Joe---There's a heck of a lot I don't about this, but I'll share what I
think I know.
Engines with hydraulic lifters have NO clearance between the valve stem
and rocker. The takeup is automatic as wear occurs, like some other
hydraulic parts on the automobile.
Engines with solid lifters, such as the TR, must be adjusted with some
clearance (lash) to be sure that the valve is free to close completely
with spring pressure.
It would seem that to get the increased breathing and cylinder purging
offered by a performance cam, more valve overlap is needed, along with a
longer time with the valves being open.
This is logical enough, but why the additional clearance? Smoother
transition between the cam follower and cam lobe when coming off the
base circle?
To further confuse the issue, cam grinders want their cams first
measurment taken at different places when coming up the lobe ramp!
As written earlier, if you reduce the lash the valve overlap will
increase. It could make the engine a little more difficult to tune, at
least at idle speeds. Exhaust emissions would increase, for what that's
worth to you. And there might be more cam wear with the valves starting
to open earlier on the ramp. I have tried reducing the lash when using
the performance cams, and didn't like the lower vacuum signal and
reduced tune-ability.
Dick Taylor
Isky Z-19
S-2
Stock grind 240 and 256
I can't think of any reason why an increased rocker ratio should call
for more lash when using a stock cam.
Others looking in may offer an explanation for all of this.
Dick
From: Lizirbydavis@cs.com
How are the valve stem/rocker arm clearances established? Why does the
clearance need to be increased with higher ratio rocker arms and higher
lift cams? I have a GP2 cam and 1.65:1 roller rockers. The valve gear
clatter with the recommended clearances is much more that I expected.
What harm will come from closing up the clearances? (not that I would!)
Joe Davis
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