In a message dated 97-09-10 01:28:55 EDT, 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
Bob,
I used the longer rods in my engine during the last rebuild, and there was a
noticeable rise in torque on the dyno. If you are interested, I may be able
to find the two dyno sheets. As to why, exactly, I am unsure, but it was a
trick picked up at the dragstrip from <gasp> chevy 454 owners, who used the
454 truck block and rods as the basis for their engines. The
wear/releliability theory was explained as you discovered...in addition, the
longer rod has less angle on the crankpin during the power stroke, and so
more power is transmitted to the crank.
On my engine, the block was decked square and the amount that the pistons
stuck out of the block measured. I had SCE machine solid copper head gaskets
.090 thick, so that minimum piston to head clearance was maintained.
No problems have surfaced with this setup.
Any more info would be appreciated......
Regards, Ray
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