Stupid, stupid SP1300 ideas

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Re: Stupid, stupid SP1300 ideas

Post by Marc » Wed May 31, 2017 9:58 pm

I don't know that I'd attribute it to a "harmonic", it's rather simpler than that. The two crankshaft throws on either side of the center main bearing are side-by-side, so there's more mass where the rods attach than there is 180° across - plus that of the rods & pistons attached to them.
Think of two little girls swinging a jump-rope between them - that's pretty much what the crank is trying to do, and the center main bearing housed in the magnesium-alloy block isn't stiff enough to control it when the force reaches a certain point.
Adding counterweights across from the rod throws helps by reducing how much the crank "whips" - the additional mass provides a counterbalancing effect which reduces the extreme load on the center main.

Having the two crankshaft "throws" on either side of the center main is not a characteristic unique to the boxer engine design, inline fours have the same layout. But nearly all of them have a main bearing supporting the crankshaft between each rod throw - and a stiffer engine block as well. All VW waterpumper cranks with a stroke of 80mm or more are fully counterweighted. On the smaller engines such as the 1457 they get away with a non-CW cast crank due to the five main bearings, but those are an anomaly (I can attest that they're still able to run 8000+ RPM with a proper balance job). Here's an 86.4mm 1588cc Diesel crank (essentially the same as the one used in the 1715cc & 1780cc gas engines) ... ft.jpg&f=1

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Re: Stupid, stupid SP1300 ideas

Post by Piledriver » Thu Jun 01, 2017 2:33 am

So will a WBX or T4 crank without counterweights.... The T4 benefits greatly from larger mains tho.
Its too bad going "all T4 mains" in a T1 or WBX requires a std uncut case.
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Re: Stupid, stupid SP1300 ideas

Post by petew » Thu Jun 01, 2017 3:32 am

Like farmer said, a radically lightened piston/rod will do the same job. Which is what JPM did with his 1603cc motor. Hmmm, interesting. :)

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