Neil, changing the lift of the cam does not effect geometry. If you change the
distance between the cam and the rocker, i.e. by decking the block, milling the
head, that's a different matter. When you go to a cam with increased lift, the
heel of the cam where you set clearence does not change(unless the grinder has
to remove metal from the heel to get the lift you want). You may need to change
pushrod length if you cut the head or block substantially, to the point that you
have to shim the rocker shaft stands to recover the proper relation between the
rocker and valve. In that case what you will be trying to do is gain the most
lift, i.e. motion that your cam and rocker will provide by ensuring that the
rocker operates so that at 1/2 lift it is at 90 degrees to the valve. You need
only draw out the path that the tip follows as it lifts the valve to understand
that you get the most lift if the rocker passes through the 90 degree position.
Assuming a vertical valve, the rocker will be moving the greatest distance for
each increment of cam lift when it is horizontal(90 degrees). John
Neil Cotty wrote:
> John,
>
> >Neil, whatever the cam lift, you should not change the pushrod length.
> John
>
> Can you explain in more detail as to why not? The two _blueprinting_ books
> (not motor rebuild books) I have both mention this as being an essential
> adjustment, when a motor has been modified in certain ways - to ensure the
> correct operating geometry of the rockers. They recommend various methods of
> adjusting the pushrods (not talking about LBC's) which I have actually
> already discussed on this BBS before - I think someone came back with using
> tubular pushrods (Bill?) which can be adjusted by machining & heat - (I
> think) - and that the stock pushrods are non adjustable. I'm not replacing
> with stock pushrods, I just need the length data.
>
> Can someone confirm this for me or am I dreaming again? <G> The net result
> of the decking/skimming etc alters the clearance between rocker arm and
> pushrod. My assumption is that increasing the clearance at the arm will
> resolve the clearance issure, but ultimately the angle of the rocker arm is
> affected by this adjustment and will affect the load placed on the rocker
> arms, and increase load on the rocker mechanism. If I am to spend the $ to
> flowbench the head, I would like to get all this spot on, but more
> importantly, as I'm considering roller rockers, I don't want these to take
> any more load than they already do. Apparently due to the increased weight,
> and different distribution of that weight, r/r's requires the use of
> slightly firmer springs to maintain the same valve bounce rpm.This is what I
> have read, and am requoting from a few sources. It's an interesting topic.
> :)
>
> Cheers,
> Neil.
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