Compression ratio and displacement
 

Okay guys, I'm new to the forums and I have a couple of noob questions I guess you could say. I've done lots of research on the 2(displacement and CR) and need help.

So we have a 3.7L engine (370z guys/gals) and our compression ratio is 11:1 which means at BDC(bottoms dead center) to TDC( top dead center) the piston will compress 1100cc units of air/fuel mixture into 100cc. So if it can take in 1100cc of air/fuel into the chamber wouldn't that mean the displacement of 1 cylinder would equal 1.1 liters (1100cc to liters conversion is 1.1L)
So if 1 cylinder equals 1.1L multiplied by 6 (6 cylinders) that would be well over a 3.7L.

Someone help!!!


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The compression ratio is correct. But why are you assuming that one cylinder can hold 1.1 liters? A cylinder starts with 3.7L/6 or about 0.617L. That gets compressed down to 0.056L for a compression ratio of 11:1.

I was under the impression that since the CR of our car is 11:1 that it would take 1100cc of air/fuel and compress it in to 100cc of air. 1100cc=1.1liter per cylinder.
Now clearly I know I'm wrong but I'm not sure why.


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No matter what the displacement, you can still have 11:1 CR.

Displacement is used to compute CR (initial volume divided by compressed volume). Using redmenace's numbers, the compression ratio is 0.617L:0.056L. Another way of stating that is 11:1 (divide both sides by 0.056).

Oh okay I see. That answers my question. Thank you guys !


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Ok so I've thought about it any one more question has emerged for me.

I know you stated no matter the displacement you can still have a cr of 11:1. So here's my question if I swapped out my Pistons to 9:1 wouldn't that decrease the total initial volume a cylinder can take in? Thus making it a smaller displacement.


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I thought displacement was Bore X Stroke? Am I slow today? Where does CR factor into the calculation of displacement?

Well from my understand CR is affected by displacement do if the CR is affected( such as different CR Pistons) so will the displacement ? I'm confused so some of my information may be wrong.


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CR has nothing to do with engine displacement.

Your displacement is the difference in area displaced by the piston from TDC to BDC multiplied by the number of cylinders, and thats it.

CR is is the ratio between that displacement plus the combustion chamber above it versus just the combustion chamber itself. The combustion chamber is often just measured in the cylinder head itself with the valves shut but also technically includes the bore x thickness of the headgasket and plus or minus how much the piston sits above or below the deck at TDC.

You can also consider dynamic compression ratio, such as how any fuel that makes it into the chamber in liquid form cannot be compressed therefore increases compression ratio while it is in liquid form. But 99.99% of the time anyone is talking about CR, they are talking about static compression ratio.

Okay so since the displacement will never change does that mean in order for a 370z motor to have a CR of 9:1 it would equal 0.617L:0.068L ?


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Whos good at math? Tell me if this is right. I feel like math should tell us combustion chamber size if we assume the compression ratio is exactly 11:1. Each cylinder is 616cc displacement to get our 3696cc total displacement.

(616 + X) / X = 11
X should be our combustion chamber size.

616 / 10 = X

(616 + 61.6) / 61.6 = 11

So when the piston is at BDC we have 616cc cylinder displacement and the combustion chamber is 61.6cc for a total of 677.6cc area. Compressed 11:1 would leave the 61.6cc chamber.



So to solve for your 9:1 ratio...
(616 + X) / X = 9
616 / 8 = 77cc
(616 + 77) / 77 = 9

So you need to make the combustion chamber 15.4cc larger without changing the bore or stroke. To do this, you would dish the surface of the piston a total displacement of 15.4cc.

Ami right?

EDIT: these equations would include the headgasket area and piston dome/relief in the combustion chamber volume, and provide final chamber size, not just the measured CCs in the cylinder heads.

That's sounds right to me. I know that compression ratio is affected by piston size and that makes sense to me. Anyone else have anything to add?


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This isn't the best link for a detailed explanation but I think you are correct on the general idea and the maths as well.

RK Tek 2-Stroke Heads: Piston Deck Height and its Importance

I know one trick for reducing compression to try and prepare a motor for boost (not an advisable one, mind you) was to use a thicker head gasket to add a little more deck height.

My calculations above are including the headgasket into the combustion chamber displacement... which I believe is correct in theory but not correct in practice. The headgasket at 1mm thickness would be adding about 7-8cc to the combustion chamber numbers I came up with.

To add .5mm thicker headgasket would be like 3-4cc extra chamber volume.

(616 + 65) / 65 = 10.47

So that is a pretty significant reduction in compression ratio.