Several of you responded to my initial plea about, "What's the deal
with valve lash?" Most of you wanted to know the answer to my
question. I don't know the answer, but here's some stuff to chew on.
Valve Lash serves two purposes.
1a) It provides a mechanical means to ensure that the valve is closed,
and doesn't hang open on the compression stroke. (e.g. If you
valve lash is too tight, the valves will not close)
1b) The length of time that the valve is closed also provides a means
to dissipate heat from the valve.
2) Slight changes WRT the timing of the cam and performance.
My original question, was why does the cam manufacture state such
large valve lash adjustments (sometimes) when delivering a cam? The
answer is: I dunno. I gotta talk to a good cam person.
I'm told that cam manufactures will run computer models on their cams,
and will come up with the best valve lash to give their cam the best
performance. Some will even dyno the cam to find the best gap.
Personally I find both of these methods dubious as there are so many
other variable to consider, I don't really see how optimum valve lash
can be obtained. In other words, valve lash on performance cams is a
black art.
With regards to timing, reducing the valve lash will 1) advance the
cam giving better low end timing, and 2) increase duration.
Increasing valve lash will retard the cam putting power in the higher
RPMs. This will likely only move the timing a few degrees at most,
but that depends on the cam grind.
You also will give up a small amount of lift if you have a large valve
lash. For example if your cam provides .400 inch of lift at the valve
and you set the valve lash at .015 instead of .010, you will lose
1.25% of your total lift. That is, you will have .395 lift instead of
.400. Probably not enough to make a difference... unless your at the
track and you need all you can get. Of course valve timing might be
more beneficial than total lift. Only your lap times might tell you
the answer.
Cam manufactures know that valve lash can create a noisy valve train,
and have come up with quieting ramps. This is mearly a ramp on the
cam lobe which allows the rocker to sneak up on the valve stem. Once
in contact with the stem, the cam ramps up very quickly to open the
valve as fast as the cam manufacture believes is correct.
My cam specs .015 on intake and .018 on exhaust. I'm going to set it
at .012 intake and .014 exhaust to see if my valves will quiet up a
bit. I rarely run my car over 5000 rpm, so I'd rather have the
timing advanced, giving me better low rpm performance anyway.
(Probably marginal, but I'll take it. :-)
The bottom line is to keep the following in mind WRT valve lash:
tighter valve lash looser valve lash
----------------- -----------------
less seat time (hotter valves) cooler valves
advances the cam retards the cam
quieter noisier
more valve lift less valve lift
increased duration less duration
In the extreme, really tight valves are very bad, in that they never
close, and you WILL burn an exhaust valve. The valve will get too
hot, warp, bend, break, etc. A general rule is to never go tighter
than the OEM specification. I would question a cam that used a spec
*tighter* than stock.
On the other extreme, very loose valve lash will cause the rocker to
SMASH into the top of the valve stem and mushroom the valve stem.
This is bad as the top of the valve stem *was* hardened, and once it's
mushroomed, it's time for new valves.
To sum it up, and given the ideas/chart above, it seems to me that
tighter clearances WRT valve lash are more beneficial than loose
specs. So *WHY* do cam manufactures use looser specs on performance
cams? Temperature probably has something to do with it, but come on,
.018 thou?!? That's alot of heat. :-)
The 1993 Prelude Si has an intake valve spec of .002. There are no
hydraulic lifters here either, it's solid from the cam to rocker to the
valve stem.
Anyway, those are my thoughts. I'd love for someone to tell me what
I'm talking about. :-)
Cheers,
-Scotty '75TR6
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