Date: Fri, 31 Jul 1998 13:48:39 -0700
Reply-To: Dave Bayer <bayer@SYBASE.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Dave Bayer <bayer@SYBASE.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
| 
