DESMO Discussions (continued, here) Thread

[DewCatTea-Bob]

Think of the combined weight of valve, opening and closing rocker, shims and collets flying up and down in an unrestrained manner, even only 2mm at revs. How much loose hammering would occur to all this gear?
Collets and shims and the valve would cop a hiding?
If springs were not fitted.
It would need to be a strong spring to stop the clatter, (inertia), then what's the point in desmodromics?

____ Since you didn't 'quote' whatever has particularly inspired your post-response, I'm thus left to assume that you were meaning to address the "UPDATE" section within my previous-post. _ So here follows merely THAT section, just as it was originally posted.....

__ UPDATE - The new-thought which Kev's posted-wording (inadvertently) inspired, is concerning what BENEFICIAL, (as opposed to merely neutral), side-effect could possibly result from such proposed running without normally restricted-control of the valves...
__ Well we know that the purpose of valve-springs, and also that of the closing-lobes, is to prevent 'valve-float' ... but what-if not only the return-springs were removed, but also that greater-bulk of the closing-lobes as well ? - With just the closing-ramp left remaining intact !?
Normally when the valve has reached the maximum lift-height of the opening cam-lobe, the springs & closing-lobe's track-face then hold-down & restrict the valve from further advancing to any higher-point than that dictated by the opening-lobe's designed-height ! _ So what if that (normally)- set restriction was completely removed ?
Well then as the revs climb higher, such a less restricted valve would then likewise be launched harder and thusly become more apt to open-up & outward to an even higher-point. - (And of-course we're aware that when the valve-head is raised further out of the way, increased air-flow is then the result.)
The ADDITIONAL available valve-lift would sort of work like a reed-valve - becoming more aptly to increase as RPMs built-up higher,, and-so this would be the beneficial-effect of valve-float, (without it's negative-effect, since the closing-ramp would still prevent THAT aspect).
Of-course the available increased lift wouldn't be much more than just 2mm, since the closing-cam track-face is minimumly raised above the shaft, when near the point of max.valve-lift.
And rocker-springs would need to be required so as to make-sure a gap doesn't develop which could possibly allow the C-clip retainers to escape. -
(So THAT's the only possible reasoning for the original inclusion of the extended closing-lobe track-face - [for in the event that no closure-springs are employed], that THEN there's SOMETHING-else to make-sure those valve-keepers can't develop any chance of squeezing-out from their intended location.)

__While it may possibly be true that I haven't devoted enough thought to this newly inspired conception, I do believe that my posted-wording had fairly well covered pretty-much all the ins & outs well enough concerning the conception. _ However YOUR post seems to be relatively coming-from it's very-own self-concocted view-point which hasn't fully appreciated all the particulars which had meant to have been adequately detailed-out,
yet still seemingly overlooked & not properly considered. _ So-thus I'm really not too-sure as to exactly what's actually motivated your post-comments.
__ I actually never intended to proclaim that the resulted-effect of removing TOO MUCH of the closing lobe's later-end would be ACTUALLY desirable,, rather, I only meant to neutrally report on the 'what-if' resulted-outcome of such an extreme circumstance, (regardless of the final end-result turning-out to be positive or negative, overall).
However your post's overall bottom-line seems to be coming-from the view-point that my simple-report needs to be defended as if it's 'bottom-line' had reached a POSITIVE-conclusion (when in fact, there was not yet any final-conclusion proclaimed).
__ Even so, I'll next address the stated-points of your post, in a straight-forward neutrally-inclined manor, (as I really have nothing to defend in any positive for-or-against view-point).


" even only 2mm at revs. How much loose hammering would occur to all this gear? "

____ Would this "hammering" (as you've selected to call it), control-handling actually be any greater than if everything was still actually under the control of a (somewhat wilder) cam ? ... The mater of whether the valve-train actually gets hammered or not, would depend on the closing-lobe ramp-slope's ability to scoop-away & take-up the excess lash (in a non-hammering fashion). _ That, in combination with the strength-amount of the closure-springs, would determine the amount of excess-lash to handled.


