Ok, yeah I misunderstood the first post, my apologies. The parenthetical "not centrifugal" confused me.

Having understood what you're saying, I'm still not sure I agree, but I need to think about it some more...I'm not sure how the centrifugal force of a rotating mass can counteract a linear force; they would only be opposed to each other at a single point in each revolution. Plus, the crank journals are offset from the axis of rotation, so that needs to be counteracted too, and the mass of the large end of the rod is also generating centrifugal force...so at the very least, a portion of what the counterweight it balancing is centrifugal and not linear.

Ok, so after further thought, I'm convinced that the counterweight is NOT counteracting linear inertial forces from the rod and piston. The reason is this: if you were to graph said inertial forces acting on the axis of rotation vs crank angle, you would get a sinusoidal plot. However, if you graph the centrifugal force of the counterweight acting on axis of rotation, you would get a straight line. You cannot cancel out a sinusoidal forcing function with a straight line.

If you tried to cancel the inertial forces with the counterweight, you would only succeed in shifting the load on the bearings to a different crank angle, because while the counterweight may cancel the inertial forces in the very narrow range of crank angles when it is mostly opposed to them, there would be nothing to counter the extra weight of the counterweight for the rest of the revolution, it would end up loading the bearings then.

One JDM/Canada version of the BP-DE actually had a 7250 redline IIRC. Main bearings were never an issue with the cast crank on EGT's and Proteges. Theory vs practice. I have been repeatedly reassured by the BP Protege crowd that the cast crank is good.

My normally aspirated experience in my white '90 is that the cast crank motor is MUCH more lively than my other 1.8 Miatas. You'd have to drive it to believe it. I suppose some of it could be the DE cams but the engine spins up much more freely and it has an eager "dog at the end of a leash" feel to it. It wants to rev. Oddly once it does rev up to 6-7k it doesn't feel like it has as much there, but it acts like it wants to get there faster (to redline). Does that make sense?

I put a MBSP with a NB2 oil pan on the engine before I swapped it into my '90 miata. I figured that couldn't hurt.

BTW, the cast crank isn't just a little lighter. It's a lot lighter. 36lbs vs 26lbs.

With the light crank and FW in my car it revs like a freaking motorcycle.

There's no such thing as a counterweight that perfectly eliminates bending moment. Even though you counter a sinusoidal reciprocating force with a centrifugal force, it still cuts the magnitude of the peaks.

Generally, counterweights can counteract bending moments from both linear acceleration and centrifugal forces. It gets more specific with inline-4 engines, and the relationship of counterweighting to "balance" (if any) is also different for every engine configuration.

What about US cars? Some JDM miata engines had a 7500 Redline, but not the US versions.

Theory vs practice, show me people making big power on cast cranks and spinning them at high RPM. In my experience, I can keep miata main bearings happy to 8500, but at 9000 they don't last. Do the protege cranks do 9000 reliably?

I wouldn't use one of these for big power. Madjak uses a cast crank in his car though.

No oem B series crank is going to buzz along happily at 9000rpms for any length of time.

It's a tradeoff. Wherever peak loading occurs, you'd want to minimize it.

If you seriously need to be at 9k I'd do this...
https://supermiata.com/Billet-Crank-Mazda-BP.aspx

Pat doesn't buy expensive parts, he makes them.

btw, I had a GTJ (Lou Fidanza) built XK 4.2 motor that was built to redline at 8500 and survive missed shifts at 9k. The old Jag had huge long stroke vs bore. We calculated that the piston speed was equivalent to 14k rpms relative to an oversquare motor! :) Oh the noise!

What are the final drive and transmission gear ratios in the Protege you are talking about being so lively?

Not protege, '90 Miata. Stock 5-speed and diff. The engine came from a '97/'98 Protege BP-DE AT.

FWIW that part number comes back NLA with some quick googling