Date: Fri, 31 Jul 1998 20:12:34 -0500
Reply-To: "John H. Rodgers" <inua@QUICKLINK.NET>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: "John H. Rodgers" <inua@QUICKLINK.NET>
Subject: Re: what are ratio rockers?
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
Correct me if I'm wrong, but as I recall, rocker ratio combined with the
design of the cam grind determine when the valves open, how long they remain
open, and when they close. All together affecting the size of the fuel/air
charge, thus affecting the force of the power stroke....ie, more torque to
the crank.
John Rodgers
'85 GL Driver
-------- REPLY, Original message follows --------
Date: Friday, 31-Jul-98 01:48 PM
From: David Bayer \ Internet: (bayer@sybase.com)
To: Vanagon \ Internet: (vanagon@vanagon.com)
Subject: Re: what are ratio rockers?
>| OK, can someone explain to me exactly what ratio rockers are and how they
>| contribute to greater power? I presume they change the valve timing,
right?
All rocker arms are "ratio rockers". What people are talking about
is higher ratio rocker arms. The ratio referred in the name, is the ratio of
lift prodcued on one side of the arm to the height travel on the other (the
end pushed on by the pushrod). This can usually be directly measured by
measuring the length of one side of the rocker arm from the center of the
adjusting screw to the center of the big hole (which is usually filled with
the rocker shaft) and comparing it to the length of the center of the
pushrod cup to the center of the big hole...
Now what do higher ratios do? A high ratio arm will move the valves
more for an equal amount of pushrod/cam lift than a lower ratio rocker arm
would. This of course translates to more valve lift. The timing of open, max
open, and close do not change, but one thing to remember is that the valve
will seem to open quickier. I have yet to find a book from which I can
determine how this might affect torque (but then I have not made it to a
public library to find some old 50s books on engine theory - there are no
books stocked at any of the local bookstores which actually detail this....
Ok, so the valves lift higher, so what? Well, this potentially
allows the engine to breath better at all engine speeds, that is if the
heads and exhaust can flow more air. By creating more room for air to travel
there is less air friction around the valves and the engine can pull to the
max flow of the exhaust and intake system. Now, since the valves are
increasing the speed at which they open (and hence they make it to the same
ligt height as they would if the cam were advanced in time), you might also
change the tuberlence characteristics and possibly the amount of workable
stroke you have - again, these are things I believe one needs to find the
correct texts to really understand and to date, I have not found them (and
thus that last statement is not too qualified)...
Anyway, that's an intro to the topic that I have gained so far in
considering this bolt on change myself. Another to note: I was took the
stock ratio on the wasserboxer is 1.25. I don't know if true as my engine is
not back together enough to measure pushrod lift and subsquent valve lift.
The ratio when I measured as I described above came out to about 1:1 or
maybe 1.1:1 but the geometry of the arm can affect subsquent valve lift...
dave
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