" If springs were not fitted. "

____ As I believe has already been established, the closing-lobes (just as they are as stock), must remain included AND-ALSO kept set to a minimum-clearance, if no closure-springs are employed !


" It would need to be a strong spring to stop the clatter "

____ Right,, the-stronger, the quieter, (AND-also, as the revs decrease, lowering the amount of float, [thus also lowering the amount of extra lash] ).


" then what's the point in desmodromics? "

____ Ahhhh,, to make absolutely-SURE the valves are kept out of harm's way ?
Or are you thinking, primarily for clatter-noise reduction ?


Good/Fun-Cheers,
-Bob
PS.
I'm wondering what Kev's (or anyone-else's) take is on this now ?

[machten]

Sorry Bob, haven't had a chance to think about this yet. I've had to actually DO some spannering today rather than think about it. (See the Vento cam thread related to that). That being said, I'm through that for the moment and will see soon if I can keep treading water whilst being out of my depth!

Kev

[graeme]

Hi Bob,
Don't get me wrong here with my "devils advocate" comments. I find this interesting and thought evocing, and I see and understand your thinking re the un nessary complete closing lobe and letting the cylinder pressure seal the valve against the seat. And your thinking regarding the overlap where it may be possibe for an un controlled exhaust valve to blow back open to accept some charge that has escaped down the exhaust if the header and reverse cone end were tuned to allow this to happen. Forced back into the cylinder by the reversing of the pressure wave in the pipe.
BUT
This can be done with a specifically ground cam which could control the valves to allow this to happen at a given rev range, (with a tuned exhaust) without the valves being allowed to do as they wish.

Bob wrote,
"However your post's overall bottom-line seems to be coming-from the view-point that my simple-report needs to be defended as if it's 'bottom-line' had reached a POSITIVE-conclusion (when in fact, there was not yet any final-conclusion proclaimed"

It was not meant to assume you were stating a definate fact, I'm aware your comments since about page 3 of this post are to provoke thought, and that is a good thing.
But this thread is or was about Desmo cams that were produced by the factory, their faults, and other desmo cams that were produced by V2 etc. and I thought how to sort the problems.


Graeme

[Harvey]

As posted

by DewCatTea-Bob » Sat Mar 09, 2013 9:37 am

And that's because there's still ANOTHER factor which tends to keep the exhaust-valve closed ! ...
__ At higher engine-revs (above kick-starting RPM), especially with wilder-cams (like the DESMO-cams), the 'momentum' of air-flow is not to be so much assumed as being merely a relatively weak force, (as it's that particular force alone, which is depended upon to allow a wild-cam to better fill the cylinder at higher RPMs !).
That air-flow momentum-force is another factor which helps to close (and hold closed) the ex.valve...
So when the piston's intake-stroke is creating negative-pressure within the cylinder (of a RUNNING engine), the intake-charge was already waiting (behind the intake-valve) in an overly-positive state of pressure, so-thus THAT volume of air really WANTS to fill the cylinder, (and helps to push the ex.valve shut),,

The inlet tract is not in a “overly-positive state of pressure” when the valve is to open, it is at the end of the cycle when the valve is closing that there is a positive pressure at the valve.

whilst the momentum of the mass of the exhaust-gasses,
(which had just been pushed-out and thus isn't interested in stopping & going BACKward into the cylinder [especially when there's another opening with air that's more than glad to do so instead]),
creates a vacuum which strongly tends to suck the ex.valve closed (just as a reed-valve would certainly do), and until the main mass of that expelling ex.gas has escaped the exhaust-plumbing, it's sucking-force only tends to pull the ex.valve shut !

When the inlet valve opens the exhaust valve still has about 60* till it closes. By that time the negative pressure that was in the exhaust pipe has dissipated in starting the inlet gas to move. So no sucking of the valve to keep it closed.

