Simple compression question - Team Camaro Tech
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post #1 of 6 (permalink) Old Aug 28th, 03, 05:17 AM Thread Starter
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Something I never really thought I needed to know, but maybe I should.

Say an engine has a compression ratio of 10:1. What does that ratio stand for? What is 10, and what is 1?

Cylinder size/cubic inch of a given cylinder to combustion area size? Just my guess.

?
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post #2 of 6 (permalink) Old Aug 28th, 03, 08:01 AM
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Josh
 
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Volume of combustion chamber, head gasket and cylinder above the piston at BDC compared to that at TDC.

J Early
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post #3 of 6 (permalink) Old Aug 29th, 03, 04:03 AM Thread Starter
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Not following.

So we know if we decrease the size of the combustion chamber, and cylinder volume stays the same, compression will go up.

But that ratio of the volume above BDC and TDC remains constant. Right?

So how does this change the compression ratio with that method?
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post #4 of 6 (permalink) Old Aug 29th, 03, 07:03 AM
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The area above the piston is smaller so you are stuffing (compressing) the same volume into a smaller space. This is why you are raising the compression ratio.

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post #5 of 6 (permalink) Old Aug 29th, 03, 07:07 AM
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Al
 
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For the sake of simplicity, let's work with a cylinder that displaces 9 cubic inches, and a combustion chamber that displaces 1 cubic inch. When the piston is at BDC, the total of the cylinder and the chamber is 10 ci. When the piston is at TDC, the total volume remaining is 1 ci. 10 divided by 1 = 10, hence a 10:1 comp ratio.

Now, let's reduce the volume of the combustion chamber by 50% with a real aggressive milling job on the head. Now, at bdc, we still have 9 ci in the cylinder, but only .5 ci in the chamber. With the piston at BDC, our total is 9.5 ci. With the piston at TDC, our total is .5 ci. 9.5 divided by .5 = 19, and a 19:1 comp ratio.

If we go the other way, and put on a new set of heads with 2 ci chambers, we have a total at BDC of 11 ci, and 2ci at TDC. 11 divided by 2 = a CR of 5.5:1.


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post #6 of 6 (permalink) Old Aug 29th, 03, 08:34 AM Thread Starter
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Thanks BPOS. [img]graemlins/beers.gif[/img]
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