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...
Thanks,
Doug Braun
'72 Spit
/// spitfires@autox.team.net mailing list
/// or try http://www.team.net/cgi-bin/majorcool
|