Welcome to the fun of camshaft specifications and marketing claims. There is no
industry standard as to how and where lift measurements are to come from. While
many of them do in fact use .005 lift at the follower as the benchmark, this is
far from an absolute. Many of them use anything that strikes their fancy.
Your observations about the effects of extra valve lash are correct, it also
reduces overlap, tending to make the cam more docile and streetable.
Douglas Braun & Nadia Papakonstantinou wrote:
> Hello,
>
> Last year I installed "fast road" reground cam.
> Recently while replacing the timing chain I actually measured the
> timing. I was surprised to find that the timing was quite
> a bit more mild than advertised. The cam was supposed
> to have the intake valve opening 22 degrees before TDC, and
> the exhaust valve closing 28 degrees after TDC. But I
> actually measured about 16 and 22 degrees for these parameters.
> (This could not be due to an error in the timing gear alignment,
> since the errors are equally advanced and retarded.)
>
> The stock MK IV cam has 18 degrees for these two specs,
> so it looks like my cam could be only a tiny tiny bit more
> radical than stock.
>
> But I also noticed that it takes about 6-8 degrees of
> crankshaft rotation to take up the clearance in the valvetrain
> and actually start opening the valve.
> So this makes me wonder: Is cam timing supposed
> to be measured at the lifter, or at the valve?
> If the timing specs are meant to be measured at the lifter,
> my cam is probably working as advertised, but if they are
> supposed to be measured at the valves, it is no different
> than the stock cam. Does anyone know?
>
> Also, the cam instructions say to set the intake valve clearance
> to .012", and the exhaust to .014" instead of the stock .010".
> Does anyone know the reason for this, and what would happen
> if I set it to .010"? It seems to me that increasing the clearance
> simply reduces the duration and lift of the cam...
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