Could you draw a picture?
On Tue, 9 Sep 97 15:24:58 PDT rpalmer@ames.ucsd.edu (Bob Palmer) writes:
>Guys,
>
>Recently, one of you out there mentioned using 289 rods in a 302 to
>improve
>torque. At least I remember something along those lines. Here's
>another
>subject we could probably spend days on and perhaps get nowhere.
>However,
>there's been a lot of hype in various hot rod magazines, etc. about
>using
>longer rods and the beneficial effect on piston velocity, torque, and
>horsepower which I want to challenge as being 99.44% baloney.
>Actually, I
>think we've about buried the "Hot Tiger" topic and it's time to look
>for
>another windmill to tilt.
>
>A little background on the rod length issue can be found in the
>Wilson,
>Sadler, and Miches book "Kinematics & Dynamics of Machinery",
>especially
>section 3.7.1 starting on page 151 which deals with the "In-Line
>Slider
>Crank Mechanism" which is what the piston/ror/crankshaft is. One
>thing I
>found interesting that I hadn't realized is that the motion of the
>piston is
>not sinusoidal. It's actually the sum of two sine waves, the primary
>one
>being the frequency of crank rotation and a second being twice this
>frequency (result of side-to-side motion of crank). The result of
>this is
>that the piston spends less time near the top of its motion and
>consequently, the acceleration of the piston is greater on either side
>of
>top dead center than around the bottom of the stroke. This is all
>very
>interesting, but what are the practical consequences? The
>aforementioned
>section graphs piston acceleration for various ratios of rod length to
>
>stroke. Piston acceleration decreases as rod to stoke ratio
>increases.
>This means, since F=ma, that the forces acting on the rod also
>decrease with
>increasing length. This is far from a complete analysis of the forces
>
>acting on the rod, but at least this is a start. As a practical
>matter,
>longer rods weigh more, cost more, the engine block has to be taller,
>etc.,
>etc. I haven't done an exhaustive survey, but most production engines
>have
>rod/stroke ratios around 1.7:1 or so.
>
>Expanding on this line of thinking, I think a solid case can be made
>for
>longer rod length in terms of reducing stress and allowing higher rpm
>operation. I presume this is why the BOSS 302 used the longer 289
>rods, but
>maybe those Ford engineers had something else in mind. Is there any
>corresponding case for improving horsepower or torque? I remain
>unconvinced
>of the latter assertion,regardles of how often it is made. Before
>resorting
>to more specious arguments, is there any verifiable dynamometer data
>to
>support this assertion. I know there are some very impressive motors
>out
>there with extra long rods. This doesn't necessarily prove that the
>extra
>rod length is any benefit in terms of performance; perhaps just helps
>keep
>it together at high rpms.
>
>Just thought I'd toss these thoughts out for a few of you engineering
>types
>to chew on. Let me know what you think or what facts you may have to
>add to
>the picture.
>
>Bob
>
>
|