sailaholic wrote:The sump is different to suit the girdle. from memory all nbc have the right sump. Early sumps can be modified to suit. Details, along with factory and upspeced bearing support plates can be found on the miataroadster website.
Can someone please explain how with the same block height a different stroke is achieved? I'm imagining that the piston is pulled further down into the block at the bottom of the stroke? Hence more bending load on the rods?
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The piston travels further up and down. The piston comp height is made shorter so the pistons don't poke out the top, and they travel further down the bore.
Put simply, engines with larger stroke are harder on rods and pistons because the accelleration is higher (piston travels further in the same amount of time) and the inertia is greater (being thrown out further) and the angles are greater (when the crank is at 90deg rotation the big end sits out further) so the rod and piston is stressed more from the great angles.
As stated earlier, the B6 rod is dimentionally the same as the BP, so in a BP the rod is stressed further than a B6. It's not an ideal way of doing it, but it was the way Mazda decided, and in standard form it doesn't cause issues, in fact even in modified form its not the end of the world.
There are a number of different theories on rod ratios etc. I find a rod ratio of about 1.6-1.8 works best on these style of engines.
A standard BP has a rod ratio of 1.5657 (133mm long rod / 85mm stroke = 1.5657)
A standard B6 has a rod ratio of 1.5909 (133mm long rod / 83.6mm stroke = 1.5909)
The B6 is closer to where it needs to be than the BP, hence less rod dramas.