear height will be incorrect. You need to get
> the rocker height halfway within the stroke of the valve if you seek
> maximum mechanical effect and minimum sideways loads on the valves and
> valve guides. If the cam you get increases the valve's opening from
> standard by 0.125" (3mm), it's likely the rocker shaft pedestals will
> need to be reduced by 0.623" (1.5mm).
> ===========================================
Well, first off - there _must_ be a typo here. I believe 1.5 mm is more
like about 0.0623. If you were to take .623 off your rocker pedestals,
there would be precious little material left!
BTW, most folks wind up adding shims to the pedestals to get the reground
cams into the adjustment range of the rocker adjustment screws. You
_might_ need to take material from the pedestals, but that would be more a
function of how large the base-circle of the regrind is more than the
gross lift. In other words, there is no sweeping single answer to this
question - you need to bolt everything together and then make some
measurements and then machine and re-assemble and test.
As far as max. cam lifts, I'm sure you will find grinds that offer more
than .425 net lift - but most of these really are not suitable street
grinds. To wit, most of the flow info that I've been privvy to show no
increase in max flow above about .430 lift. So, lift > .430 _may_ have
absolutely no benefit in a normally aspirated engine.
As far as side loading etc., he is spot on. This is why want cam bearings
and why you need to check your valve guide for wear from time-to-time.
My thoughts on this - buy a set of roller rockers from Goodparts if you're
going for insane lift with your cam. The engineering is already done.
'Nuff said.
rml
------------------------------------------------------------------------------
Bob Lang Room N42-140Q | This space for rent
Consultant MIT unix-vms-help |
Voice:617-253-7438 FAX: 617-258-9535 |
------------------------------------------------------------------------------
|