Normally when the valve has reached the maximum lift-height of the opening cam-lobe, the springs & closing-lobe's track-face then hold-down & restrict the valve from further advancing to any higher-point than that dictated by the opening-lobe's designed-height ! _ So what if that (normally)- set restriction was completely removed ?
Well then as the revs climb higher, such a less restricted valve would then likewise be launched harder and thusly become more apt to open-up & outward to an even higher-point. - (And of-course we're aware that when the valve-head is raised further out of the way, increased air-flow is then the result.)Of-course the available increased lift wouldn't be much more than just 2mm, since the closing-cam track-face is minimumly raised above the shaft, when near the point of max.valve-lift.

Not So. In the case of a 40mm inlet valve. Port throat area-valve stem=968mm2. The valves open area at 8.1mm lift= this open throat area. So it does not matter know much more lift you have, the valve throat won’t flow any more. It is the amount of time that the valve throat flows at its maximum flow that counts.

[Harvey]

Hmmm,,,,
I find it hard to imagine any gain (considering the amount of work involved to remove a large piece of lobe) as opposed to just lapping the closing lobes and forever more being done with worrying about any future binding issues?
Interesting none the less.

The V2 cams with the phasing issue, did they have a V2 part number stamped in them?

Graeme

I can't remember a number on it, may have had one. That cam was not the final 450 V2 cam,.

[DewCatTea-Bob]

the intake-charge was already waiting (behind the intake-valve) in an overly-positive state of pressure,

The inlet tract is not in a “overly-positive state of pressure” when the valve is to open, it is at the end of the cycle when the valve is closing that there is a positive pressure at the valve.

____ You are most-certainly not incorrect Harvey, (DEPENDING-on exactly WHICH 'air-pressure' we are actually in reference to !). ...
__ In order for my chosen-wording to have made completely CORRECT-sense, I should've made ALL such details better established.
In defense of my (rather incomplete) post-wording, I now point-out that there are actually more than one 'pressure' which could've been the 'point-of-reference pressure' being referenced-to, in my statement - (those others being 'cylinder-pressure' & 'exhaust-pressure', in addition to 'atmospheric-pressure'). _ However, the particular pressure-difference which I was actually in reference-to, is 'intake-PORT pressure' relative-to 'atmospheric-pressure',, (which you apparently correctly assumed).
HOWEVER, it seems that you need to be brought up to speed on the matters of in.port-pressures... While it is very quite true that in.port-pressure is at it's MAXimum just-AFTER the in.valve has closed (due to ram-effect), that pressure then only BEGINS to become relieved as a created pressure-wave moves backward through the intake-tract (at about the speed of sound, from the closed valve-head). _ And while that pressure-wave is on it's way out (to become fully released, once it's wave-front reaches the open-end of the intake-tract, [to THEN become exposed to the atmosphere's pressure]), it's pressure is STILL-remaining greater than that of the atmosphere !
__ Now IF I had been in reference to an engine running at low-RPMs,, THEN, the point which you've made would then very-likely be completely-valid indeed, (as in such case as THAT, the in.port pressure-wave would've then had TIME to become exposed to the atmosphere). _ BUT at higher-RPMs, (which I had quite definitely referenced to), that pressure-wave doesn't get enough time to reach-through back-out all the way to the atmosphere BEFORE the intake-valve is once-again opened ! _ (Thus my questioned-claim is actually indeed certainly VALID !)
__ And that's the added-pressure effect which wilder cams like DESMO-cams depend-on, so as to help better fill the cylinder ! _ And that's a substantial contribution of why engine-torque builds-up as revs increase, cuz that pressure-wave gets less & less time to allow the intake-port's rammed-up air-pressure to re-diminish (all the way back-down the in.port & out the inlet) before the in.valve opens again.
And that's why wilder cams produce the power-effect which is often referred-to as "coming-ON-the-cam", (as those cams open the in.valve earlier, so as to take greater advantage of the intake-charge's rammed-pressure,, NOT ONLY before it gets completely released, but MORE-so before it even becomes very diminished !).
So-thus as the revs build-up higher & higher, the in.valve then repeatedly opens to even-better catch that rammed-up air-pressure sooner & sooner before the retreating wave-front gets much time to further bleed-off the intake-port's built-up max.pressure (which had occurred just-after the in.valve closed on the previous 4s.cycle). _ Thus-therefore leading to that 'rush' of building power that ya feel as the engine "comes-on-the-cam" !
That effect could never occur IF the pressure within the intake-port were to INSTANTLY-diminished (back to mere atmospheric-pressure), much faster than the speed of sound !

