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John Deere Tractors Discussion Forum

Raising Compression

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BUDD

12-14-2003 05:26:21

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Can anyone tell me how much I need to take off the head or block to gain 30 lbs compression on a 5 5/8" bore with a 6 3/4" stroke? I need to clean the block up and I would like to gain this much compression in doing this.

Thanks for all input BUDD ('';'')

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gschilling

12-16-2003 07:01:42

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Re: Raising Compression in reply to BUDD, 12-14-2003 05:26:21

Hi Budd - Let me start like this: Compression ratio is nothing but a free volume ratio between the cylinder volume at BDC to that at TDC (this includes the head). It is NOT a ratio of pressures as is often believed. This is because the volumetric effeciency of any given engine is never 100% (with volumetric efficiency being the ratio between the volume of air at atmospheric pressure and temperature drawn into the cylinder during the induction stroke to the volume actually swept by the piston). From the changes that exist in volumetric efficiency from year to year, and engine to engine, it is easy to see that at the same compression ratio, the compression pressure will be much lower in an engine with lower volumetric efficiency than one with a high VE. For our tractors, I would guess that the volumetric efficiency ranges from 70% on upward.

With that said, lets greatly simplify things for a moment. The amount to shorten your bore to achieve a 30 psi increase will depend on what your existing compression ratio is. For sake of an example, I will use 4.71:1 which is what one of my tractors is. If you want to increase the compression by approx. 30 psi, you will need to shorten the block by approx. .4 inches (with your bore and stroke). If your compression ratio is not 4.71:1, but say 7.0:1, then the amount you will need to shorten the block by is closer to .2 inches (assuming the same bore and stroke). These numbers are nothing but very rough and dirty ballpark numbers generated to give you an idea of the magnitude of the change that is needed. You also have to be sure that whatever change you make can be physically accomodated by what is there to begin with. If you would like to pursue this further, it is OK to send an e-mail.

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