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Discussion Starter #1
I would think that crank throw would be the only thing that would change compression?

I know I'm wrong somehow since so many different compression ratio pistons are sold, but wouldn't changing pistons and rods only change the area that was swept, i.e. higher piston would more completely clear the chamber, but would still be higher at the bottom of the intake stroke so it wouldn't pull in any more air to compress harder. Whereas lowering the piston would clear less air out on the exhaust stroke and still have the same throw to bring the same amount of new air in on the intake stroke (exempting what is left over after the exhaust stroke). So again the compression should be the same right?

Please someone explain the error in my thought process.
 

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compression formula

to find compression ratio you must take V1+V2+V3+V4 and divide by V2+V3+V4.

V1= bore X bore X stroke X.7854
V2= bore X bore X compression height of the piston X .7854
V3= bore X bore X head gasket compressed height X .7854
V4= chamber volume - piston dome volume


since V4 will give you an answer in CC instead of CI, you must multiply V4 by .064 to get CI.


I hope this helps you some.
 

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Discussion Starter #4
Thanks, but I understand how engines work, just was confused by how the same throw of a crank would compress the air more or less with different pistons.

I figured it out though: No matter what piston(or rod) you put in it will push and pull the same volume of air, lets say 1 liter, but your full cylinder air capacity will be slightly more than that liter. Raising the piston leaves less room to compress the air therefore higher compression-- there's actually less cylinder volume, but the swept volume is static so this is not a disadvantage. Lowering the piston leaves more room to compress a slightly higher volume, but that same 1 liter is all that's swept.

Example:
Raised piston
1.1 liter cylinder capacity with 1 liter throw=11 to 1 compression

Lowered piston
1.2 liter cylinder capacity with 1 liter throw= 6 to 1 compression

full capacity / (full capacity - swept volume)=compression

Pretty simple now that I think it through.

I also now realize its just what you said BF$ :p
 

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you gotta look at it this way,

at the TDC, you have the minimum air volume, but it is not completely 0, say this is 10cc and the whole displacement from BDC is 80cc, it would be 8:1 CR, but if you make it so that TDC volume is
8cc compared to 10cc, it would make it 10:1 CR,

Make sense?

I hope you understand.
 

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Discussion Starter #6
Yea, thanks, I understand now. I feel silly for not getting it before I asked. So easy now that I do understand :D
 

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no, that's normal I think.

I thought that the same way you did, so.

Now, let's get our 8.5CR forged Ross pistons!!
 

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Why do you guys make things so complicated? :)

The compression ratio is simply the ratio of the volume in the cylinder when the piston is at BDC to the volume when the piston is at TDC. The *difference* in those two volumes is the swept (or displacement) volume. The volume when the piston is at TDC is the combustion chamber volume (to simplify). If you increase the displacement by overboring, then CR goes up. You can try moving the piston up or down relative to the rod pin but there really is no room to do this.

For the turbo engine:

Displacement or Swept Volume:

Stock Bore = 91.1 mm; 3.587 in.
Stock Stroke = 76 mm; 2.992 in.
Stock Displacement
= (Bore/2)^2 x PI x Stroke

= 3.587/2 x 3.587/2 x 3.14159 x 2.992
= 30.235 CI (Total = 181.41 CI)
- or -
= 91.1/2 x 91.1/2 x 3.14159 x 76
= 0.495382 L (Total = 2.9723 L)

Stock Compression Ratio = 8.0: 1
Total Volume equals swept volume (SV) plus combustion chamber volume (CCV) which together are the 8 parts (in the 8 to 1 ratio). At TDC, CCV represents 1 part (in the 8 to 1 ratio). So SV equals 7 parts.

Stock Combustion Chamber Volume = 30.235/7 = 4.319 CI (or 0.070769 L)


Example for my engine with a 0.050" overbore.

Rebuild bore = 92.38 mm; 3.637 in. (was 0.050 in. over stock)
Rebuild Displacement
= 31.084 CI (Total = 186.5 CI)
= 0.50940 L (Total = 3.0564 L)

Rebuild Compression Ratio = (SV + CCV) / CCV
= (31.084 + 4.319) / 4.319 = 8.197:1 = 8.2:1
 

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Hey, Jeff.

Thank you for your calculation. hehehe, very simple, isn't it?

I was just at the Porsche Dealer, and was looking at the New GT-2.

It has 460ps, and it has ....9.4:1 CR!!!!

It also has 3.6 liter engine, and they said its internal could handle untill it reaches 640ps or so. only with boost up.

It sounded really good, and the response is just like (or maybe faster) than that of High CR NAs, ex. honda prelude's H22,... man, it spooled up so quick, (well, it sounded like the engine had absolutely no lag....)

So, is it possible to bring our engine CR to 9 or so?, or do we have to do more to the engine?
 

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Jeff, I like your equation, but it doesn't take deck clearace into consideration. You said you could try to move the piston up or down in the bore using different wrist pin heights, and I really can't agree with that. You absolutly need at least .035" deck clearace with steel rods (.045" aluminum). The only time this should be changed is when you change rod length, stroke of the engine, or decking of the block. Changing the compression height of the piston (moving the wrist pin up or down) affects V2. Any increase in V1 or decrease in V2, V3, or V4 will increase compression. Since V2 is unchangeable, your left with increasing bore size, increasing stroke, using a thinner head gasket, or finding heads with a smaller combustion chamber (possibly mill some metal off the bottem of the heads like I did with my 2.2L, although that might not be possible with our engines).
 

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OH HOW MY BRAIN HURTS.......after reading that!:D:D All that stuff is like learning a new language with me. I can change the oil and that's about it. ;)

Great info guys!;) Now I know if I even need any "engine guru's" I can get some help from you guys when/if I ever decide to start exploring my engine. That will be the day, huh!:D
 

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BF$ said:
Jeff, I like your equation, but it doesn't take deck clearace into consideration. You said you could try to move the piston up or down in the bore using different wrist pin heights, and I really can't agree with that
I agree and do NOT suggest a person try that. Just saying it was possible. :) Knowing deck clearance is not needed for the simple equations above. Many folks mill hundredths off their heads without problems. I had a few thousands taken off mine to flatten it out. Milling the allowed amount does not apprecialbly affect compression.
 

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You can try moving the piston up or down relative to the rod pin but there really is no room to do this
A few people might've got the wrong impression from what you said. At least it is cleared up for most people. Sorry TFXP-Zeke01
didn't mean to fry anyones brain. I must also agree with you that milling a few thousanths off the head will not change compression that much. But some cars can really benefit from it. Mopar Performace sells shorter timing belts for 2.2L engines. I can't remeber how much I personaly milled off mine, but I do remember that vavle to piston clearance was dangerously close and the even the shorter belt seemed way too long (oops).
 
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