51 Pacemaker Resurrection

squirrel

RPM is usually what kills connecting rods (it tears the big end apart), although if you make enough power, you can buckle one.

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squirrel said:

RPM is usually what kills connecting rods (it tears the big end apart), although if you make enough power, you can buckle one.

That's the voice of experience talking right there!  A long rod is prone to buckling, although there's not as much angle as you'd see on a modern OHV.  Big ends are pretty big at 2-1/2"; lots of frictional loss at elevated RPM's as well.  Rotational force is distributed over more area, but I don't know the strength of the big end.  ARP bolts would help.  

PS - I cuisinarted my first rod at 12.  It was a shock to pull the head off and look at the crank journal.  I remember looking up at the sky, thinking it magically escaped the motor somehow.  It didn't - I found it and the remnants of the piston in the pan.  

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Ken U-Tx said:

squirrel said:

RPM is usually what kills connecting rods (it tears the big end apart), although if you make enough power, you can buckle one.

That's part of the story......piston weight and piston speed in feet per minute is also a very significant factor. The 4.5" stroke 308 has a average piston speed of 3,375 feet per minute whereas the 3.875" stroke 232 has an average piston speed of 2,906 feet per minute. The reason I say "average piston speed" is because the speed of the piston is not constant throughout the stroke, it is rapidly accelerating to even higher feet per minute speeds and then rapidly decelerating to a stop before reversing direction in the reciprocating cycle. The resulting G forces on the piston and rod assembly's mass puts tremendous pressure on the big end and rod bearings that is born by a thin layer of oil. It is therefore easy to see the importance of using as light a piston as can be used without having issues (piston slap from too short a skirt length, etc). Rod bolts can stretch under high loads, and this naturally can result in the bolts failing and then the rod coming loose, usually beating an exit through the cam side of the block. On the 262 and 308, you should be gearing the car to keep the rpms in the peak torque range at the speeds you want to cruise in. Gear them down, not rev them up.

Absolutely right!  I did work with McLaren Engines back in the day, and they went so far as to calculate the weight of the oil film on the journals when balancing an engine for RPM, then calculating the speed/load at which the hydrodynamic pressure of the film was exceeded.  

All my performance engines receive ARP bolts and studs.  I've seen high speed film of rotating elements as part of my dyno work with GM, and it is amazing to see how much components move in operation.

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Ken U-Tx said:

I neglected to state that the average piston speeds stated above were for 4500 rpm. I do know that destructive harmonic resonance sets in at the 4800-5000 rpm band on the stock 308 crank......In real world driving, you should never have to exceed 4000 rpm with a 308. 4000 rpm with a Hydramatic and 3.07 Dana 44 will give about 110 mph.

I'm looking at final drive ratios with a 0.70 top gear ratio, and it looks like a 4.10 is the hot ticket with O/D, as freeway cruising at 70-75 is in the fat part of the TQ curve in the mid 2's.  The car has an O/D in it, an O/D in the trunk and the seller furnished a 200R4 to sweeten the deal.  it's a guess at this point, but the car likely has a 4.55 gear, but I won't know until I get under that part of it - and - I need to get the tires to hold air for more than an hour at a time first!  

The harmonic resonance speed is good to know as well, thanks!