It's not a universal feature, but in many cam designs you want the initial
valve acceleration to be extreme (think POP the valve off the seat) so the
sonic pulse down the pipe is crisp and the exhaust gases will start evacuating
through a large hole rather than a small crack. The cam has a ramp to it, even
if not by design, then by geometry, and letting the lash and slack in the
lifting system take up the ramp allows the cam to get into the meat of it's
opening cycle before the valve begins to open.
I don't have dyno experience on this with car motors, but I saw it often on
bike engines. We'd tighten the lash to pick up some overlap and the power and
torque would both decline a little. Add lash and it would increase until you
added too much, then it would go down again. It's a small amount so II never
could really separate the effect from the variation inherent in repeated dyno
runs (anyone who has pulled a lot of runs knows what I mean by this) but I
still "knew" it was there, so I always set lash to the tight side of a cam
builder's spec.
-----Original Message-----
From: fot-bounces+billb=bnj.com@autox.team.net on behalf of Greg - Lunker
Hilyer
Sent: Fri 1/4/2008 7:32 AM
To: Allen Washatko
Cc: Friends of Triumph
Subject: Re: [Fot] Roller Rockers/Valve Lash
I'm also interested in any input from the list. When installing a
TR-666 Isky I was shocked with the recommended .028" [hot] lash.
Convinced that it was a misprint, I called their tech line and was
assured this was the correct number. Then at the very end of the
conversation the tech happen to mention "oh yah, all of our lash
specs are +/- .010" in actual use" !!!
I sure wasn't going to experiment with .038" or anywhere near. Even
at .018 the clatter sounded pretty extreme to me. I've been running .
015 with no apparent problems. I mentioned this to our Larry Young
who tried explaining to me how my adjustment was slowing
acceleration. I thought my tighter adjustment would be of benefit as
it would be giving the valve x amount more lift.
I'll look forward to more info.
Greg Hilyer
TR4 #314
Albuquerque NM
On Jan 4, 2008, at 6:51 AM, Allen Washatko wrote:
> FoT,
>
> As long as we are on this topic, how do you establish valve lash for a
> specific cam/rocker combination? The specs that came with my cam set
> valve lash at .020 intake and .022 exhaust. When I set the lash at
> these specs, there was extreme wear at the rocker tip and at the
> end of
> the valve stem. I then reduced the lash to .015/.016 but after taking
> the motor apart this winter, there was still noticable wear at the
> rocker tip. Any thoughts on this?
>
> Allen
>
> Allen & Jody Washatko
> 1971 TR6 - Number 6
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