# compression ratio

Discussion in 'General Tech and Maintenance' started by 22crazy, Dec 3, 2003.

1. ### 22crazySenior Member

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whats up? I was just wondering how the compression ratio of a motor is calculated manually? What are the formulas? What imformation is required? thanx.

2. ### pissedoffsolRETIRED

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Calculations are based on the engine's bore, stroke, cylinder head chamber volume, piston design, deck clearance, and head gasket properties.

// calculate variables

swept_volume = ((bore / 2) * (bore / 2)) * 3.1415926535 * stroke / 1000 * 4;
piston_dome_displacement = piston_dome_displacement * 4;
combustion_chamber_volume = combustion_chamber_volume * 4;

total_CCV = combustion_chamber_volume * 4;
total_BDC = ((bore / 2) * (bore / 2)) * 3.1415926535 * stroke * 4;

displacement = (total_CCV + total_BDC + total_HGV) / 1000;

// calculate compression ratio

compression_ratio = tdc_volume / bdc_volume;

hows that?

3. ### ryanwolfe911Senior Member

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I'm lost. lol I must have been sleeping that day in school

4. ### 22crazySenior Member

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thanks. I was just curious to know if anyone knows all the variables listed above for a jdm h22? I think the motor is from a 1993.

Prehaps i should also tell you what this is for. A friend of mine has a 93 jdm h22 in a 1997 civic DX coupe and plans to boost the shit out of it. the idea is what a friend ours did; he has a 3.1L VR6 turbo running about 450~550 hp. What he did was basicially put a thicker headgasket to lower the compression ratio by a relatively large amount. The reason i asked for the calculations, is that i just wanted to figure out how thick of a head gasket i would need to lower the compression to about 9:1 on the h22. Any thoughts about this setup?

5. ### pissedoffsolRETIRED

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I so don't understand this at all.

if hes out to make serious power, hes going to need a stand-alone anyway.

why lower the static compression ratio, and feed it more boost to equal the same effective compression ratio as stock and less boost?
10:1 is PERFECT for what he wants. there is no need to drop it down to 9:1. all he needs is good 93-94 octane gas, and a good tune.

moreover, you're friend is going to blow up. a thick headgasket on a stock h22 with any serious power is going to go boom. the sleeves are weak by design.

on top of all that, a headgasket that drops a motor a Full point in compression will be liek an inch thick.

6. ### 22crazySenior Member

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Thats cool, well im just going on what i hear and what my friend and his buddies said they were gonna do. I know ur a smart guy pissedoffsol and im not trying to insult ur intelligence, and i also appreciate u informing me, but ive heard of alot of people putting custom/thicker head gaskets to run hi-boost and not having to modify there internals (ie. pistons and sleeves). Im also assuming that these custom head gaskets are probably made out of strong materials so that they dont blow. Also arent the jdm h22 closed deck? wouldnt this help out with handling a higher pressure?

7. ### Citizen_InsaneSenior Member

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Does this mean I can run boost on a stock b16? <is hopeful>

-Kai

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