Big Blocks: What breaks first?

[03Sssnake]

Setup usually determines the failure. But I’d say the first thing to go will be top end lifters/rocker arms, cam.

I know on my buddies drag week cars they throw new lifters in before they leave and bring an extra set as well as they are the first to go. They will also put a new cam in depending on mileage, unfortunately a broken cam ruined his drag week last year.

Cheap bottom end parts will go first. Eagle crank - Chinese garbage didn’t like 1000hp.
View attachment 1699957

Big cam, a lot of RPM. Shit will eventually give. View attachment 1699958

great points, spend the money on American made parts! I definitely agree on top end, shaft mount rockers, link bar lifters etc...

 

[James Snover]

Probably, but the question is why? Is it rod failure due to stretch? Is the inertia of the the piston/rod combo too much for the oil film to keep the assembly from touching the bearing and spinning it causing bearing failure? Is the wrist pin boss on the piston too weak to control all that mass on the intake stroke? Lets assume no boost now, NA only.

Example: In the case of the Cheby 454, its got a 4.0" stroke. For comparison, the Boss 302 has a 3.65" stroke and can be revved out to 8k. A little over a 1/4" more stroke and the 454 shouldnt be revved past 6500 or so? Is it stroke at all thats the limiting factor?

Lets turn our attention to the size of the piston. There's obviously a significant difference between the 3.63" of the 302 versus the 4.25" of the 454. The .62" larger diameter of the 454 certainly adds more mass to each piston. Does this double or triple the forces on the rod/rod bearing/wrist pin compared to the 302? Or is is a harsher more exponential like increase in force?

In this theoretical exercise, the pistons are forged aluminum, the rods are forged steel, the crank is forged, the wrist pins are tool steel, all rod and head bolts are ARP2000 material, the ring gaps have been set for the appropriate level of gap, the valve springs are race quality capable of controlling the large mass of Big Block valves (Ford or Chevy or Hemi) and the fuel is correctly metered E85 or race fuel which eliminates pre-ignition or detonation.

What you got SVTP? What fails and why.

Hmmm... @James Snover what say you good sir?

The stresses on a moving part rise with the square-cube of the mass and size of the component. In real life, that means your 3.65 rod which is happy at 8500rom, then add that 1/4 inch, and now, just from that 1/4 inch, the stresses at 8500 turn it into a grenade. Add in larger pistons, larger wrist pins, larger bearings ... and when you scale an engine up to Diesel Locomotive V16, it's max redline is 800rpm. 801, and it's junk. There is no way around the square-cube law, it's a vicious, cold-hearted MF'er that always wins.

Also keep in mind the speed of the rod and piston are changing constantly throughout the stroke. As the piston approaches TDC or BDC, it is decelerated. As the piston nears the mid-point of the stroke, it is being accelerated. So at 8,500 RPM, the rods, wrist pins, oil films, bearing, have to handle two dead stops and two maximum accelerations. The only thing that makes it possible is the geometry of the rod to the crankshaft providing a smooth transition between dead stop and max acceleration.

 

[Silverstrike]

Yes on all the valve train stuff going first, all you have to do is look at all the old Nascar races real early 60's to late 70's and it always seemed to be a valve, rocker or push rod that bought the farm first if it involved the engine quitting. The block, pistons, and bottom end was pretty much bullet proof.