> Can you explain the theory behind this for us? Is there a variation that
> works for other cams?
Hugh :
This works for any cam that is 'symmetrical', ie has the intake open the
same number of degrees before TDC as the exhaust opens after TDC. I believe
all of the Triumph factory cams were this way, but there might be
exceptions. One exception is the TR8, which of course was not a S-T
designed motor. Probably half of the aftermarket cams (especially the older
profiles) are this way, but you have to check the timing specification to be
sure. These are frequently listed as a sequence of four numbers, eg 72-on
TR6 stock cam was 18-58-58-18. In this case, we need to know if the first
number (intake opens) and last number (exhaust closes) are the same.
Another feature of _all_ camshafts is that they have very definite ramps in
the valve motion. They cannot just 'snap' the valve open and shut, they
have to gently (well, relatively <g>) lift the valve off it's seat, then
accelerate it smoothly, and decelerate it to it's open position. Closing
works the same way.
Armed with that, it should be easy to see how the test I outlined works. At
TDC between the exhaust and intake stroke, the intake valve should be just
opening and the exhaust should be just closing. Both will be slightly open,
this is known as the overlap period. The exact amount that they will be
open by varies with different camshaft profiles, but to check cam timing,
all we need do is see that they are the same. By increasing the lash to
.050" or so, we make it easy to measure the amount of lift with a simple
feeler gage.
Let me know if I've missed anything.
Randall
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