Originally Posted by
Chr1s
Well, this is a forum and everybody is open to an opinion, so here's mine.
In a forced induction motor, you have a condition where the engine is always under positive pressure. In a naturally aspirated engine, we often find a vacuum or an atmospheric condition present depending on how efficient the motor is.
So what? Well think of the process the rods and rod bolts see, the changing ingress of stresses is what i'm talking about in particular; Tension and compression.
Compression occurs when the piston is "pushing" down on the rod, this induces stress in the rod near the wrist pin, this kind of stress will lead to a bending type of failure. There is no stress on the rod bolts at this point, if anything, this is where the importance of oil pressure between the bearing and journal is needed to avoid contact. For an increase in BMEP (power) there is the requirement for an increase in oil pressure. But this is another story.
Tension occurs as the piston is "pulling" on the rod and peaks in stress when the crank changes direction, imagine sprinting as fast as you can and then stopping and trying to change direction as fast as you can, the inertia would be huge. As mentioned earlier, a failure at this point would typically yield a rod split in two. What has rod bolts got to do with this stress? Well when the rod is under tensile stress, the bottom half of the rod cap is resisting this force due to the oil pressure again between the bearing and the journal. This resistant force is transmitted to the rod bolts which results in, you guessed it, tensile stress. Now we've realised that this kind of motion isn't present on the compression stroke, hence the term compression, the rod is subject to compressive stress, when the power stroke is occuring, the compressive stress is releived and the rod is in a happy state, so this rules HP out of the equation for stresses implied on rod bolts. Obviously the rod has to have a suitable second moment of inertia to sustain large compressive stresses. Although I will guarantee you that the tensile stress acting on the rod is much greater than the compressive.
It should be common sense now to see how a rod is affected now by the greater pressure differential as it sweeps from TDC on the intake stroke (overlap, MAP, tensile) in a naturally aspirated motor.
Why HP has nothing to do with how strong a rod bolt is? Because the tensile stress present on the rod bolt is what kills them, and the faster you spin the engine, the greater this becomes. THIS is why I say, RPM kills rod bolts.
Generally speaking, higher HP engines require a greater RPM to make this higher HP, this is why items fail in this manner and people just resort to "oh it's because of the power" mentality. Remember your simple Physics.
Oh by the way, you may have been building engines for the past 15 years, which no offense, means jack all to most educated people, although keep in mind you might be talking to an ex F1 engine developer one day.
