Jim,
I agree with your premise that you can't change the cam profile with rocker
ratio, but I believe you can change the effective duration at the valve.
Since valve travel and acceleration are related to the cam profile by the
rocker ratio, the valves will actuate more quickly with the higher ratio
rockers. If you have .010 of valve lash, a 1.5:1 rocker will require .0067"
of lift at the cam to obtain "zero" lash. If a 1.6:1 rocker is used, .0063"
of lift at the cam is required to get to "zero" lash. Since the lower number
occurs earlier on the cam lobe, your duration has been slightly increased.
I'm not sure how significant this would be, but it will effect overlap and
duration to some degree. My guess is that the increase in valve lift and
acceleration with the increased ratio provide most of the performance
increase.
I agree that compression tests can't really be used to compare different
engine set-ups. There are too many other variables in the equation.
Walt
74 TR6
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net]On
Behalf Of Jim Swarthout
Sent: Monday, November 25, 2002 7:46 AM
To: 6pack@autox.team.net
Subject: RE: Roller rockers continued
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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