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Old 10-20-2004, 06:52 PM   #1
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Cam Timing At Lash Point?

Is cam timing at lash point the same as cam timing at .006? The GM Hotcam is measured at "Lash Point." The Comp Cams cam guy didn't even know what the heck this was. He thought it was duration measured from .000 lash ramp to .000 lash ramp. Different manufacturers use different methods to get the so called "Advertised" duration. Is this GM's way of saying .006?
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Old 10-20-2004, 07:07 PM   #2
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Re: Cam Timing At Lash Point?

The industry standard uses 0.006" as a zero lash point.

I think this is to ensure all of the play has been eliminated through the
valve train when setting hydraulic lifters.

If the camshafts are to be compared, the must be a defined start and end
point (hence the 0.006" and 0.050" references).


Someone please confirm.
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Old 10-20-2004, 08:39 PM   #3
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Re: Cam Timing At Lash Point?

Quote:
Originally Posted by Zero_to_69
The industry standard uses 0.006" as a zero lash point.

I think this is to ensure all of the play has been eliminated through the
valve train when setting hydraulic lifters.

If the camshafts are to be compared, the must be a defined start and end
point (hence the 0.006" and 0.050" references).


Someone please confirm.
IMO, there is really no "Industry Standard" for measuring total duration.

"Advertised Duration" used by some cam companies may use .006 valve lift point. Engine Analyzer PRO uses .003 valve lift to quote total duration. They say that this corresponds quite closely to many cam companies' "Advertised Durations".

In my mind, "Lash Point" when applied to solid lifter systems, is the point at which all the lash has been taken up by the opening/closing ramps on the cam lobes, and the lifter/pushrod/rocker/valve is just starting to move. GM applies the term to both solid and hydraulic cams, but they also use very long hydraulic ramps, especially on street cams.

There are so many variables in the ramps (not the "flank" or portion of the lobe that does the majority of the lifting) of OEM and aftermarket cams that two cams with 220 degree @ .050 duration might have "Advertised" or "Total" durations from 270 to over 320 degrees.

For example, the Gen V 502 hydraulic flat cam (14096209) has 220 deg @.050 duration and 328 degrees at lash point. The old LS-6 mechanical (solid) flat (3904362) has 242 deg @ .050 and 307 degrees at lash point on the intake with .024 lash. The exhaust is also 242 @ .050, with 298 at lash point and .028 lash. The .004 difference in lash looks like it makes about 4.5 degrees difference on both opening and closing, if one assumes opening and closing ramps are similar.

"Curiouser and curiouser." Alice must have been into cams!
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Old 10-20-2004, 09:02 PM   #4
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Re: Cam Timing At Lash Point?

Does this mean that the GM LT4 Hotcam could have more "overlap" than the Comp Cams XE 224/230? In other words, even though it only measures 218/228 @ .050 112 LSA could it have more actual duration the XE 224/230 with 112 LSA?
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Last edited by chrism400; 10-20-2004 at 09:05 PM.
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Old 10-21-2004, 07:56 PM   #5
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Re: Cam Timing At Lash Point?

I'll take whack at this......

224/230 vs. 218/228 duration is measured at .050 of TAPPET lift. Both of these cams being hydraulic cams, with theoretically zero lash, .050 durations will be directly comparable IN THEORY.

However, that doesn't take into account the actual shape of the ramps from "zero" to .050 of tappet lift. If the Hot cam is really lazy from "zero" to .050 and the XE is really quick, then yes, the Hot cam could have more ADVERTISED duration. It still has less .050 duration, however.

IF it has more advertised duration then it would have more overlap as measured at advertised duration. It will still have less overlap at .050 duration.

Last one I'll throw out here..... GM does their cam ratings "different" from everyone else. I would not be able to adequately describe this to you, but don't try to compare GM specs to aftermarket specs. I don't know how GM measures stuff, but it doesn't line up with other company's way of doing things.
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Old 10-21-2004, 07:56 PM
 
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