Re: Valve Float

[n9ads@domain.elided (Howard R Pletcher)]

On Tue, 13 Jan 1998 21:06:38 -0600 "John Stricker" <jstricke@domain.elided>
writes:
>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
>
I have no great insight into this as the engines were designed before I
was around, but it seems to me this failure pattern is probably the
result of designing an engine that would go out and run at 3800-4000 rpm
down the highway pulling the rated horsepower out of the engine for hours
on end in a dump truck or a van with a huge frontal area.  Compared to an
automobile engine that may occasionally reach its max RPM and spends most
of its life at 2-3000 RPM, I would expect the truck engine to have a much
stronger bottom end as it seems that would be the area stessed by its
normal operation.  I wouldn't say the valve train was designed to fail
first, but it likely would be the weaker link in the chain.

John's comments illustrate the reason governors were used on some of the
medium truck engines.  Given that the truck was geared so that it
operated at maximum rated RPM at 55 or 65 mph on the highway, which was
necessary to get the horsepower needed, many drivers, particularly the
less trained ones, just put their foot to the floor and let it run.  If
it got to 4500 RPM or even 5000 RPM downhill, that was all the better as
far as they were concerned.  Hence the governor was the owner's way of
protecting his investment.  The driver could put a brick on the throttle,
but it still wouldn't go over 3800 RPM or whatever the governor was set
for unless it was a really steep hill that would push the engine to
higher RPMs.  Since gearing and driving conditions were different in
pickups and Scouts, the governor wasn't needed here, except when Tom
wants to race Cameros with his Scout.

Howard Pletcher
Howteron Products Scout Parts