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1997 civic ex
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Discussion Starter · #1 ·
Im a little new to engine building and i'm building a z6 block. I was trying to figure out my compression ratio with everything that i picked out and my static compression ratio is coming out to 7.71:1, and my dynamic is 11.90:1 on 8 pounds of boost. Can someone explain the difference between the static and dynamic ratios? im a little lost.


Thanks in advance.
 

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Static is the ratio of cylinder volume to combustion volume. Dynamic is that same ratio but only counting from when the valves are closed and the piston is actually compressing the mixture. Or at least that is my understanding
 

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93 4d lx, z6,ported, port matched, compression bumped, balanced, manual swap
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Dynamic accounts for the cylinder being already charged with pressure before the piston begins to move upwards, it can also be used to account for valve closing angles in certain applications where the intake valve is open until after the piston starts its upwards travel.
 

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Easiest way I have found for folks learning.

Static, Elastic, Dynamic.

Static is compression measured against a sealed combustion chamber, the ratio of volume from bottom of the stroke to the top of the stroke.

Elastic compression is measured with the camshaft in place, and this is ALWAYS lower than static. The intake valve might be open slightly during initial exhaust to use the exhaust flow to suck through some more air and help prefill the chamber for the next cycle. This is how some VERY well crafted naturally aspirated engine builds can create positive pressure effects. Of course, that also assumes a perfectly matched camshaft, intake, and exhaust setup. Elastic compression can change with the camshaft being timed differently, or if valve lash is adjusted. Hell, some D series engines have come into my hands, claiming the engine is junk and worthless, onlly to find a simply nut on the lash stud was loose, and the cylinder was bleeding off compression and acting like a cracked valve.

Dynamic as I assume (as I am NOT well educated on this) is when artificial volume is added to the chamber. Superchargers, turbo chargers, chemical supercharging like nitrous, whatever brings either more oxygen or higher pressure oxygen into the chamber.

This is my take on those terms over the last 20 years, and I could definitely be completely off the marker. But I am 99% sure on static and elastic.



That all said...

7-8 :1 compression is very low. You are giving up a lot of potential power and engine response, on top of the engine being hard to start when cold, thoguh lower compression means you likely will never over-exert a starter ever again.

With you being new to engine building, it very well might be worth using off the shelf parts. your 7.7:1 ratio seems like it is a vitara based piston, and you are skipping the longer connecting rods to help bring the compression up to a more usable level.

It might be worth finding off the shelf rods and pistons and going with that package. Something like cheap maxpeeding rods and PG6 oem pistons that are sent in to get valve reliefs cut a little deeper, then the tops polished up, and the skirts treated with an anti-friction coating. THe PG6 pistons bring up overall compression, which you can lower by using a thicker Cometic headgasket to drop it down to around 9:1 or so. 9.5:1 is a great goal for compresssion if you are intent on keeping power under 300whp or so. The PG6 pistons are a great mild all motor compression level on any of the 88-00 D16 engines, and the use of a thicker headgasket to allow lowering it to YOUR wants and needs means you are flexable, while still getting a good honda spec'd piston.
 
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