creates a vacuum which strongly tends to suck the ex.valve closed

When the inlet valve opens the exhaust valve still has about 60* till it closes. By that time the negative pressure that was in the exhaust pipe has dissipated in starting the inlet gas to move. So no sucking of the valve to keep it closed.

____ Sorry Harvey, but I can-NOT agree with you any at all, on that statement of yours !
While that which you've stated would certainly be true IF the exhaust-port was opened directly to the atmosphere, it actually takes a relatively much greater time for ALL the mass of the expelled ex.gases to be thrown-out (from the cylinder) all the way to the tip-end of the ex.system, (which is part of why the ex.cam has to continue holding-open the ex.valve, [so that the moving-gas doesn't pull the valve shut along-with or behind it] ) !
AND-also, on-top of that effect,, once most-all of the fresh-ex.gas has just been expelled from the ex.system,, a vacuum (relative to atmospheric-pressure), pressure-wave then travels back up the ex.pipe (near the speed of sound) towards the ex.valve, which of-course creates a relatively-great NEGATIVE-pressure which not only MORE greatly attempts to suck the ex.valve shut, but also greatly assists at pulling-in the intake-charge (inward past the in.valve) ! _ THAT effect is often referred-to as "coming-on-the-pipe".
(However that desirable effect also comes-along with a following (undesirable)- positive pressure-wave, which occurs after the exhaust-system once-again becomes fully re-pressurized [as the atmospheric-pressure becomes re-established within]. _ So by then, ya desire the valves to be closed [or-else there'll then become a loss, which is part of why mild-cams & wild-cams perform their best nearer to opposite-ends of the RPM-range].)
All this tech.stuff is referred-to as valve-timing & ex.system 'tuning' (to best suit one-another).
__ So now back to my original claim,, the ex.valve has no (overwhelming) reason to get pulled-open by the piston's downward-action, even when there's no valve-springs or closing-lobe to prevent that undesired action,, as the mass of the expelled ex.gas creates all that's necessary to keep the ex.valve closed (long enough) without the added-help.
____ Incidentally,, concerning DESMO-cams, your statement: "When the inlet valve opens the exhaust valve still has about 60* till it closes.", is rather OFF,, as when the in.valve is about to open, the ex.valve still has another 135-degrees before it closes !
__ And also,, IF your statement: "By that time the negative pressure that was in the exhaust pipe has dissipated in starting the inlet gas to move.", was actually true, THEN there would be no sense in purposely keeping the ex.valve remaining open (for continuing to perform that particular function) for so long, (as is ACTUALLY practiced).
Or are you really meaning to declare that the designer (of the DESMO-cam.model) was wrong to have purposely set the ex.valve to stay open for so long as it does ? _ Cuz if not, then certainly leaving the ex.valve open that late (during the intended high-revs), must STILL even then be purposely-intended to help perform the function of helping to draw-in the intake-charge !

(And of-course we're aware that when the valve-head is raised further out of the way, increased air-flow is then the result.)

Not So. In the case of a 40mm inlet valve. Port throat area-valve stem=968mm2. The valves open area at 8.1mm lift= this open throat area. So it does not matter know much more lift you have, the valve throat won’t flow any more. It is the amount of time that the valve throat flows at its maximum flow that counts.

