Gary McCormick wrote:
> If you plot the motion of the connecting rod/piston combination it becomes
>quite apparent
> that not only the piston, but the conn rod also comes to a complete stop at
>each end of
> the stroke. The rotating motions of the wrist pin and the big end of the rod
>are
> relatively mild factors to deal with compared to these sudden halts and
>reversals
> occurring twice each stroke. This is where your oiling system and your motor
>oil's film
> strength really matter. Not only does combustion pressure on the power stroke
>delivers a
> punch that wants to press the bearing surfaces right through the oil film in
>the
> interstitial area between one surface and the other, but the force inherent in
> decelerating and re-accelerating the combined mass of the piston and conn rod
>produces
> similar forces in both ends of the rod at the other end of the stroke.
In fact the largest loads placed on the rod/crank/wristpin/piston assembly don't
have to do with compression loads at all. While the largest loads do occur at
TDC, the compression loads from combustion pale next to the tension loads at TDC
on the exhaust stroke. At any other time the tension loads are offset by either
compression above the piston, or pressure from combustion. But at TDC on the
exhaust stroke there is no significant pressure pushing down on the piston to
offset the tension loading that is generated by the stoppage of upward movement
and the reversal of direction. This is when parts fail, almost without
exception. The biggest blower, the largest turbo, nitro, nitrous, none of it
manages to change this simple fact. The tension loads on the rod/crank/wristpin
assembly from the reversal of direction at TDC on the exhaust stroke is the
largest load these parts will ever see in operation. Usually it is during a no
load overrev, such as when shifting. BTW the main reason the largest loads
aren't seen at BDC is because of the geometry of the crank/rod configuration. I
worked on engines for years without ever realizing this. Corky Bell pointed this
out to me in an interview I did with him about 10 years ago.
--
Marc Sayer
82 280ZXT
71 FJ510
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