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| cam/valve lash, help me understand https://slantsix.org/forum/viewtopic.php?t=50334 |
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| Author: | CNC-Dude [ Sat Nov 17, 2012 10:33 am ] |
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This post seems to get more confusing as it goes on. Lets say you have a Slant cam ground for the stock .901 lifter and stock 1.5 rockers. If you then build the engine with a 1.000" Race Hemi style lifter and try to start plotting all your valve openings and closings, its not going to correspond with the cam card, it can't. The Hemi lifter has a .100 larger diameter footprint on the cam lobe and will make all the opening events happen faster than the card says because the outer edge of the now larger OD lifter transfers the motion to the valvetrain components earlier than the .901 lifter does at the same point in relation to the timing events. In the same way swapping to a 1.6 rocker will make those timing events happen faster than a 1.5 rocker will. |
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| Author: | kielbasa [ Sat Nov 17, 2012 11:55 am ] |
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Well something is off becuase i know my #s are correct |
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| Author: | robertob [ Sat Nov 17, 2012 11:21 pm ] |
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I am pretty sure this is an incorrect assumption. Lifter diameter determines maximum lift velocity possible with a given profile. However, when the lifter starts to move should be exactly the same with a given profile and two different lifter diameters, since the contact area at that point only the center of the lifter is touching the cam lobe. Quote: This post seems to get more confusing as it goes on. Lets say you have a Slant cam ground for the stock .901 lifter and stock 1.5 rockers. If you then build the engine with a 1.000" Race Hemi style lifter and try to start plotting all your valve openings and closings, its not going to correspond with the cam card, it can't. The Hemi lifter has a .100 larger diameter footprint on the cam lobe and will make all the opening events happen faster than the card says because the outer edge of the now larger OD lifter transfers the motion to the valvetrain components earlier than the .901 lifter does at the same point in relation to the timing events. In the same way swapping to a 1.6 rocker will make those timing events happen faster than a 1.5 rocker will.
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| Author: | robertob [ Sat Nov 17, 2012 11:25 pm ] |
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Quote: Well something is off becuase i know my #s are correct
I am pretty sure that you are right here, but I can't find the source to back me up. Basically the lifter diameter determines what cam lobe design is optimal/possible, but it does not determine the exact timing with a given lobe. The 0 to 0 duration will be the same because of the lash ramps and the flanks of the lobe.Checking a given cam lobe with a dial indicator, a Chevy .842 lifter and a Race Hemi 1.00 lifter should all give the same results. If a lobe is designed to take advantage of a larger diameter lifter and used with a smaller one like your VW example, the nose of the lobe COULD run off the end of the lifter and be destroyed quickly. If a lobe is designed for a smaller lifter and used with a larger one (like the Comp Cams for slant six cam) nothing will happen, it just won't give the maximum possible performance. The nose of the lobe will hit the lifter inboard of the outer edge and wipe a shorter distance across it. |
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| Author: | CNC-Dude [ Sun Nov 18, 2012 9:30 am ] |
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Quote: I am pretty sure this is an incorrect assumption.
Doc said the same thing as I descibed here on the first post on page 2 of this topic......
Lifter diameter determines maximum lift velocity possible with a given profile. However, when the lifter starts to move should be exactly the same with a given profile and two different lifter diameters, since the contact area at that point only the center of the lifter is touching the cam lobe. Quote: This post seems to get more confusing as it goes on. Lets say you have a Slant cam ground for the stock .901 lifter and stock 1.5 rockers. If you then build the engine with a 1.000" Race Hemi style lifter and try to start plotting all your valve openings and closings, its not going to correspond with the cam card, it can't. The Hemi lifter has a .100 larger diameter footprint on the cam lobe and will make all the opening events happen faster than the card says because the outer edge of the now larger OD lifter transfers the motion to the valvetrain components earlier than the .901 lifter does at the same point in relation to the timing events. In the same way swapping to a 1.6 rocker will make those timing events happen faster than a 1.5 rocker will. |
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| Author: | robertob [ Sun Nov 18, 2012 9:49 am ] |
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Not exactly. But think about it - with a lobe designed for an .843 lifter, the nose must contact the lifter on the face. So at no point is the contact between the lifter and cam further than .4215 from the center. If you put a dinner plate over the cam nothing changes. I can imagine a cam lobe shape such that this is not true, but I am pretty sure it would not be an automotive cam lobe shape. I would love to be proven wrong on this I spent a ton of time last night reading about cam geometry and I am almost positive lifter diameter has no effect on duration numbers with the same lobe. Now if we are talking about two different lobes designed to maximize velocity with two differen lifter diameters, yet with the same timing, that is a different story. |
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| Author: | SlantSteve [ Sun Nov 18, 2012 1:08 pm ] |
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Just a random thought to throw in here guys, but wouldn't base circle and the overall circumference of the lobe also come into play here? If you had the same profile on a fresh billet and a regrind on an old cam with a smaller base circle wouldn't that come into play in the same way? Personally, an approx .050" larger dia lifter , that's only .025" on each side from centre, doesn't sound like much for the average street profile, for a racing application I'm sure it would be a different story. Out of interest, I overhauled aircraft engines for many years, regrinding cams is allowed to a specified limit, as are refaced lifters, but I often saw a small loss in power, but still within acceptable parameters, but for some critical engines in somewhat underpowered choppers we always fitted unground cams and lifters and the operators claimed a noticeable difference... Forgive my rambling!! Steve |
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| Author: | Dart270 [ Mon Nov 19, 2012 4:06 am ] |
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I think that the lobe never significantly contacts the very edge of the lifter. If it does, I believe your lifter will go bye bye quickly. Larger lifters allow for faster ramp and higher lift cams with no destruction. This is my understanding but I could be convinced I am mistaken. Lou |
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