On Thursday, 13 January 2022 at 19:13:17 UTC+11, Xeno wrote:
> On 12/1/22 11:51 am, Xeno wrote:
> > On 12/1/22 12:51 am, Noddy wrote:
>
> >
> > What you haven't allowed for here is valve *float* and valve
> > *overshoot*, both being candidates for contact with a piston. Valve
> > float occurs when the inertia of the valve when decelerating down the
> > cam is greater than the ability of the valve spring to keep the valve
> > (train) in contact with the cam. IOW, it stays open slightly more/longer
> > than it should at the high point and closing ramp of the cam. The head
> > of the valve is sticking further into the cylinder than it should at
> > that point. The other is valve overshoot. This occurs when the valve
> > reaches/passes the highest *acceleration* point and starts to go over
> > the top of the cam into the deceleration phase. The momentum of the
> > valve (train) keeps the valve moving on its opening path and, as a
> > result, the valve opens *more* than it would had it remained on the cam..
> > Both the scenarios, all other things being normal, only occur when the
> > engine is running at excessive RPM levels. This is most likely to occur
> > at WOT on a long downhill run. Guess what, that's where it *did happen*..
> >
> >> while I wasn't at the track to see where Les was running his mixtures
> >> and have no idea how hot it was getting I'd happily bet that Les was
> >> running the thing fairly conservatively given that he had a habit of
> >> doing so and was familiar with the setup of the vehicle. Either the
> >
> > Wouldn't matter what the mixtures were at had the valve head been given
> > a tap by the piston.
> >
> >> valve guide clearance was too tight or the valve failed prematurely,
> >> but either way given that I had nothing to do with the head it had
> >> nothing to do with me.
> >
> > You were doing the head fitting, everything to do with you.
> >>
> >> Some important points to bear in mind here are that a piston to valve
> >> clearance issue is a mechanical issue that is evident from the first
> >> time the engine is turned over in extreme cases, and certainly from
> >> the first time the engine reaches operating temperature in borderline
> >> cases.
> >
> > Nope. Valve overshoot and valve float are *dynamic issues* and will
> > never occur when the engine is being rotated slowly by hand or operating
> > in a low RPM range.
> >
> >> However in *all* cases it is *not* a problem that will suddenly
> >> magically appear after an engine has done some time at full noise like
> >> this one did, and if Clasener and and his clog wearing puppy dog had
> >> *any* idea at all they would *know* this.
> >
> > Valve float and overshoot *only appear* when the engine is at full noise
> > or, more correctly, excessively high RPM levels.
>
> Rather than try to explain to a nuff nuff like you Darren, I'll let this
> website do the talking. They discuss valve float, bounce and loft. They
> use the term "loft" rather than "overshoot", but we'll get by.
>
> https://blog.trendperform.com/spintron-secrets-valve-float-bounce-and-loft-explained
>
> Let’s picture the scenario. The engine is running smoothly on
> the dyno but as the revs rise, maybe past 7,000rpm, the power
> suddenly drops and the airflow plunges. Or perhaps you’re out
> on track, and the motor hits an invisible wall, refusing to
> make more power. In either case, the valvetrain could be out
> of control. In short, the valves – and in particular the
> intake valves – are hitting the seat hard and bouncing open
> again, perhaps multiple times.
> This is valve bounce, the most damaging of several phenomena
> that collectively comprise valve float, which is when a valve’s
> opening and closing phases don’t match the cam lobe profile in
> the way that they should. At best, you’ll stop making power as
> the rpm rises. At worst, the valve will make contact with the
> piston, or it – or another element of the valvetrain – will
> break.
>
> At worst, the valve will make contact with the piston? And we're only
> talking about valve bounce, a phenomena that only occurs at high RPM.
> Got clearance when you turn the engine over by hand but at high RPM
> levels those valve springs can't hold that valve closed after it's
> slammed down on the seat. If you don't have *enough* clearance, it may
> come to grief at high RPM but as a daily driver, no worries. Dynamic
> effects Darren.
>
> “People often think that valve float means that the parts are
> coming out of contact, but that’s not necessarily true,” he
> begins. “It’s actually a combination of valve bounce and valve
> loft. Valve loft is when the dynamic lift at high speed is
> *higher than the measured static lift*. Many times, the parts
> are actually still in contact. Valve bounce is when the valve
> opens again after closing and certainly means that the engine
> will not make power – and will break things.”
>
> And will break things - like valve heads that drop off.
>
> The schematic showing loft and bounce is rather interesting. Note that
> the loft occurs at two points, after the valve has closed but also at
> the top of the cam lobe. IOW, the *dynamic lift* is greater that the
> *static lift*. When you check just to see clearance by turning the
> engine over, even 0.005" will be sufficient and nothing will contact
> that shouldn't be in contact. But at high RPM, you need to allow a
> greater margin, ~0.060" - 0.080", being fairly typical.
