Walt,
I looked at my sent items. I had touchi spelled correctly the first
time. I used the apostrophe key to add the pronunciation symbol while
typing and MS word automatically corrected it to the proper
representation, as shown in your post. I have no idea why the (e') was
replaced it with an i on this list! Maybe I should not have enabled rich
text; I guess this post will be a test as I will leave it as rich text
as before to see if I can duplicate the problem. I know it seems
ridiculous but I want to know why the text changed!
I have more to add about rockers, although many others may be completely
bored with the subject.
I think my next suggestion may stir a lot of comments!
Jim,
I agree, I suspect the difference in valve timing is negligible. As you
say, there is probably a lot more variability to the adjustment. I
wonder if
the decrease in cylinder pressure is due to slightly higher valve lift
allowing more "backflow" into the intake and exhaust passages. We may
never
know! :-)
Ah, the wonders of fluid dynamics. It's a wonder anyone got a cam shaft
design right before high speed computers. I guess they blew up a lot of
engines.
Walt
ps. by the way, it's "touchi" :)
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net]On
Behalf Of Jim Swarthout
Sent: Monday, November 25, 2002 3:36 PM
To: 6pack@autox.team.net
Subject: RE: Roller rockers continued
Walt,
I had to think about it for a few minutes, but you're right. I had not
considered the fact that, given the same initial valve lash (.010), the
higher ratio rocker will contact the valve stem earlier and release
later.
Touchi.
Several things though when considering the original post. Something to
do with a decrease of 30 PSI per the compression gauge reading.
As you pointed out the valve lash would only need to be altered by
.0004" to completely negate the effect on duration. 4 tenths of one
one-thousandth of an inch would be the difference in the amount of drag
felt as the feeler gauge was slipped between the rocker and valve stem.
I thought about using my CAD program to simulate the difference in dwell
degrees, but I'm certain that it would be negligible. Minutes, maybe
only seconds of a degree...I think. Engines are not precise enough to be
affected by a .0004" difference in valve lash, or the subsequent
difference in duration that may be caused.
Jim
-----Original Message-----
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
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