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Hi everyone. Hopefully this should be a relatively simple question here.

I'm running a compression test on my car and I am using a Bentley Manual (very good mechanical Bibles car people use that depict how to take apart the entire car) as a reference for the PSI I should be shooting for.

A compression test basically works by plugging in a special gauge into each cylinder spark plug hole, while cranking the engine in order to get a pressure reading. This pressure reading is used to assess the condition of the engine internals.

Assume that temperature and altitude are constant, while rubber hose expansion is nonexistent.

My concern is that the Bentley Manual doesn't state the length/diameter of the extension hose that they used, and I know that this affects the readings. Obviously the longer the hose, the more air, the lower the readings.

I was debating between two extensions, a 28 cm and 15 cm. After doing some math though, it seems that these differences should be quite insignificant. Here is my math, with a brief explanation below:

http://img64.imageshack.us/img64/4543/ecui.png [Broken]

Basically I calculated the volume of each of the two extensions, and then calculated that as a percentage of the total cylinder volume. The delta between these turned out to be 0.18%, a very small amount. Then I just used a value of a 150 PSI compression reading and found I should be expecting a difference of only ~0.3 PSI due to the length differences.

Does this sound about right? Any help is greatly appreciated. Thanks.

I'm running a compression test on my car and I am using a Bentley Manual (very good mechanical Bibles car people use that depict how to take apart the entire car) as a reference for the PSI I should be shooting for.

A compression test basically works by plugging in a special gauge into each cylinder spark plug hole, while cranking the engine in order to get a pressure reading. This pressure reading is used to assess the condition of the engine internals.

Assume that temperature and altitude are constant, while rubber hose expansion is nonexistent.

My concern is that the Bentley Manual doesn't state the length/diameter of the extension hose that they used, and I know that this affects the readings. Obviously the longer the hose, the more air, the lower the readings.

I was debating between two extensions, a 28 cm and 15 cm. After doing some math though, it seems that these differences should be quite insignificant. Here is my math, with a brief explanation below:

http://img64.imageshack.us/img64/4543/ecui.png [Broken]

Basically I calculated the volume of each of the two extensions, and then calculated that as a percentage of the total cylinder volume. The delta between these turned out to be 0.18%, a very small amount. Then I just used a value of a 150 PSI compression reading and found I should be expecting a difference of only ~0.3 PSI due to the length differences.

Does this sound about right? Any help is greatly appreciated. Thanks.

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