Jim, I understand what you are saying about overall lift yet I think that
where you are possibly wrong is amount of valve lift during overlap. If the
stock rockers open both valves at overlap, say .002, then the new ratio may
open them .003 at overlap, hence some cylinder pressure loss at overlap.
IMHO.
>From: "Jim Swarthout" <jswarth1@tampabay.rr.com>
>Reply-To: "Jim Swarthout" <jswarth1@tampabay.rr.com>
>To: <6pack@autox.team.net>
>Subject: RE: Roller rockers continued
>Date: Mon, 25 Nov 2002 07:45:34 -0500
>
>Dick,
>
>
> "We know that the increase in the ratio will hold both valves
>open longer during the overlap segment of the stroke, while cranking".
>
> Let's all think about this for a moment. Imagine this scenario:
>both prior to and after installing rocker arms of a different ratio.
>
>Remove the valve cover. Pick a valve, any valve. Slowly turn the crank
>until the chosen valve and rocker just make contact, "Zero lash". Using
>the timing pointer at the crankshaft make a mark on the damper at
>"Zero". Now rotate the crank allowing the valve to go through a complete
>cycle until returning again to the fully closed position, "zero lash".
>Make another mark on the damper at "Zero". This represents the total
>duration expressed in degrees, (Relative to the crankshaft position,)
>that the valve is open!
>
>This value, in degrees, can only be changed by altering the cam lobe
>profile! Imagine two camshafts with the same duration, but different
>lifts. The valves will remain open or closed for the exact number of
>degrees, but one will cause the valves to open further. Rockers of a
>different ratio create the same effect. You will notice a greater valve
>opening, but at the same duration. Duration is ground into the lobe
>profile and can not be changed other than regrinding the cam.
>
>Repeat the above using rockers of a different ratio, and I can assure
>you that the original marks made on the crankshaft, (with a different
>ratio rocker), will again represent the exact moments of opening and
>closing.
>
>On another note: Compression tests are normally used to evaluate the
>condition of the engine with regards to uniformity, not necessarily an
>exact gauge reading. Next time you do a compression test, try this!
>Assuming the cylinder being tested is in fairly good condition; check
>the gauge reading with the carbs, both shut and fully open. If you ever
>have to remove the intake manifold, run a compression test. Again based
>on the cylinder being in good condition you should get different gauge
>readings for each test!
>
>
>Jim
>
>
>
>
>Jim S.---Looks like we have agreed to disagree with the loss of pressure
>(with the RR ratio increase)
>
>Here are my thoughts. We know that the increase in the ratio will hold
>both valves open longer during the overlap segment of the stroke, while
>cranking. Now you have a time-against-pressure sequence that has
>occurred, where I believe this could cause lower readings. I think the
>key words here are at Cranking Speeds.
>
>Do we still disagree? I'm willing to learn new things!
>
>Dick
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