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The variables would be quite a long list, and wouldn’t venture away very far from any other V8 engine, given similar parameters such as lobe design, duration, lift, rocker ratio, rpm, etc.
Are you inquiring about a specific LS engine?
best would be ls1 with stock cam, 1999 to be pecise. as you say, any pushrod v-8 would be fine, the above is a zetec ford.
whats revealing is the peak at abot 30° after intake valve open. im also trying to understand where the second peak at close comes from. the valve bangs onto the seat, but why that would affect valve tip force is beyond me. the rocker would accelerate away from the valve, no? (i can see the second peak between cam and lifter due to inertia of the components.)
edit: there will always be a substantial difference between calculated and measured values.
thats interesting, because i understand many applications go the opposite route (up to 1:2 ratio). there are physical limits of how a cam can be ground, unless you use hollow grinds that are difficult to do. (no idea if these even exist for ls engines.)
yes, but it opens slowly. what factors are there? clinder pressue, spring pressure, accleration (max about 30° after ivo and before evc?), inertia, what else?
Yes, but it opens slowly. What factors are there? Cylinder pressure, spring pressure, acceleration (max about 30° after ivo and before evc?), inertia, what else?
im looking for real (measured) data.
And, who do you think would see a real reason to measure such data? If it were such vital or important data, it would be commonly and easily available.
My point is, what is to be gained from such knowledge as far as durability or component life goes?
As has been said, the answers to your questions would be very similar regardless of the cam, lifter, pushrod, and rocker arm used. It mostly comes down to spring rate and pressure.
please look at the graph i posted. do you see any similarity to spring pressure? there is a plethora of studies on the subject out there for good reason, i just havent found one on a ls engine yet. (perhaps because our engines are such "dinosaurs"?)
what can be gained? everything you need to know to develop a functioning valve train: forces and speeds on contact points (e.g. for tribological evalution), stiffness and material requirements of components, insight into their longitudinal and transverse vibration as well as damping properties. what do you think engineers do? just dream something up?
example: fatigue strenght of a component depends on amplitude of load and number of cycles (to simplify). you need to know by how much effective load and number of cycles exeed a simple model due to vibration to chose a material.
edit: have you ever looked at a graph of real valve acceleration? might be quite revealing.
hey, that would be greatly appreciated. as to the relvance: there is so much b.s. floating around on valvetrain set up (pedestral/stud height, sharpie pattern) and im looking to get an informed opinion on it.
engine: stock 1999 ls1 (c5). according to my info: "472/479, 199/207, 117" cam. im measuring 276/281 on the lobes and 473 exh. valve lift (intake got lost somehow). the only other info i would have is pushrod, spring, keeper and retainer weight. spring is supposedly 75/230 lbs. (i dont remember where that came from.)
yes, but it opens slowly. what factors are there? clinder pressue, spring pressure, accleration (max about 30° after ivo and before evc?), inertia, what else?
im looking for real (measured) data.
Cylinder pressure dwarfs all other concern. Post combustion pressures are in thousands of psi. Ex valve diameter is 1.6". If cylinder pressure is 1000 psi, which would be VERY low for post combustion, that's 2000 lbs approx on the ex valve. If you have super-ultra-extreme springs, they're not half that at max lift. Inertia, etc, that's rounding error by comparison.
Second to combustion pressure I worry about bouncing off the seat secondly.
It's a cool topic for sure. If I'm wrong it will have been worth it to learn something.
"what factors are there? clinder pressue, spring pressure, accleration (max about 30° after ivo and before evc?), inertia, what else?"
ill add: friction, change of angles pushrod-rocker/rocker-valve.
lets see if i can upload this (i dont remember what kind of engine it was):
no, it doesnt upload for some reason. ill try later.
its a graph basically showing seat pressure being about equal to keeper pressure at ivo and quickly falling to zero before the acceleration jerk of the valve. it also shows the banging at close being about half of max seat pressure, but is not reflected in valve tip pressure (as expected). this is why i dont get the second peak in the graph in first post.
The example by Darth is correct.
I would ADD the importance of exhaust valve seat pressure, often forgotten.
His math used states the area of the exhaust valve, "R" squared x 3.1416.
Thus a common EMAP in a "turbo" engine (39psi) will reduce the exhaust valve seat pressure by 60 .lbs.
This could allow seat bounce.
I kept chewing on this. I can see another potential cause for a spike on tip load, and that is pushrod flexion. not so much that it flexes, but it "pole-vaults" the valve at the top of the lobe for a momentary loss of load (pushrod springs straight and throws the valve even against spring pressure), followed by crashing back down when the spring recovers. It would hit like a hammer, so to speak. Usually, I design valvetrains to avoid this, but I do know that some people do this on purpose for lift area. Usually the people that do this on purpose also rebuild between events....
But still, typically the highest load by far on the valvetrain is the Exhaust valve opening event.
how true. by deviating from midlift/square rocker geometry you can open the valve faster. but guess what, its going to get slammed on the seat faster.
btw, i guess the "dynamics" of the valvetrain are often underestimated. a real acceleration graph can show "vibrations" of 50% of the average value throughout the whole event. jerk and even quirk curves with large spikes. its surprising the system somehow survives.
this is the graph i was talking about above:
it doesnt upload. does that happen often? (i havent been here for a while.)
uploading graph: oh, wrong graph, but interesting anyway.
so the graph i mentioned will not up load, same png-format, same file, very weird.
anyway, to put this into perpective, the original question implicitly implied a v8 pushrod engine at wot around peak power. im pretty sure now the largest valve tip forces are around 30° after valve open and 30° before valve close at 10-15% lift. maybe think about this when following the omnipresent instructions to center the rocker contact point on the valve.
of course if looking at the engine idling the highest pressure will be dictated by spring force (thats why many taxi engines have problems with the trunions). if you look at a slow running fi diesel engine at wot, cylinder pressure will have the largest impact on the exhaust side. valve bounce can input the largest forces into the system, but not on the valve tip, i guess.
Hi Dian, sure all this data is fine.
What is your reason ?
My ADVICE is the find ways the reduce valve train inertia.
Remember the valve train needs to be accelerated.
A lite valve well reduce required forces, spring pressure requirement and allow the engine to "spin up" faster.
07-14-2022, 07:41 PM
