This got bounced for being over the limit. 4KB indeed. I type with BOTH hands.
>>The benefit of reducing rocker friction isn't overall engine efficiency,
just as reducing the rocker weight isn't an overall weight reduction issue.
Skip that second donut and you'd be way ahead for a lot less money. It's all
about making life easier for the tappet, camshaft, and the valve so that you
can either make the head flow better or rev the engine higher. So to answer
the original question as succinctly as possible, you can do much, much better
than 2% with roller rockers if the overall engine has been designed to have
roller rockers. If not, then you won't.
A high performance racing engine designed for standard rockers probably won't
benefit from roller rockers to a large degree. But a good cam designer knowing
exactly what's going to be in the engine can shape a more radical profile for
a roller rocker than a stock one, accelerating the valve more harshly, and
holding it at maximum lift longer. Just as they can if they know you're going
to have roller followers (only moreso). The head porting guy that knows you're
using a roller rocker with properly and precisely set valve geometry can
eliminate the valve guide from the intake port, knowing that the remaining
stub is going to be enough.
As with any other flowing system, as the valve gets open further, it has less
influence on flow. Its why you don't use a gate valve to throttle and why
carbs flow about the same from 3/4 to full opening. It's pretty well
understood that the value of the higher rocker ratio isn't really ultimate
lift, it's acceleration during the early part of the opening cycle. Given that
rocker ratio doesn't influence duration of the cam, or the point that it
begins to open, it does have an effect on early flow since the valve is
accelerating faster.
The problem with conversing with me about this is that I'm re-reading the
Engine Builder's Handbook, and trying to translate all the great V8 knowledge
in that book to puny tractor motors. A frustrating practice that makes it
impossible for me to look at any individual part without considering the
entire system.
-----Original Message-----
From: fot-bounces+billb=bnj.com@autox.team.net on behalf of Allen Washatko
Sent: Wed 12/26/2007 4:41 PM
To: Bill Babcock; Friends of Triumph
Subject: Re: [Fot] Roller Rockers
Bill,
That being said, the dyno tests involved a fully developed race engine
run with stock rockers and again with roller rockers. The difference
was 2 h.p. - maybe a 1-2% difference overall. Hardly close to a 10%
gain as some promise as a bolt on improvement (I would assume that this
figure is for a stock situation). It is clear that the pin bearing and
roller will reduce friction but the increased leverage will also add
more work at the tappet end. As in everything, there is a limit to
improvement. As machines become more refined, the next increment of
improvement usually becomes smaller and more difficult to obtain. My
point in asking the original question was whether or not roller rockers
really added more horsepower due to a longer valve duration assuming
that all the other bases were covered - friction aside. I would assume
that there is a limit to just how much air/fuel mixture you can suck
through a Triumph head as fully developed as it may be. The dyno test
would seem to suggest that.
Allen
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