Why is the b30 the highest cR?

[HerbE30]

Just thinking of motor swaps, frankenbuilds, straight swaps, FI. Anywho, started looking into the b30 and saw that with the second highest displacement, with the lowest cR?

The 2.2 is at 10.8
The 2.5 is at 10.5
The 3.0 is at 10.2
The 3.2 is at 11.5


Can someone education this hillbilly a little?

[nando]

if you increase displacement but keep the same compression volume (head chamber) will increase compression ratio if the pistons & squish zone are the same shape.

basically, you're compressing a larger cylinder volume into approximately the same amount of space.

the 3.2 you're talking about I assume is the S54, that has a higher compression ratio because it was designed that way and isn't really relevant. The other two have a higher ratio than the 3.0 so I'm not really sure what you are looking at.

The compression ratio being tied to displacement really isn't applicable if it's not the same engine. It does apply if you for example took an M20 and built a stroker using stock pistons and shorter rods, assuming the compression volume stays the same.

[terraphantm]

if you increase displacement but keep the same compression volume (head chamber) will increase compression ratio if the pistons & squish zone are the same shape.

basically, you're compressing a larger cylinder volume into approximately the same amount of space.

the 3.2 you're talking about I assume is the S54, that has a higher compression ratio because it was designed that way and isn't really relevant. The other two have a higher ratio than the 3.0 so I'm not really sure what you are looking at.

The compression ratio being tied to displacement really isn't applicable if it's not the same engine. It does apply if you for example took an M20 and built a stroker using stock pistons and shorter rods, assuming the compression volume stays the same.

Unless I'm making a stupid mistake in the math, if you increase displacement while keeping the same compression volume (volume @ TDC), compression ratio will increase.

Essentially the limit of (x + c) / c as x approaches infinity is infinity if c is constant (x being displacement, c being the head chamber, and x + c being displacement + head chamber).

However, in the case of the M54b30 vs M54b25, the rods were apparently are short enough to increase the volume of the combustion chamber, and thus reduce compression.

Edit: Actually I misread you. We agree.

[nando]

Hi terra. :p

yeah, that's what I was trying to say. lol.

[HerbE30]

This makes a lot more sense now. I see the error of my ways. haha.

The more you know!

Thanks again guys. I feel dumb now!

[nando]

nah, you should feel smart now - most people don't know how that relationship works. :)

[digger]

i'm more interested in why BMW decided to have the piston so far below the deck on the b30 as its down stupid amount like 2 or 3 mm iirc. why not have the piston come to the proper deck position and have a slight dish to control the CR

[LateFan]

Essentially the limit of (x + c) / c as x approaches infinity is infinity if c is constant (x being displacement, c being the head chamber, and x + c being displacement + head chamber).

Nice! I forwarded this to my son, who is majoring in Physics and learning to restore an E30.