Does AFM bluetop have lower comp pistons than the MAP bltp?

[firehawk]

Quote:I didn't bother with calculations but I would guess that 1mm brings more that 0.3 difference in CR.

Hmm, let's check that out. The swept volume of one cylinder is 397cm^3, so on an engine with a known 10.3:1 static CR, the effective combustion chamber volume is 397/10.3 = 38.5cm^3. For 10.0:1 CR on the same engine, the volume would have to be 397/10.0 = 39.7cm^3, giving a 1.2cm^3 difference.

If we spread that 1.2cm^3 into a slice 1mm thick, the slice's area should be 12cm^2 - like, say, 2cm x 6cm. Pretty close, but actually feels a bit smaller that what it really is. I'd estimate the dome's top to be something like 7cm x 3.5cm, in which case the difference in volume would be about 2.5cm^3 and CR would be 9.7:1.

For comparison, with 9.4:1 CR the effective combustion chamber volume should be 42.2cm^3 and thus the difference in dome volume 3.8cm^3. Assuming the same area of 7cm x 3.5cm, the height difference to a 10.3:1 CR piston should be 1.6mm, meaning that it would actually form a small dish 0.1mm below the piston's crown. The dish does look a bit deeper than that on the photos, though..

Anyway, I'm not really sure if I got my calculations right so it might actually be a good idea if someone could double-check these in case I made a blunder somewhere.