318 Connecting Rods and Crankshaft

nm9stheham said
Look into the rod materials for Eagle vs SCAT, OP for the I-beams. (I have not compared the H beams.) The Eagles are a 5140 material and the SCAT's are a 4340.. stronger. Your app is not stressing the rods so maybe no big deal. For total weight, they are nominally the same: 605 gr for the Eagle I-beams vs 602 gr nominal for the SCAT's. I have been able to call Summit and ask for the lightest set of SCAT's they had on the shelf and got some that were 30 grams lighter than nominal to help with a challenging balance setup. Which leads into balancing.....

The non-standard combination is gonna require balance work on the crank. With a 3.58 stroke, lighter rods are going to put you a long way towards being able to balance internally without adding weight to the crank, just removing it. (Adding weights drives up the balance price.) So the aftermarket rods and the 3.58 stroke are a good thing here. Going with lighter hyper pistons like the KB's plus light rods will almost guarantee being able to balance a 3.58 stroke crank internally by just drilling out weight... you just pushed the 'easy' button LOL. A 4" stroke makes it very hard to impossible to balance internally without adding weight to the crank.

Not a show stopper in any case, but with a 4" stroke, the 318 block may need a bit of notching by the pan rails; I'll venture to guess that the 3.58 stroke setup with SCAT rods will easily clear the pan rails as cast with no issues. For either stroke, the SCAT rods will help with that clearance versus stock rods, by 1/8" or more from direct measurements. I have not measured Eagles for that.

Interesting... I was looking for that example..... a small fraction of a % difference in the rod/stroke ratio of a SBM with 4" stroke 1.534 versus 1.531....I don't think that 454 bores wear out particularly fast. 1.50 is considered the lower limit of 'normal' engine design for rod-stroke ratio. FWIW, with the OP's lower RPM range of 5000 RPM max and interstate cruise RPM of 2500, it seems like a good match. But that is probably academic at this point.

Thanks for the information. I had looked at both Eagle and Scat and have been leaning toward the Scats just for the material. Honestly I had not paid attention to weight.
Balancing: Unless I bought a balanced rotating assembly form someone, which as far as I have found is limited to a 4" stroke - 390, I was going to have to balance anyway. An expense already accounted for in the budget.
Clearance at the pan rails is a concern, but not so much with a 3.58 stroke. I do not think anything will have to be done there. But mockup and measuring will tell the tale.