[Fot] Roller Rockers/Valve Lash clearance

[Bill Babcock]

No science, no assumptions of better power, I just can't stand the noise.


-----Original Message-----
From: Barr, Scott [mailto:sbarr at McCarty-Law.com]
Sent: Fri 1/4/2008 8:59 AM
To: Bill Babcock
Subject: RE: [Fot] Roller Rockers/Valve Lash clearance

<<so I always set lash to the tight side of a cam builder's spec.>>

Did you mean loose side?  Or is there an assumption, inherent in your
choice, that cam designers set their lash spec looser than it really
needs to be and, therefor, starting on the tight side of their spec gets
you back into the part of the curve where you're getting additional
power?

-----Original Message-----
From: fot-bounces+scott.barr=mccarty-law.com at autox.team.net
[mailto:fot-bounces+scott.barr=mccarty-law.com at autox.team.net] On Behalf
Of Bill Babcock
Sent: Friday, January 04, 2008 10:34 AM
To: Greg - Lunker Hilyer; Allen Washatko
Cc: Friends of Triumph
Subject: Re: [Fot] Roller Rockers/Valve Lash clearance

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.

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