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| cam/valve lash, help me understand https://slantsix.org/forum/viewtopic.php?t=50334 |
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| Author: | Doc [ Thu Nov 15, 2012 10:45 am ] |
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Bigger lifter diameter = ramp starts moving sooner and stops moving later, in relation to degrees of cam rotation. Smaller lifter is the opposit. DD |
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| Author: | kielbasa [ Thu Nov 15, 2012 11:20 am ] |
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Makes sense thanks doc |
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| Author: | CNC-Dude [ Thu Nov 15, 2012 2:57 pm ] |
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Thats why VW's use a mushroom tappet, and could be some of the reason for tighter lash. |
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| Author: | kielbasa [ Thu Nov 15, 2012 3:23 pm ] |
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Hmm... now that I'm thinking about it, my vw cam was said 31mm lifters were reccomended, but at the time i was only able to use 28mm, what is the possible consequence for doing so? |
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| Author: | Mroldfart2u [ Thu Nov 15, 2012 3:57 pm ] |
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On a VW engine, not for sure, how long has it been mis-matched? If its been a bit and the cam hasnt wiped you will probably be ok... |
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| Author: | ceej [ Thu Nov 15, 2012 4:31 pm ] |
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Another note would be effect of different Rocker Ratio with a given cam profile. As the ratio increases, so does lash requirement. At the same time, getting the intake event back to the right place pokes it's nose into the equation, so don't throw away your degree wheel.  CJ |
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| Author: | kielbasa [ Thu Nov 15, 2012 4:40 pm ] |
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Its been in there for years but only have a maximim of 10hours drive time. I've had lots of carburator problems (new weber idas like to leak past throttle shafts) and valve guide problem, the heads i have are heavily ported, and did not have enough material left to hold the guide in, so a giant hole was made for a aluminum sleeve to be inserted so it has moregmoregrabbing force, and guide was pressed into that. That sleeve eventually failed. Heads are fixed and material was added to port to hold a valveguide only. Have not ran it yet. Also did notice that one lifter on pushrod side chipped very small, does not seem it has affected lifter bore, valve clearences were checked regularly to make sure nothing wad going flat... seems ok. Curious if smaller lifter diameter could have had some play in that. Idd call manufature a bit ago and they said 28mm will be fine but 31 is the preference on my paticular cam. It is a fairly gnarly cam. |
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| Author: | CNC-Dude [ Thu Nov 15, 2012 11:23 pm ] |
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Using a bigger lifter diameter than what the cam was ground for makes the cam act bigger than it really is. So using a smaller lifter than what the cam was ground for makes it smaller than it was intended. |
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| Author: | kielbasa [ Thu Nov 15, 2012 11:42 pm ] |
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How so? I had my cam read by local shop, have a giant sheet of info, telling me lifts and degrees and all that. While dialing the cam in the case, i got exact #s as cam sheet. |
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| Author: | CNC-Dude [ Fri Nov 16, 2012 12:23 am ] |
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Using a larger OD lifter is like using a larger rocker ratio, it changes the intake and exhaust timing events on any given cam from what it was originally ground. So either your cam company was mistaken about what lifter you were to use and you ended up using the correct one for what the cam was ground for, or you had the big OD lifters thinking they were the smaller ones and it matched the cam card. But anyway, its physically impossible to use a bigger OD lifter or bigger rocker ratio and it not change the cam timing for a cam that wasn't ground for those components originally. |
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| Author: | kielbasa [ Fri Nov 16, 2012 2:07 am ] |
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I never checked cam with rockers installed, except for doing total lift and geometry. But cam was ment for either 1.25 or 1.4 ratio, my 1.4 rockers ended up rolling a 1.57. The original cam card was completely wrong from cam, off by 20 degrees on some spots and lobe lift was wrong. That was the reasoning for having cam shop read it. And i highly doubt they posted wrong lifter reccomendation. Its the "bigfoot" series lol which require bigfoot lifters. And i know i have the 28mm. The shop used a machine that used basicly a dial indicator with a rod about 1/2 inch in diameter. Smaller then my lifter... recieved same numbers as i calculated on my own |
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| Author: | Mroldfart2u [ Fri Nov 16, 2012 11:58 am ] |
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huh? either the cam/card "matched excactly" or it didnt.... I got 99 problems, and none of them are a vw..... |
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| Author: | kielbasa [ Fri Nov 16, 2012 3:15 pm ] |
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The original cam card which was packaged with camshaft was wrong, they hand wright their #s on a generic blank card. I had a machine shop read the cam and got correct #s, which matched up to my own specs i came up with as i was mocking it all up in the case. This topic would not be vw specific i would think as I'm asking about lobe vs lifter diameter. |
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| Author: | Mroldfart2u [ Sat Nov 17, 2012 1:14 am ] |
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Quote: The original cam card which was packaged with camshaft was wrong, they hand wright their #s on a generic blank card. I had a machine shop read the cam and got correct #s, which matched up to my own specs i came up with as i was mocking it all up in the case.
Well I understand now what you are asking, but now even i am more confused. CNCDUDE said basically a smaller lifter would have smaller values of a cam vs a larger lifter on the same lobe/same cam? (or did he)?.... (here is where it gets murky, You are running a cam designed for a larger lifter but are using small lifters, and cam was checked with what you thought was a "follower" half the size of either, but the specs are the same as what you have came up with with it installed) All i have is HOW does that happen if CNC is right? And maybe thats really the underlying question..... This topic would not be vw specific i would think as I'm asking about lobe vs lifter diameter. Or maybe i missed the whole episode.... 
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| Author: | kielbasa [ Sat Nov 17, 2012 10:26 am ] |
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That's why i brought it uo, because what he says is 100% wrong in my situation. I am fairly confused my self. |
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