I was just thinking about it, I had my motor (sohc d16z6) bored out to about 1.8L and the pistons were a very tight fit. now, since i have a larger compression chamber, and if the compression on the pistons were the same as the stock ones, would that not mean i have a lower compression? i think the stock cr on my motor is 9.2.... so by adding another 200cc what would my compression be now?

 

Your compression is still stock. Because you needed new pistons and you asked for the stock compression ratio they comensated by making the pistons with a higher compression ratio.

 

heh, actually i didnt ask for ****... i didnt even know they did it to my car. my parents own a shop and when they took my motor in to get rebuilt the mechanics thought i would like that treat. i saw the pistons and they were huge... but i dont know the compression on them. i think its about time i do a compression test. does anyone know the formula for the psi to compression thingie? like if it reads 185 on all cylinders what does that translate to?

 

Doesn't translate to squat.

You need to actually measure the pistons and the combustion chamber.

 

I would think that you'd have a slightly higher compression.

The stroke remains the same, which means that your TDC combustion chamber volume to BDC volume ratio remains the same. This implies the same compression. However, there's still the volume due to the dome under the head that hasn't been bored out. That remains the same, but relative to the swept volume it is now slightly lower.

Therefore: slightly higher compression. How much? I don't know.

If it wasn't for that volume in the head, then your compression ratio should remain the same. You combustion chamber volume does increase, but in proportion to the swept volume.


[Modified by Lsos, 4:57 PM 4/3/2002]

 

so confusing, but what about some people i see who get 140 across all four cylinders while most people get 185 or sometimes 210 all across? your saying theres no difference between 140 and 210? if its lower compression wouldnt i be able to yield a higher number?

 

Umm...I don't really know what you are talking about, I just ran a couple volume formulas on a piece of paper.....

I'd imagine that higher compression would yield a higher number. I don't really know what this has to do with what I said, though.

I guess I'm kind of confused.


[Modified by Lsos, 10:44 PM 4/3/2002]

 

Actually, boring an engine slightly raises compression. The reason for this is that you now have moe quench surface on the outer periphery of the combustion chamber due to the fact that you have more exposed head surface.

 

while the psi ratings do vary from compression ratio to compression ratio, it isnt an acurate way to find out how much is being compressed.

the compression ratio is the volume of the cylinder w/ combustion chamber at BDC vs. the volume of the cylinder w/ combustion chamber at TDC.

the compression readings you see(145, 185, etc.) are measurements (in pounds per square inch) to give you an idea of the sealing abilities of your engine (i.e. valve seals, piston rings, head gasket, etc.). these can vary a great deal, depending on the condition of the engine.