____ Okay Harvey, you seem to have caught me with my chosen-wording not having been detailed extensively enough,, (which is most-often purposely-left to be less-detailed whenever my wording is thusly bracketed with-in the next-following notation-marks: ( )
).
__ Anyhow, it seems that you're claiming that if tested on a flow-bench,, regardless of whether the valve-head is held-open at-least 8.1mm, or completely removed from it's stem,, the flow-bench testing would remain indicative of the very-same flow-rate in either case, (since ANOTHER limiting-factor -[the "Port throat"] has already set the flow-limit) ! _ Is that correct ??
__ To put your statement in other-words... I believe you're claiming that once the created-area between the valve-head & it's seat has been lifted-upward & increased enough to become EQUAL to the same amount of area as that of the valve-seat ID's area, (minus the area-space taken-up by the valve-stem, of-course),, that THEN no further increase of the valve-lift area will allow for any further increase in air-flow. _ Is that not pretty-much the same as you meant to state ? - (Just so I'll know for-sure that I've properly understood exactly what you're claiming.)
I've checked-out your math, and in order for it to be correct, your expected 'port-throat' restriction-diameter MUST be '36.0mm' (in order to equal-out & exactly-match the same area-figuring for "8.1mm" of valve-lift. _ (So it seems you have a good handle on your math-skills.)
__ Now I must admit that that logical/mathematical reasoning SEEMS to set-the-stage as being firmly-sound & undeniably factual. _ However, if such was ACTUALLY & completely true, THEN it would make no sense for cams to have lift-heights which actually provide even greater lift-areas.
While it certainly must be true that up to that point of equality, there are of-course SUBSTANTIAL-gains derived from increased valve-head lift,, anyMORE beyond that balanced-point, will merely provide 'diminishing-returns' (up to the point where as if the valve-head has been completely taken out of the picture).
The reason that the valve-head still remains as an impediment to airflow even beyond that point of area-equality, is because the v.head forces the air to change it's flow-direction ! _ (You wouldn't expect an unrestrained intake-valve to be content with remaining set in-place [lifted-out merely to that point of area-equality], with air being forced-through the in.port & towards the valve-head,, now would you ?) _ The fact that the valve-head has to force the air-flow to change direction, is the proof that it must be continuing-on with impeding airflow, even when beyond that point of area-equality -(that you've indicated of) ! _ It's just not as great of an impediment as it is BEFORE that equality-point is reached, (as it's merely-just a turning-point between substantial & diminishing-return gains).
After-all, airflow is what it is... 'air-flow',, disrupt it's 'flow' in any way, and it becomes decreased ! _ (And ANY decreased amount, is of-course undesired [in the case of intake-valving], which is why even higher valve-lift is always sought-after.)
IF that point (which I think you meant to make note of), were actually all there is to it,, then in like-kind, it seems that airflow would not be assisted at all by a carb with a bore-size that's any wider than the diameter of the intake-port (as opposed to a carb with the SAME bore as the port). _ After-all, 'funnels' do help 'flow', and so then why shouldn't a funnel-shape in reverse with an increased outlet-area, (which is pretty-much what we're equating-to - with a valve-head opened even further), not also help-out with max.airflow ?
__ Obviously I've gotten too carried-away at this-point, as I think my viewpoint has already been made fairly well enough.
Any further arguments ?

____ I actually appreciate others who so attempt to help keep me (& my claims) on-my-toes, (like with these posts) ! _ As it helps to keep the O'noggin off junk-TV, and hopefully also helps others to contemplate something which they may-not have otherwise.


Dukaddy-DUKEs,
D.Bob

[DewCatTea-Bob]

Sorry Bob, haven't had a chance to think about this yet.

