Valve Float

["John Stricker" <jstricke@domain.elided>]

Date: Tue, 13 Jan 1998 16:29:57 -0600 (CST)
From: Steve Stegmann <stegmann@domain.elided>
Subject: Re: ihc-digest V5 #118

Tom,

Sorry about being so far behind in reading my Digests.

If your valves float, not much happens.  The motor just won't go any
faster.  There are some cars where the valves might hit the pistons but I
doubt that will happen in a Scout.  8.5:1 compression leaves too big a
combustion chamber, I imagine.

Steve


Steve,

In many cases, you may be right.  However, you may be wrong.  Think about
what is happening when the valves float.

Basically, the valves don't have enough spring pressure to close in the
given amount of time, with the ramp profile of the engine.  The valves are
effectively "stuck" (well, not really, but they are staying open).

The lifters, on the other hand will more closely follow the cam profile and
while they may not stay firmly planted against the cam they will stay closer
to it than the valves are closing.

Think about what your pushrods are doing at this time.  Clearance is WAY too
great.  In fact, it is very possible that the pushrod will unseat itself
from under the rocker arm.  This will result in all kinds of conflict with
the most common result being a bent pushrod or pushrods.

I saw this often when I worked in the IH dealership.  It is also very common
for trucks used in logging.  It was a very common engine problem in the
Black Hills.  Inexperienced drivers coming out of the hills and letting the
engine do the braking in too high a gear.  Talk to an older mechanic
sometime that had experience in a mountain area with the 345/392 engines in
the Loadstars. It seemed to me at the time that most of these were showing
rpm's in the 5500 and up range when it happened.

You're right Steve in the fact that the limiting RPM factor is in the
valvetrain first.  And that the likelihood of hitting the pistons is remote.
Then the bottom end.  I've always wondered if IH engineers intentionally
designed the engines that way to save the bottom end.  First the valves
float to limit rpm.  Then if it persists, the pushrods bend and limit it
some more.  Finally, the bottom end lets go (at a much higher rpm).  Howard
may have some insight on this.  Maybe those truck designers  weren't so dumb
after all?

John Stricker

jstricke@domain.elided

"I didn't spend all these years getting to the top of the food chain
just to become a vegetarian"