[6pack] GP2 cam idle characteristics

[Navarrette, Vance]

 	Dick, Robert:

	The valve lash setting specified by the cam manufacturer is
specified with a specific rocker ratio in mind. The "valve lash" setting
is in actuality a "cam lash" spec. For solid lifter cams you must allow
for thermal expansion of the valve train in order to insure that valve
will fully close under all conditions. This means you must allow some
gap in the valve train - this is why you must set the lash.
	The exact sizing of the lash depends on the CAM, not the valves
or rockers. This is because there is a built in ramp on the cam lobe
designed to gently take up the lash before accelerating the valve off of
its seat. The amount of lash is therefore specified to correctly
accommodate the needs of the CAM LOBE. When the manufacturer specifies
the lash AT THE VALVE, he DIVIDES that number by the ROCKER RATIO to
determine the lash at the cam lobe. Ergo, if you change the rocker
ratio, you must change the lash setting AT THE VALVE to insure that the
lash setting AT THE LOBE remains correct.
	Example: Lash is specified at 0.015" for a 1.25 rocker. I
install 1.50 rockers, so the new lash setting is 1.5/1.25 x 0.015, or
roughly 0.019". If I fail to increase my lash, not only do I risk
damaging the valves because they will not close fully when hot, I
effectively increase the duration of the cam by a few degrees, making
the idle lumpier.

	Cheers,

	Vance


    Vance Navarrette
    Cogito Ergo Zoom
    I think, therefore I go fast

-----Original Message-----
From: 6pack-bounces+vance.navarrette=intel.com at autox.team.net
[mailto:6pack-bounces+vance.navarrette=intel.com at autox.team.net] On
Behalf Of Robert Lang
Sent: Thursday, June 19, 2008 12:22 PM
To: Sally or Dick Taylor
Cc: 6pack at autox.team.net
Subject: Re: [6pack] GP2 cam idle characteristics

On Thu, 19 Jun 2008, Sally or Dick Taylor wrote:

> Vance---Why would "higher ratio rockers" require more valve lash?

I'm not Vance - he can speak for himself. But the dudes that make higher

ratio roller rockers warn you to "adjust" your valve lash based on the
rocker-arm ratio. I think it has to do with reducing the shock on the
cam
and lifter due to the larger "ramp" load - with a high-ratio rocker, the

cam is actually lifting more (or technically there is more inertial
load).

Undoubtedly, someone will find error with my reply, but the Goodparts
cam
sheets warn you to adjust base on which rocker setup you have. I am 100%

certain of that as I just read one on Sunday.

> Dick