____ Perhaps you had me OVERLY-impressed but, I actually think-not since you expressed fairly valid reasoning.
__ I had thought that perhaps you'd come-back (after inspecting a DESMO-cam), and declare that the proposed 2mm of possibly probable valve-float would likely not be obtainable after-all, cuz there's actually not enough closing-lobe track-face depth to be removed out of the way at that particular rotation-location of the camshaft.
(Which would then mean that I was falsely inspired, and never should've posted anything which had crossed my mind in the spur of the moment.)


Ducky-DUKEs,
-Bob

[Harvey]

by DewCatTea-Bob » Mon Mar 11, 2013 2:40 am

_ However, the particular pressure-difference which I was actually in reference-to, is 'intake-PORT pressure' relative-to 'atmospheric-pressure',, (which you apparently correctly assumed).
HOWEVER, it seems that you need to be brought up to speed on the matters of in.port-pressures... While it is very quite true that in.port-pressure is at it's MAXimum just-AFTER the in.valve has closed (due to ram-effect), that pressure then only BEGINS to become relieved as a created pressure-wave moves backward through the intake-tract (at about the speed of sound, from the closed valve-head). _ And while that pressure-wave is on it's way out (to become fully released, once it's wave-front reaches the open-end of the intake-tract, [to THEN become exposed to the atmosphere's pressure]), it's pressure is STILL-remaining greater than that of the atmosphere !

Well Bob I think this statement is confusing the TWO actions that take place in the inlet tract, during the intake cycle.
Looking at the inlet cycle that starts when the valve opens. The piston which was stopped at TDC, starts to accelerate down the bore, reaching max speed about 85*ATDC, creating a depression in the cylinder to start the gas flowing. Due to the fact, that the piston accelerates faster than the air that reaches its max speed, closer to BDC. The piston then starts to slow as it reaches BDC and stops, but the inertia of the air keeps the flowing, but as the piston is now rising on the compression stroke, the pressure in the cylinder keeps increasing, till it reaches its max pressure. It is this max cylinder pressure that we time the inlet valve to capture, as it closes. The inertia pressure left in the tract when the valve closes will rebound back up the tract in a series of to diminishing cycles.

The second pressure action that occurs in the inlet tract, is caused by the sound wave (that travels at about 1100ft/second) that starts as the inlet valve opens. A negative pressure sound wave, starts to move up the tract, on reaching the open end of the bellmouth it expands to be replaced be a positive wave that travels back down to reach the piston, about 90/100*ATDC. As the wave has reached a closed end, it is reflected back as the same positive pressure, up the tract to again reach the open end of the tract. As the reflected positive wave has reached an open end, it is replaced with an opposite negative wave that travels back down into the cylinder to meet the piston around BDC. As it has reached a closed end it is reflected back to the open end as the same negative pressure.
This then has a negative pressure in the cylinder from about BDC to inlet valve closing. It is this negative cylinder pressure that increases the inlet gas flow, along with the inertia pressure, to increase the cylinder filling.
Positive pressure on the outside of the cylinder, and a negative pressure inside the cylinder.

____ Sorry Harvey, but I can-NOT agree with you any at all, on that statement of yours !
While that which you've stated would certainly be true IF the exhaust-port was opened directly to the atmosphere, it actually takes a relatively much greater time for ALL the mass of the expelled ex.gases to be thrown-out (from the cylinder) all the way to the tip-end of the ex.system, (which is part of why the ex.cam has to continue holding-open the ex.valve, [so that the moving-gas doesn't pull the valve shut along-with or behind it] ) !
AND-also, on-top of that effect,, once most-all of the fresh-ex.gas has just been expelled from the ex.system,, a vacuum (relative to atmospheric-pressure), pressure-wave then travels back up the ex.pipe (near the speed of sound) towards the ex.valve, which of-course creates a relatively-great NEGATIVE-pressure which not only MORE greatly attempts to suck the ex.valve shut, but also greatly assists at pulling-in the intake-charge (inward past the in.valve) ! _ THAT effect is often referred-to as "coming-on-the-pipe".