>
> “Energy is stored in the components [as they deflect] and is then
> let go as the cam acceleration slows down and the lifter comes
> ‘to the nose’ of the cam before everything turns around and goes
> in the other direction.”
> As engine speeds rise, this can lead to valve loft. This is when
> the parts start to be ‘thrown’ as the cam acceleration comes off,
> early to mid-lift.
>
> You can see this in the graph, the baseline RPM is 1500. The valve
> (purple line) accurately follows the cam profile around the nose of the
> cam. At 8000 RPM however, the valve (red line) is being thrown off the
> cam by its own momentum (lofting) and follows a much wider profile with
> the valve spring desperately trying to pull the valve (and tappet) back
> down against the cam face. It only manages to do that about a third of
> the way down the closing ramp of the cam. That's why, when you measure
> piston to valve clearances, you allow a huge margin. Making sure the
> valves don't bind against the pistons when rotating the engine by hand,
> ie. bare minimum clearances, just won't cut it at 8000+ RPM. Loft, float
> and bounce are engine killers. People who actually are involved in
> racing, not you obviously, know all this shit and work accordingly. You,
> the absolute tyro, *think* you know it all - with obvious consequences -
> Les' engine being the perfect *proof* of that.
> >
> > Now, had you really done an auto machining apprenticeship, and studied
> > cam profiles, you would have learnt all about this and been able to
> > calculate when such RPM points occur. Certainly anyone who does
> > performance cam machining needs to be aware of cam profile limitations,
> > the effects of inertia, valve spring rates, etc. You appear only to have
> > the most simplistic idea of such matters. That tells me you *definitely*
> > never sat your arse in an auto machining apprentice class ever in your
> > life.
>
> I know you haven't been involved in racing since concepts familiar to
> race engine builders seem quite alien to you.
>
>
> Now, you made the point in your response that it would be the intake
> valve that would be affected. I beg to differ - and so does Summit Racing;
>
> https://help.summitracing.com/app/answers/detail/a_id/4765/~/what-is-valve-float%3F
>
> What is valve float?
> Definition & Description
> Valve float occurs when the valvetrain gets out of control. It
> is caused by not having enough Valve Spring Pressure. There are
> 2 types of valve float:
> Loft - When the lifter is thrown off the nose of the cam lobe.
> Bounce - When the valve bounces off its seat, before it settles.
> How is it identified?
> Mild valve float can be felt as power falling off quickly at high
> rpm.
> Severe valve float will be heard as a sputtering sound at peak rpm.
> How does it affect performance?
> Loft is the more severe issue. When the lifters loft, the valve
> spring can go into Coil Bind. This could cause bent or broken
> valvetrain parts. In severe cases, *the exhaust valve can hit the*
> *piston*. To fix a loft problem, increase your seat pressure.
> When a valve bounces off the seat, the engine looses compression.
> To fix a bounce problem, increase your open pressure.
> In most cases, New Valve Springs are the answer. Lighter valvetrain
> parts can also help with valve float issues. These parts reduce the
> weight of the valvetrain and make it easier for the springs to
> control the valves. The following parts are available for many
> applications:
> Hollow Stem Valves Titanium Retainers Titanium Valves
>
> In severe cases, Darren, *the exhaust valve can hit the piston*.
>
> Which valve was it on the 4AGE20 Darren? The *exhaust valve*.
> Oh dear, you've been screwed by the facts yet again.
>
> Let's try another site;
>
> https://www.cycleworld.com/valve-float-or-valve-loft-kevin-cameron-top-dead-center/
>
> Valve float has traditionally been regarded as undesirable because
> it can be destructive, resulting in violent contact between the
> piston, nearing the top of its exhaust stroke, and the exhaust
> valve, which is in the process of closing. The usual outcome – the
> head of the *exhaust valve* breaking off its stem and then being
> hammered into the piston crown – was given the name “penny-in-the-
> slot” by Harold Willis, Velocette’s phrase-loving rider-engineer
> of the 1930s.
>
> As I said previously Darren, you're a concrete thinker and abstract
> concepts, such as the dynamics of valve operation, plain and simple, are
> way beyond your ken. All the waffle in your post was just that, waffle.
> You were trying to baffle *other posters* with a barrage of bullshit.
> Luckily some of us with a background in the automotive field can see
> through your bullshit and, boy, do you lay it on thick.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You do indeed....The old DTWBOBTWB story. A quick scan has shown me that Google has taught you a plethora of new words and terms.
All still basic however. <G>

The ol' pussy will no doubt be impressed, he may even leave alvira and come after you. HAHAHAHA

BTW! Whodathunkit...... You and biking! (Velo`s at that :-o) together in yer twilight years. Whoo Hoo.

Watch for the next exciting??? episode of *Days of our lives* starring kryppy and co.

> --
> Xeno
>
>
> Nothing astonishes Noddy so much as common sense and plain dealing.
> (with apologies to Ralph Waldo Emerson)