Nos681 wrote:
Are there potential issues with a crankshaft when changing firing order?
Is it similar to valve springs with a larger camshaft?
Would the crankshaft be “set” in its previous firing order?
Basically you are wondering if the stresses the crank had previously endured would create a fatigue life related failure when switching the firing order. That is, materials have a yield strength and a fatigue life, both of which can cause failures. The yield strength is the amount of force required to create permanent deformation of the material, indicating a failure on the molecular level. The part has failed, and has now become a serious liability, if its not outright broken. For obvious reasons under normal circumstances you would never put a part into service that comes even close to its yield strength based on loading in normal operation. The issue therefore becomes fatigue life, which is the amount of loading below the yield strength it takes for a part to fail given enough cycles of repetition. Imagine you drive forward and backward over the same path through an unpaved field. Eventually you start to create ruts. At some point you create ruts so deep that you probably could drive the route without having to even use the steering wheel to keep on track. The molecules in a metal do this exact same thing when the metal is loaded the same way over and over again. Eventually the bonds and lattice structure start to weaken from this repeated back and forth (not having to use the wheel is a bad thing in this example). At some point there will be a catastrophic failure of the lattice at a load much lower than the yield strength due to the weakening that's occurred over many, many cycles of loading. In an engine that's a loading cycle probably measured in billions or even trillions of cycles, but considering how often the loading happens in a running engine (once per revolution) it starts to be concern. This is why NASCAR teams assign a life value to every part in the engine and change out those parts before they get to the point where they've had fatigue life related failures with those parts in the past.
That's the theory, but in the real world, If switching between the 5.0 order and the 289/320 order I would say a fatigue life related failure is unlikely. The crank is already undergoing the same stresses in terms of cylinder firing; its just that you are swapping two pairs of cylinders in that order. So the question becomes whether or not there's some fatigue life related weakness from shear stress from the specific order, but honestly, since you are changing the loading if anything you've probably at least marginally increased the part's fatigue life because you are now in essence charting a new path through the field. In a typical street engine I certainly wouldn't worry about it. In a high performance engine likely you aren't using stock crank anyway, so its also likely a non factor. If you're planning to use a stock, high mileage crank in a performance build and are worried about failure I'd say my concern would stem primarily from what the crank is made from and its previous life rather than concerns about the firing order swap. The problem there is an issue of trying to use low performance parts in a high performance engine, which is simply a bad idea from the drive.
Upon further review something else dawned on me. If you look at the 5.0 firing order it still has two cylinders on the same rod journal firing one right after the other; its just that instead of 1&5 its become 4&8. Interestingly 4&8 are on the journal closest to the thrust bearing and the rear bulkhead of the engine. Apparently that was preferable to the location of that event in the old order, and it is interesting that in the SBC firing order the two pairs of cylinders that fire one right after each other are on either side of the center main journal, which is also the location of the thrust bearing in the SBC. I don't think this is coincidence.
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