Ok same problem combining two different exhaust pressures.
When the exhaust valve opens, TWO actions start. First the gas pressure of about 70psi is released into the pipe, this pressure ‘plug’ is pushed down the pipe by the pressure behind it, pushing the stationary gas in the pipe ahead of it. As this pressure front moves down the pipe, a low pressure is developed behind it, dropping the pressure in the cylinder. If the pipe is long enough, the low pressure in the cylinder will last till the inlet valve opens. This is just the inertia of the ‘plug’ travelling at about 300 ft/second, but when it reaches the open end of the pipe it is lost and the low pressure created by it, diminishes.

The second action is again produced be sound waves travelling at about 1600ft/second. This starts with the BANG sound that the escaping gas produces as a positive pressure, that travels down the pipe to reach the open end, so it changes sign to be a negative pressure, that travels back up the pipe to reach the cylinder as the inlet valve is opening.

It is not possible for the two pressures to be combined at their peak pressures, due to the fixed length of the pipe and the difference speed of the two pressures. Thus in the modern higher speed engine we have a pipe length to allow, the inertia pressure to be used around BDC to 90* ABDC, to reduce the pumping losses from the piston having to push the gas out. We then rely on the sound wave pressure to induce the inlet flow.

__ Anyhow, it seems that you're claiming that if tested on a flow-bench,, regardless of whether the valve-head is held-open at-least 8.1mm, or completely removed from it's stem,, the flow-bench testing would remain indicative of the very-same flow-rate in either case, (since ANOTHER limiting-factor -[the "Port throat"] has already set the flow-limit) ! _ Is that correct ??

Well I should have said “that there is a very slight increase for the next 1to2mm, due to the valve head in the flow, as you said, but its effect on the amount that the throat will flow is too small to be a factor in the cylinder filling”.

__ Now I must admit that that logical/mathematical reasoning SEEMS to set-the-stage as being firmly-sound & undeniably factual. _ However, if such was ACTUALLY & completely true, THEN it would make no sense for cams to have lift-heights which actually provide even greater lift-areas.

As I said, “ Does not matter know much more lift you have, the valve throat won’t flow any more. It is the amount of time that the valve throat flows at its maximum flow that counts.”
The whole point in having a greater lift, than the throat can flow, is the amount of time that the throat is flowing its max flow. We can see this in this diagram of a pair of cam profiles one at 9mm the other at 12mm lift, 240* duration, from another engine that has the port max flow at 9mm, but close enough to see the effect.
The 9mm lift cam reaches its max flow in time to start closing again, having max flow for about 10* The 12mm lift reached its max flow at 9mm then continued to 12mm then back to 9mm. The port has been at max flow for about 90*
The desmodronics explot this in the racing cams by having a very fast opening profile to reach the max flow as fast as the valve gear can stand.

____ I actually appreciate others who so attempt to help keep me (& my claims) on-my-toes, (like with these posts) ! _ As it helps to keep the O'noggin off junk-TV, and hopefully also helps others to contemplate something which they may-not have otherwise.

Yes Bob I agree, TV sucks when the Moto GPs are not on.
Cheers.

[machten]

____ Perhaps you had me OVERLY-impressed but, I actually think-not since you expressed fairly valid reasoning

I think you've cottoned on to me now Bob! After Harvey's posts I know I'm out of my depth now, but I am very much interested and enjoying trying to digest the information.

Carry on, chaps!

Kev

[DewCatTea-Bob]

Well Bob I think this statement is confusing the TWO actions that take place in the inlet tract, during the intake cycle.

Positive pressure on the outside of the cylinder, and a negative pressure inside the cylinder.

____ Okay Harvey, rather than quote your entire first-section of your post, I've merely re-posted only it's first & last sentences (simply for section-reference).
____ Anyhow, it now seems apparent that I was mistaken to assume that YOU seemed to need to be "brought up to speed", because rather, (with respect to all that detail which you've now posted), it's I who's not up-to-speed.
__ I recall (from back in the late '60s) when I was first briefed-through all this kind of stuff, that I didn't find it all real easy to get a firm grasp of,, but by the mid-70s, I thought I had a pretty firm grasp of all the more pertinent aspects of it (and still think I do). _ However I sure don't think I would at all qualify to teach a class of students who NEED to learn all the involved detailed facts !
So-thus my post was only meant to allow others to become aware of the more major aspects of this stuff, (as I've forgotten about all the finer & less significant details, anyhow).
____ It's quite nice that YOU've found our w.site and are will to share your rather advanced knowledge on such.

Well I should have said “that there is a very slight increase for the next 1to2mm, due to the valve head in the flow, as you said, but its effect on the amount that the throat will flow is too small to be a factor in the cylinder filling”.
As I said, “ Does not matter know much more lift you have, the valve throat won’t flow any more.

____ I of-course agree that it's effect is not very significant to be any major factor, but I don't totally agree that merely-just "1to2mm" more (beyond the 8.1mm) is all the further the valve-head has to be lifted out of the way, in order to become the vary-same as if no-longer at all existent.
I should've made certain note in my last-post that I had actually never MEANT for it to be assumed that I believed that even HIGHER valve-lift could somehow possibly CAUSE airflow to become even FURTHER-increased through & past the intake-port's bottle-neck/throat-passage,, but rather in PROPER-respective, I had ACTUALLY meant that the valve-head's presents STILL provokes yet another flow-reduction aspect which leads-to the throat's maximum-flow CAPABILITY to be somewhat rather CURTAILED down-to a lesser amount than it otherwise WOULD flow with the valve-head COMPLETELY-gone (& entirely out of the flow-path !),, EVEN AFTER the valve's 'VALVING-function' has petered-out (by the 8.1mm lift-point) and the v.lift-height continues-on to an even higher lift-amount.
__ The reasoning is that if the valve-head is still held close enough to the port-throat outlet to cause any diversion of the flow of air, then certainly it must absorb & reflect some of the inertial-energy of the moving air as it's being redirected, and in-turn creates a pressure-front which builds-up atop-against the blocking-area of the v.head, which in-turn causes the creation of somewhat greater air-pressure right in front of the port-outlet which of-course naturally tends to oppose & impede the otherwise FREE-flowing airflow coming-out of the port-throat. _ So-therefore the amount of valve-lift doesn't merely additionally-restrict ONLY-just the intake-track's maximum-restriction ([of max.airflow], in the port-throat), when lifted less than the 8.1mm equality-point,, but it -(the valve-head lift-amount), also must continue-on to cause interference with (otherwise totally) FREE-flow, even well beyond the 8.1 lift-point.
__ I'd expect such flow-interference to continue-on remaining an existent-factor, (however insignificant), until the 40mm valve-head becomes lifted-up out of the way by perhaps as MUCH as 20 to 40mm (in order to finally become as insignificant as if no-longer existing at all).
__ I'd think that a flow-bench tester (flowing at it's TOP flow-pressure capability, & with extreme measurement detection), could prove this likely difference (between 8.1 & 40mm lift), one way or the other.

It is the amount of time that the valve throat flows at its maximum flow that counts.”
The whole point in having a greater lift, than the throat can flow, is the amount of time that the throat is flowing its max flow.

____ I hadn't understood exactly what you had actually meant to convey before,, but now, yes, of-course that certainly makes perfect-sense indeed !
And it's a major-point which my seemingly stupid post-statement did-not take into account. _ So thanks for catching THAT obvious oversight Harvey !
(Even-so however, [while your mentioned 'time' factor is vastly more important !],, I still believe that there's the OTHER, [although relatively insignificant], positive-benefit from even-further increased valve-lift.)


Enlightened-Cheers,
D.Bob